2018-03-21 19:22:34 +00:00
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.. _page_migration:
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==============
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2006-02-01 11:05:38 +00:00
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Page migration
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2018-03-21 19:22:34 +00:00
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==============
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2006-02-01 11:05:38 +00:00
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2020-09-02 22:52:47 +00:00
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Page migration allows moving the physical location of pages between
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nodes in a NUMA system while the process is running. This means that the
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2006-02-01 11:05:38 +00:00
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virtual addresses that the process sees do not change. However, the
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system rearranges the physical location of those pages.
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2020-09-02 22:52:47 +00:00
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Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>`
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for migrating pages to or from device private memory.
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The main intent of page migration is to reduce the latency of memory accesses
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by moving pages near to the processor where the process accessing that memory
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is running.
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Page migration allows a process to manually relocate the node on which its
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pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
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2020-09-02 22:52:47 +00:00
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a new memory policy via mbind(). The pages of a process can also be relocated
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from another process using the sys_migrate_pages() function call. The
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migrate_pages() function call takes two sets of nodes and moves pages of a
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process that are located on the from nodes to the destination nodes.
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2006-03-15 03:50:20 +00:00
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Page migration functions are provided by the numactl package by Andi Kleen
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(a version later than 0.9.3 is required. Get it from
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2020-09-02 22:52:47 +00:00
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https://github.com/numactl/numactl.git). numactl provides libnuma
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which provides an interface similar to other NUMA functionality for page
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migration. cat ``/proc/<pid>/numa_maps`` allows an easy review of where the
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2008-08-15 07:40:18 +00:00
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pages of a process are located. See also the numa_maps documentation in the
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proc(5) man page.
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Manual migration is useful if for example the scheduler has relocated
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a process to a processor on a distant node. A batch scheduler or an
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administrator may detect the situation and move the pages of the process
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nearer to the new processor. The kernel itself only provides
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2006-06-23 09:03:55 +00:00
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manual page migration support. Automatic page migration may be implemented
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through user space processes that move pages. A special function call
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"move_pages" allows the moving of individual pages within a process.
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For example, A NUMA profiler may obtain a log showing frequent off-node
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accesses and may use the result to move pages to more advantageous
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locations.
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2006-02-01 11:05:38 +00:00
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Larger installations usually partition the system using cpusets into
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sections of nodes. Paul Jackson has equipped cpusets with the ability to
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2009-02-04 09:12:08 +00:00
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move pages when a task is moved to another cpuset (See
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:ref:`CPUSETS <cpusets>`).
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Cpusets allow the automation of process locality. If a task is moved to
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a new cpuset then also all its pages are moved with it so that the
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performance of the process does not sink dramatically. Also the pages
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of processes in a cpuset are moved if the allowed memory nodes of a
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cpuset are changed.
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2006-02-01 11:05:38 +00:00
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Page migration allows the preservation of the relative location of pages
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within a group of nodes for all migration techniques which will preserve a
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particular memory allocation pattern generated even after migrating a
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process. This is necessary in order to preserve the memory latencies.
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Processes will run with similar performance after migration.
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Page migration occurs in several steps. First a high level
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2006-03-15 03:50:20 +00:00
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description for those trying to use migrate_pages() from the kernel
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(for userspace usage see the Andi Kleen's numactl package mentioned above)
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and then a low level description of how the low level details work.
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2006-02-01 11:05:38 +00:00
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2018-03-21 19:22:34 +00:00
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In kernel use of migrate_pages()
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================================
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1. Remove pages from the LRU.
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Lists of pages to be migrated are generated by scanning over
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pages and moving them into lists. This is done by
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2006-03-15 03:50:20 +00:00
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calling isolate_lru_page().
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2020-09-02 22:52:47 +00:00
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Calling isolate_lru_page() increases the references to the page
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so that it cannot vanish while the page migration occurs.
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2020-09-02 22:52:47 +00:00
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It also prevents the swapper or other scans from encountering
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the page.
