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f04a5d5d91
Patch series "mm: place pages to the freelist tail when onlining and undoing isolation", v2. When adding separate memory blocks via add_memory*() and onlining them immediately, the metadata (especially the memmap) of the next block will be placed onto one of the just added+onlined block. This creates a chain of unmovable allocations: If the last memory block cannot get offlined+removed() so will all dependent ones. We directly have unmovable allocations all over the place. This can be observed quite easily using virtio-mem, however, it can also be observed when using DIMMs. The freshly onlined pages will usually be placed to the head of the freelists, meaning they will be allocated next, turning the just-added memory usually immediately un-removable. The fresh pages are cold, prefering to allocate others (that might be hot) also feels to be the natural thing to do. It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: when adding separate, successive memory blocks, each memory block will have unmovable allocations on them - for example gigantic pages will fail to allocate. While the ZONE_NORMAL doesn't provide any guarantees that memory can get offlined+removed again (any kind of fragmentation with unmovable allocations is possible), there are many scenarios (hotplugging a lot of memory, running workload, hotunplug some memory/as much as possible) where we can offline+remove quite a lot with this patchset. a) To visualize the problem, a very simple example: Start a VM with 4GB and 8GB of virtio-mem memory: [root@localhost ~]# lsmem RANGE SIZE STATE REMOVABLE BLOCK 0x0000000000000000-0x00000000bfffffff 3G online yes 0-23 0x0000000100000000-0x000000033fffffff 9G online yes 32-103 Memory block size: 128M Total online memory: 12G Total offline memory: 0B Then try to unplug as much as possible using virtio-mem. Observe which memory blocks are still around. Without this patch set: [root@localhost ~]# lsmem RANGE SIZE STATE REMOVABLE BLOCK 0x0000000000000000-0x00000000bfffffff 3G online yes 0-23 0x0000000100000000-0x000000013fffffff 1G online yes 32-39 0x0000000148000000-0x000000014fffffff 128M online yes 41 0x0000000158000000-0x000000015fffffff 128M online yes 43 0x0000000168000000-0x000000016fffffff 128M online yes 45 0x0000000178000000-0x000000017fffffff 128M online yes 47 0x0000000188000000-0x0000000197ffffff 256M online yes 49-50 0x00000001a0000000-0x00000001a7ffffff 128M online yes 52 0x00000001b0000000-0x00000001b7ffffff 128M online yes 54 0x00000001c0000000-0x00000001c7ffffff 128M online yes 56 0x00000001d0000000-0x00000001d7ffffff 128M online yes 58 0x00000001e0000000-0x00000001e7ffffff 128M online yes 60 0x00000001f0000000-0x00000001f7ffffff 128M online yes 62 0x0000000200000000-0x0000000207ffffff 128M online yes 64 0x0000000210000000-0x0000000217ffffff 128M online yes 66 0x0000000220000000-0x0000000227ffffff 128M online yes 68 0x0000000230000000-0x0000000237ffffff 128M online yes 70 0x0000000240000000-0x0000000247ffffff 128M online yes 72 0x0000000250000000-0x0000000257ffffff 128M online yes 74 0x0000000260000000-0x0000000267ffffff 128M online yes 76 0x0000000270000000-0x0000000277ffffff 128M online yes 78 0x0000000280000000-0x0000000287ffffff 128M online yes 80 0x0000000290000000-0x0000000297ffffff 128M online yes 82 0x00000002a0000000-0x00000002a7ffffff 128M online yes 84 0x00000002b0000000-0x00000002b7ffffff 128M online yes 86 0x00000002c0000000-0x00000002c7ffffff 128M online yes 88 0x00000002d0000000-0x00000002d7ffffff 128M online yes 90 0x00000002e0000000-0x00000002e7ffffff 128M online yes 92 0x00000002f0000000-0x00000002f7ffffff 128M online yes 94 0x0000000300000000-0x0000000307ffffff 128M online yes 96 0x0000000310000000-0x0000000317ffffff 128M online yes 98 0x0000000320000000-0x0000000327ffffff 128M online yes 100 0x0000000330000000-0x000000033fffffff 256M online yes 102-103 Memory block size: 128M Total online memory: 8.1G Total offline memory: 0B With this patch set: [root@localhost ~]# lsmem RANGE SIZE STATE REMOVABLE BLOCK 0x0000000000000000-0x00000000bfffffff 3G online yes 0-23 0x0000000100000000-0x000000013fffffff 1G online yes 32-39 Memory block size: 128M Total online memory: 4G Total offline memory: 0B All memory can get unplugged, all memory block can get removed. Of course, no workload ran and the system was basically idle, but it highlights the issue - the fairly deterministic chain of unmovable allocations. When a huge page for the 2MB memmap is needed, a just-onlined 4MB page will be split. The remaining 2MB page will be used for the memmap of the next memory block. So one memory block will hold the memmap of the two following memory blocks. Finally the pages of the last-onlined memory block will get used for the next bigger allocations - if any allocation is unmovable, all dependent memory blocks cannot get unplugged and removed until that allocation is gone. Note that with bigger memory blocks (e.g., 256MB), *all* memory blocks are dependent and none can get unplugged again! b) Experiment with memory intensive workload I performed an experiment with an older version of this patch set (before we used undo_isolate_page_range() in online_pages(): Hotplug 56GB to a VM with an initial 4GB, onlining all memory to ZONE_NORMAL right from the kernel when adding it. I then run various memory intensive workloads that consume most system memory for a total of 45 minutes. Once finished, I try to unplug as much memory as possible. With this change, I am able to remove via virtio-mem (adding individual 128MB memory blocks) 413 out of 448 added memory blocks. Via individual (256MB) DIMMs 380 out of 448 added memory blocks. (I don't have any numbers without this patchset, but looking at the above example, it's at most half of the 448 memory blocks for virtio-mem, and most probably none for DIMMs). Again, there are workloads that might behave very differently due to the nature of ZONE_NORMAL. This change also affects (besides memory onlining): - Other users of undo_isolate_page_range(): Pages are always placed to the tail. -- When memory offlining fails -- When memory isolation fails after having isolated some pageblocks -- When alloc_contig_range() either succeeds or fails - Other users of __putback_isolated_page(): Pages are always placed to the tail. -- Free page reporting - Other users of __free_pages_core() -- AFAIKs, any memory that is getting exposed to the buddy during boot. IIUC we will now usually allocate memory from lower addresses within a zone first (especially during boot). - Other users of generic_online_page() -- Hyper-V balloon This patch (of 5): Let's prepare for additional flags and avoid long parameter lists of bools. Follow-up patches will also make use of the flags in __free_pages_ok(). Signed-off-by: David Hildenbrand <david@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> Reviewed-by: Vlastimil Babka <vbabka@suse.cz> Reviewed-by: Oscar Salvador <osalvador@suse.de> Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com> Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Mike Rapoport <rppt@kernel.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Haiyang Zhang <haiyangz@microsoft.com> Cc: "K. Y. Srinivasan" <kys@microsoft.com> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Michal Hocko <mhocko@suse.com> Cc: Scott Cheloha <cheloha@linux.ibm.com> Cc: Stephen Hemminger <sthemmin@microsoft.com> Cc: Wei Liu <wei.liu@kernel.org> Cc: Michal Hocko <mhocko@kernel.org> Link: https://lkml.kernel.org/r/20201005121534.15649-1-david@redhat.com Link: https://lkml.kernel.org/r/20201005121534.15649-2-david@redhat.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.