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mm: document how to use struct page

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Be really explicit about what bits / bytes are reserved for users that
want to store extra information about the pages they allocate.

Link: http://lkml.kernel.org/r/20171220155552.15884-8-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matthew Wilcox 2018-01-31 16:19:06 -08:00 committed by Linus Torvalds
parent 036e7aa49f
commit be50015d7e
1 changed files with 23 additions and 1 deletions
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24
include/linux/mm_types.h
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@ -31,7 +31,29 @@ struct hmm;
* it to keep track of whatever it is we are using the page for at the
* moment. Note that we have no way to track which tasks are using
* a page, though if it is a pagecache page, rmap structures can tell us
* who is mapping it.
* who is mapping it. If you allocate the page using alloc_pages(), you
* can use some of the space in struct page for your own purposes.
*
* Pages that were once in the page cache may be found under the RCU lock
* even after they have been recycled to a different purpose. The page
* cache reads and writes some of the fields in struct page to pin the
* page before checking that it's still in the page cache. It is vital
* that all users of struct page:
* 1. Use the first word as PageFlags.
* 2. Clear or preserve bit 0 of page->compound_head. It is used as
* PageTail for compound pages, and the page cache must not see false
* positives. Some users put a pointer here (guaranteed to be at least
* 4-byte aligned), other users avoid using the field altogether.
* 3. page->_refcount must either not be used, or must be used in such a
* way that other CPUs temporarily incrementing and then decrementing the
* refcount does not cause problems. On receiving the page from
* alloc_pages(), the refcount will be positive.
* 4. Either preserve page->_mapcount or restore it to -1 before freeing it.
*
* If you allocate pages of order > 0, you can use the fields in the struct
* page associated with each page, but bear in mind that the pages may have
* been inserted individually into the page cache, so you must use the above
* four fields in a compatible way for each struct page.
*
* SLUB uses cmpxchg_double() to atomically update its freelist and
* counters. That requires that freelist & counters be adjacent and