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mm: improve struct page documentation

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Rewrite the documentation to describe what you can use in struct page
rather than what you can't.

Link: http://lkml.kernel.org/r/20180518194519.3820-12-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.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 2018-06-07 17:08:53 -07:00 committed by Linus Torvalds
parent 4da1984edb
commit 97b4a67198
1 changed files with 19 additions and 21 deletions
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40
include/linux/mm_types.h
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@ -33,29 +33,27 @@ 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. If you allocate the page using alloc_pages(), you
* can use some of the space in struct page for your own purposes.
* who is mapping it.
*
* 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 the page using alloc_pages(), you can use some of the
* space in struct page for your own purposes. The five words in the main
* union are available, except for bit 0 of the first word which must be
* kept clear. Many users use this word to store a pointer to an object
* which is guaranteed to be aligned. If you use the same storage as
* page->mapping, you must restore it to NULL before freeing the page.
*
* 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.
* If your page will not be mapped to userspace, you can also use the four
* bytes in the mapcount union, but you must call page_mapcount_reset()
* before freeing it.
*
* If you want to use the refcount field, it 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.
*
* If you allocate pages of order > 0, you can use some of the fields
* in each subpage, but you may need to restore some of their values
* afterwards.
*
* SLUB uses cmpxchg_double() to atomically update its freelist and
* counters. That requires that freelist & counters be adjacent and