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2006-02-01 11:05:38 +00:00
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2006-06-23 09:03:55 +00:00
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2. We need to have a function of type new_page_t that can be
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passed to migrate_pages(). This function should figure out
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how to allocate the correct new page given the old page.
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3. The migrate_pages() function is called which attempts
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2006-06-23 09:03:55 +00:00
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to do the migration. It will call the function to allocate
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the new page for each page that is considered for
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moving.
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2006-02-01 11:05:38 +00:00
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2018-03-21 19:22:34 +00:00
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How migrate_pages() works
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=========================
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migrate_pages() does several passes over its list of pages. A page is moved
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if all references to a page are removable at the time. The page has
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already been removed from the LRU via isolate_lru_page() and the refcount
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is increased so that the page cannot be freed while page migration occurs.
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Steps:
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1. Lock the page to be migrated.
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2018-04-10 23:36:56 +00:00
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2. Ensure that writeback is complete.
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2015-11-06 02:50:02 +00:00
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3. Lock the new page that we want to move to. It is locked so that accesses to
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this (not yet up-to-date) page immediately block while the move is in progress.
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2015-11-06 02:50:02 +00:00
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4. All the page table references to the page are converted to migration
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2015-11-06 02:49:30 +00:00
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entries. This decreases the mapcount of a page. If the resulting
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mapcount is not zero then we do not migrate the page. All user space
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processes that attempt to access the page will now wait on the page lock
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or wait for the migration page table entry to be removed.
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2018-04-10 23:36:56 +00:00
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5. The i_pages lock is taken. This will cause all processes trying
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to access the page via the mapping to block on the spinlock.
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6. The refcount of the page is examined and we back out if references remain.
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Otherwise, we know that we are the only one referencing this page.
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2015-11-06 02:50:02 +00:00
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7. The radix tree is checked and if it does not contain the pointer to this
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page then we back out because someone else modified the radix tree.
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2015-11-06 02:50:02 +00:00
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8. The new page is prepped with some settings from the old page so that
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accesses to the new page will discover a page with the correct settings.
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9. The radix tree is changed to point to the new page.
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2018-04-10 23:36:56 +00:00
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10. The reference count of the old page is dropped because the address space
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reference is gone. A reference to the new page is established because
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the new page is referenced by the address space.
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2018-04-10 23:36:56 +00:00
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11. The i_pages lock is dropped. With that lookups in the mapping
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become possible again. Processes will move from spinning on the lock
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to sleeping on the locked new page.
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12. The page contents are copied to the new page.
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13. The remaining page flags are copied to the new page.
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14. The old page flags are cleared to indicate that the page does
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not provide any information anymore.
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15. Queued up writeback on the new page is triggered.
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2020-09-02 22:52:47 +00:00
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16. If migration entries were inserted into the page table, then replace them
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with real ptes. Doing so will enable access for user space processes not
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already waiting for the page lock.
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17. The page locks are dropped from the old and new page.
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Processes waiting on the page lock will redo their page faults
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and will reach the new page.
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18. The new page is moved to the LRU and can be scanned by the swapper,
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etc. again.
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2006-03-15 03:50:20 +00:00
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2018-03-21 19:22:34 +00:00
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Non-LRU page migration
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======================
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mm: migrate: support non-lru movable page migration
We have allowed migration for only LRU pages until now and it was enough
to make high-order pages. But recently, embedded system(e.g., webOS,
android) uses lots of non-movable pages(e.g., zram, GPU memory) so we
have seen several reports about troubles of small high-order allocation.
For fixing the problem, there were several efforts (e,g,. enhance
compaction algorithm, SLUB fallback to 0-order page, reserved memory,
vmalloc and so on) but if there are lots of non-movable pages in system,
their solutions are void in the long run.
So, this patch is to support facility to change non-movable pages with
movable. For the feature, this patch introduces functions related to
migration to address_space_operations as well as some page flags.
If a driver want to make own pages movable, it should define three
functions which are function pointers of struct
address_space_operations.
1. bool (*isolate_page) (struct page *page, isolate_mode_t mode);
What VM expects on isolate_page function of driver is to return *true*
if driver isolates page successfully. On returing true, VM marks the
page as PG_isolated so concurrent isolation in several CPUs skip the
page for isolation. If a driver cannot isolate the page, it should
return *false*.
Once page is successfully isolated, VM uses page.lru fields so driver
shouldn't expect to preserve values in that fields.
2. int (*migratepage) (struct address_space *mapping,
struct page *newpage, struct page *oldpage, enum migrate_mode);
After isolation, VM calls migratepage of driver with isolated page. The
function of migratepage is to move content of the old page to new page
and set up fields of struct page newpage. Keep in mind that you should
indicate to the VM the oldpage is no longer movable via
__ClearPageMovable() under page_lock if you migrated the oldpage
successfully and returns 0. If driver cannot migrate the page at the
moment, driver can return -EAGAIN. On -EAGAIN, VM will retry page
migration in a short time because VM interprets -EAGAIN as "temporal
migration failure". On returning any error except -EAGAIN, VM will give
up the page migration without retrying in this time.
Driver shouldn't touch page.lru field VM using in the functions.
3. void (*putback_page)(struct page *);
If migration fails on isolated page, VM should return the isolated page
to the driver so VM calls driver's putback_page with migration failed
page. In this function, driver should put the isolated page back to the
own data structure.
4. non-lru movable page flags
There are two page flags for supporting non-lru movable page.
* PG_movable
Driver should use the below function to make page movable under
page_lock.
void __SetPageMovable(struct page *page, struct address_space *mapping)
It needs argument of address_space for registering migration family
functions which will be called by VM. Exactly speaking, PG_movable is
not a real flag of struct page. Rather than, VM reuses page->mapping's
lower bits to represent it.
#define PAGE_MAPPING_MOVABLE 0x2
page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
so driver shouldn't access page->mapping directly. Instead, driver
should use page_mapping which mask off the low two bits of page->mapping
so it can get right struct address_space.
For testing of non-lru movable page, VM supports __PageMovable function.
However, it doesn't guarantee to identify non-lru movable page because
page->mapping field is unified with other variables in struct page. As
well, if driver releases the page after isolation by VM, page->mapping
doesn't have stable value although it has PAGE_MAPPING_MOVABLE (Look at
__ClearPageMovable). But __PageMovable is cheap to catch whether page
is LRU or non-lru movable once the page has been isolated. Because LRU
pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
good for just peeking to test non-lru movable pages before more
expensive checking with lock_page in pfn scanning to select victim.
For guaranteeing non-lru movable page, VM provides PageMovable function.
Unlike __PageMovable, PageMovable functions validates page->mapping and
mapping->a_ops->isolate_page under lock_page. The lock_page prevents
sudden destroying of page->mapping.
Driver using __SetPageMovable should clear the flag via
__ClearMovablePage under page_lock before the releasing the page.
* PG_isolated
To prevent concurrent isolation among several CPUs, VM marks isolated
page as PG_isolated under lock_page. So if a CPU encounters PG_isolated
non-lru movable page, it can skip it. Driver doesn't need to manipulate
the flag because VM will set/clear it automatically. Keep in mind that
if driver sees PG_isolated page, it means the page have been isolated by
VM so it shouldn't touch page.lru field. PG_isolated is alias with
PG_reclaim flag so driver shouldn't use the flag for own purpose.
[opensource.ganesh@gmail.com: mm/compaction: remove local variable is_lru]
Link: http://lkml.kernel.org/r/20160618014841.GA7422@leo-test
Link: http://lkml.kernel.org/r/1464736881-24886-3-git-send-email-minchan@kernel.org
Signed-off-by: Gioh Kim <gi-oh.kim@profitbricks.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: John Einar Reitan <john.reitan@foss.arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2016-07-26 22:23:05 +00:00
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2022-06-07 19:38:48 +00:00
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Although migration originally aimed for reducing the latency of memory
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accesses for NUMA, compaction also uses migration to create high-order
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pages. For compaction purposes, it is also useful to be able to move
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non-LRU pages, such as zsmalloc and virtio-balloon pages.
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mm: migrate: support non-lru movable page migration
We have allowed migration for only LRU pages until now and it was enough
to make high-order pages. But recently, embedded system(e.g., webOS,
android) uses lots of non-movable pages(e.g., zram, GPU memory) so we
have seen several reports about troubles of small high-order allocation.
For fixing the problem, there were several efforts (e,g,. enhance
compaction algorithm, SLUB fallback to 0-order page, reserved memory,
vmalloc and so on) but if there are lots of non-movable pages in system,
their solutions are void in the long run.
So, this patch is to support facility to change non-movable pages with
movable. For the feature, this patch introduces functions related to
migration to address_space_operations as well as some page flags.
If a driver want to make own pages movable, it should define three
functions which are function pointers of struct
address_space_operations.
1. bool (*isolate_page) (struct page *page, isolate_mode_t mode);
What VM expects on isolate_page function of driver is to return *true*
if driver isolates page successfully. On returing true, VM marks the
page as PG_isolated so concurrent isolation in several CPUs skip the
page for isolation. If a driver cannot isolate the page, it should
return *false*.
Once page is successfully isolated, VM uses page.lru fields so driver
shouldn't expect to preserve values in that fields.
2. int (*migratepage) (struct address_space *mapping,
struct page *newpage, struct page *oldpage, enum migrate_mode);
After isolation, VM calls migratepage of driver with isolated page. The
function of migratepage is to move content of the old page to new page
and set up fields of struct page newpage. Keep in mind that you should
indicate to the VM the oldpage is no longer movable via
__ClearPageMovable() under page_lock if you migrated the oldpage
successfully and returns 0. If driver cannot migrate the page at the
moment, driver can return -EAGAIN. On -EAGAIN, VM will retry page
migration in a short time because VM interprets -EAGAIN as "temporal
migration failure". On returning any error except -EAGAIN, VM will give
up the page migration without retrying in this time.
Driver shouldn't touch page.lru field VM using in the functions.
3. void (*putback_page)(struct page *);
If migration fails on isolated page, VM should return the isolated page
to the driver so VM calls driver's putback_page with migration failed
page. In this function, driver should put the isolated page back to the
own data structure.
4. non-lru movable page flags
There are two page flags for supporting non-lru movable page.
* PG_movable
Driver should use the below function to make page movable under
page_lock.
void __SetPageMovable(struct page *page, struct address_space *mapping)
It needs argument of address_space for registering migration family
functions which will be called by VM. Exactly speaking, PG_movable is
not a real flag of struct page. Rather than, VM reuses page->mapping's
lower bits to represent it.
#define PAGE_MAPPING_MOVABLE 0x2
page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
so driver shouldn't access page->mapping directly. Instead, driver
should use page_mapping which mask off the low two bits of page->mapping
so it can get right struct address_space.
For testing of non-lru movable page, VM supports __PageMovable function.
However, it doesn't guarantee to identify non-lru movable page because
page->mapping field is unified with other variables in struct page. As
well, if driver releases the page after isolation by VM, page->mapping
doesn't have stable value although it has PAGE_MAPPING_MOVABLE (Look at
__ClearPageMovable). But __PageMovable is cheap to catch whether page
is LRU or non-lru movable once the page has been isolated. Because LRU
pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
good for just peeking to test non-lru movable pages before more
expensive checking with lock_page in pfn scanning to select victim.
For guaranteeing non-lru movable page, VM provides PageMovable function.
Unlike __PageMovable, PageMovable functions validates page->mapping and
mapping->a_ops->isolate_page under lock_page. The lock_page prevents
sudden destroying of page->mapping.
Driver using __SetPageMovable should clear the flag via
__ClearMovablePage under page_lock before the releasing the page.
* PG_isolated
To prevent concurrent isolation among several CPUs, VM marks isolated
page as PG_isolated under lock_page. So if a CPU encounters PG_isolated
non-lru movable page, it can skip it. Driver doesn't need to manipulate
the flag because VM will set/clear it automatically. Keep in mind that
if driver sees PG_isolated page, it means the page have been isolated by
VM so it shouldn't touch page.lru field. PG_isolated is alias with
PG_reclaim flag so driver shouldn't use the flag for own purpose.
[opensource.ganesh@gmail.com: mm/compaction: remove local variable is_lru]
Link: http://lkml.kernel.org/r/20160618014841.GA7422@leo-test
Link: http://lkml.kernel.org/r/1464736881-24886-3-git-send-email-minchan@kernel.org
Signed-off-by: Gioh Kim <gi-oh.kim@profitbricks.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: John Einar Reitan <john.reitan@foss.arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2016-07-26 22:23:05 +00:00
|
|
|
|
2022-06-07 19:38:48 +00:00
|
|
|
If a driver wants to make its pages movable, it should define a struct
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|
|
|
movable_operations. It then needs to call __SetPageMovable() on each
|
|
|
|
|
page that it may be able to move. This uses the ``page->mapping`` field,
|
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|
|
|
so this field is not available for the driver to use for other purposes.
|
mm: migrate: support non-lru movable page migration
We have allowed migration for only LRU pages until now and it was enough
to make high-order pages. But recently, embedded system(e.g., webOS,
android) uses lots of non-movable pages(e.g., zram, GPU memory) so we
have seen several reports about troubles of small high-order allocation.
For fixing the problem, there were several efforts (e,g,. enhance
compaction algorithm, SLUB fallback to 0-order page, reserved memory,
vmalloc and so on) but if there are lots of non-movable pages in system,
their solutions are void in the long run.
So, this patch is to support facility to change non-movable pages with
movable. For the feature, this patch introduces functions related to
migration to address_space_operations as well as some page flags.
If a driver want to make own pages movable, it should define three
functions which are function pointers of struct
address_space_operations.
1. bool (*isolate_page) (struct page *page, isolate_mode_t mode);
What VM expects on isolate_page function of driver is to return *true*
if driver isolates page successfully. On returing true, VM marks the
page as PG_isolated so concurrent isolation in several CPUs skip the
page for isolation. If a driver cannot isolate the page, it should
return *false*.
Once page is successfully isolated, VM uses page.lru fields so driver
shouldn't expect to preserve values in that fields.
2. int (*migratepage) (struct address_space *mapping,
struct page *newpage, struct page *oldpage, enum migrate_mode);
After isolation, VM calls migratepage of driver with isolated page. The
function of migratepage is to move content of the old page to new page
and set up fields of struct page newpage. Keep in mind that you should
indicate to the VM the oldpage is no longer movable via
__ClearPageMovable() under page_lock if you migrated the oldpage
successfully and returns 0. If driver cannot migrate the page at the
moment, driver can return -EAGAIN. On -EAGAIN, VM will retry page
migration in a short time because VM interprets -EAGAIN as "temporal
migration failure". On returning any error except -EAGAIN, VM will give
up the page migration without retrying in this time.
Driver shouldn't touch page.lru field VM using in the functions.
3. void (*putback_page)(struct page *);
If migration fails on isolated page, VM should return the isolated page
to the driver so VM calls driver's putback_page with migration failed
page. In this function, driver should put the isolated page back to the
own data structure.
4. non-lru movable page flags
There are two page flags for supporting non-lru movable page.
* PG_movable
Driver should use the below function to make page movable under
page_lock.
void __SetPageMovable(struct page *page, struct address_space *mapping)
It needs argument of address_space for registering migration family
functions which will be called by VM. Exactly speaking, PG_movable is
not a real flag of struct page. Rather than, VM reuses page->mapping's
lower bits to represent it.
#define PAGE_MAPPING_MOVABLE 0x2
page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
so driver shouldn't access page->mapping directly. Instead, driver
should use page_mapping which mask off the low two bits of page->mapping
so it can get right struct address_space.
For testing of non-lru movable page, VM supports __PageMovable function.
However, it doesn't guarantee to identify non-lru movable page because
page->mapping field is unified with other variables in struct page. As
well, if driver releases the page after isolation by VM, page->mapping
doesn't have stable value although it has PAGE_MAPPING_MOVABLE (Look at
__ClearPageMovable). But __PageMovable is cheap to catch whether page
is LRU or non-lru movable once the page has been isolated. Because LRU
pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
good for just peeking to test non-lru movable pages before more
expensive checking with lock_page in pfn scanning to select victim.
For guaranteeing non-lru movable page, VM provides PageMovable function.
Unlike __PageMovable, PageMovable functions validates page->mapping and
mapping->a_ops->isolate_page under lock_page. The lock_page prevents
sudden destroying of page->mapping.
Driver using __SetPageMovable should clear the flag via
__ClearMovablePage under page_lock before the releasing the page.
* PG_isolated
To prevent concurrent isolation among several CPUs, VM marks isolated
page as PG_isolated under lock_page. So if a CPU encounters PG_isolated
non-lru movable page, it can skip it. Driver doesn't need to manipulate
the flag because VM will set/clear it automatically. Keep in mind that
if driver sees PG_isolated page, it means the page have been isolated by
VM so it shouldn't touch page.lru field. PG_isolated is alias with
PG_reclaim flag so driver shouldn't use the flag for own purpose.
[opensource.ganesh@gmail.com: mm/compaction: remove local variable is_lru]
Link: http://lkml.kernel.org/r/20160618014841.GA7422@leo-test
Link: http://lkml.kernel.org/r/1464736881-24886-3-git-send-email-minchan@kernel.org
Signed-off-by: Gioh Kim <gi-oh.kim@profitbricks.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: John Einar Reitan <john.reitan@foss.arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2016-07-26 22:23:05 +00:00
|
|
|
|
2020-08-12 01:31:51 +00:00
|
|
|
Monitoring Migration
|
|
|
|
|
=====================
|
|
|
|
|
|
|
|
|
|
The following events (counters) can be used to monitor page migration.
|
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|
|
1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a
|
2022-01-14 22:08:37 +00:00
|
|
|
page was migrated. If the page was a non-THP and non-hugetlb page, then
|
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|
|
|
this counter is increased by one. If the page was a THP or hugetlb, then
|
|
|
|
|
this counter is increased by the number of THP or hugetlb subpages.
|
|
|
|
|
For example, migration of a single 2MB THP that has 4KB-size base pages
|
|
|
|
|
(subpages) will cause this counter to increase by 512.
|
2020-08-12 01:31:51 +00:00
|
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|
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|
|
2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
|
2020-09-02 22:52:47 +00:00
|
|
|
PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages,
|
2022-01-14 22:08:37 +00:00
|
|
|
if it was a THP or hugetlb.
|
2020-08-12 01:31:51 +00:00
|
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|
|
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|
|
3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.
|
|
|
|
|
|
|
|
|
|
4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
|
|
|
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|
|
|
|
|
|
5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had
|
|
|
|
|
to be split. After splitting, a migration retry was used for it's sub-pages.
|
|
|
|
|
|
|
|
|
|
THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or
|
|
|
|
|
PGMIGRATE_FAIL events. For example, a THP migration failure will cause both
|
|
|
|
|
THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.
|
|
|
|
|
|
mm: migrate: support non-lru movable page migration
We have allowed migration for only LRU pages until now and it was enough
to make high-order pages. But recently, embedded system(e.g., webOS,
android) uses lots of non-movable pages(e.g., zram, GPU memory) so we
have seen several reports about troubles of small high-order allocation.
For fixing the problem, there were several efforts (e,g,. enhance
compaction algorithm, SLUB fallback to 0-order page, reserved memory,
vmalloc and so on) but if there are lots of non-movable pages in system,
their solutions are void in the long run.
So, this patch is to support facility to change non-movable pages with
movable. For the feature, this patch introduces functions related to
migration to address_space_operations as well as some page flags.
If a driver want to make own pages movable, it should define three
functions which are function pointers of struct
address_space_operations.
1. bool (*isolate_page) (struct page *page, isolate_mode_t mode);
What VM expects on isolate_page function of driver is to return *true*
if driver isolates page successfully. On returing true, VM marks the
page as PG_isolated so concurrent isolation in several CPUs skip the
page for isolation. If a driver cannot isolate the page, it should
return *false*.
Once page is successfully isolated, VM uses page.lru fields so driver
shouldn't expect to preserve values in that fields.
2. int (*migratepage) (struct address_space *mapping,
struct page *newpage, struct page *oldpage, enum migrate_mode);
After isolation, VM calls migratepage of driver with isolated page. The
function of migratepage is to move content of the old page to new page
and set up fields of struct page newpage. Keep in mind that you should
indicate to the VM the oldpage is no longer movable via
__ClearPageMovable() under page_lock if you migrated the oldpage
successfully and returns 0. If driver cannot migrate the page at the
moment, driver can return -EAGAIN. On -EAGAIN, VM will retry page
migration in a short time because VM interprets -EAGAIN as "temporal
migration failure". On returning any error except -EAGAIN, VM will give
up the page migration without retrying in this time.
Driver shouldn't touch page.lru field VM using in the functions.
3. void (*putback_page)(struct page *);
If migration fails on isolated page, VM should return the isolated page
to the driver so VM calls driver's putback_page with migration failed
page. In this function, driver should put the isolated page back to the
own data structure.
4. non-lru movable page flags
There are two page flags for supporting non-lru movable page.
* PG_movable
Driver should use the below function to make page movable under
page_lock.
void __SetPageMovable(struct page *page, struct address_space *mapping)
It needs argument of address_space for registering migration family
functions which will be called by VM. Exactly speaking, PG_movable is
not a real flag of struct page. Rather than, VM reuses page->mapping's
lower bits to represent it.
#define PAGE_MAPPING_MOVABLE 0x2
page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
so driver shouldn't access page->mapping directly. Instead, driver
should use page_mapping which mask off the low two bits of page->mapping
so it can get right struct address_space.
For testing of non-lru movable page, VM supports __PageMovable function.
However, it doesn't guarantee to identify non-lru movable page because
page->mapping field is unified with other variables in struct page. As
well, if driver releases the page after isolation by VM, page->mapping
doesn't have stable value although it has PAGE_MAPPING_MOVABLE (Look at
__ClearPageMovable). But __PageMovable is cheap to catch whether page
is LRU or non-lru movable once the page has been isolated. Because LRU
pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
good for just peeking to test non-lru movable pages before more
expensive checking with lock_page in pfn scanning to select victim.
For guaranteeing non-lru movable page, VM provides PageMovable function.
Unlike __PageMovable, PageMovable functions validates page->mapping and
mapping->a_ops->isolate_page under lock_page. The lock_page prevents
sudden destroying of page->mapping.
Driver using __SetPageMovable should clear the flag via
__ClearMovablePage under page_lock before the releasing the page.
* PG_isolated
To prevent concurrent isolation among several CPUs, VM marks isolated
page as PG_isolated under lock_page. So if a CPU encounters PG_isolated
non-lru movable page, it can skip it. Driver doesn't need to manipulate
the flag because VM will set/clear it automatically. Keep in mind that
if driver sees PG_isolated page, it means the page have been isolated by
VM so it shouldn't touch page.lru field. PG_isolated is alias with
PG_reclaim flag so driver shouldn't use the flag for own purpose.
[opensource.ganesh@gmail.com: mm/compaction: remove local variable is_lru]
Link: http://lkml.kernel.org/r/20160618014841.GA7422@leo-test
Link: http://lkml.kernel.org/r/1464736881-24886-3-git-send-email-minchan@kernel.org
Signed-off-by: Gioh Kim <gi-oh.kim@profitbricks.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: John Einar Reitan <john.reitan@foss.arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2016-07-26 22:23:05 +00:00
|
|
|
Christoph Lameter, May 8, 2006.
|
|
|
|
|
Minchan Kim, Mar 28, 2016.
|
2022-06-07 19:38:48 +00:00
|
|
|
|
|
|
|
|
.. kernel-doc:: include/linux/migrate.h
|
