diff options
author | Matthew Wilcox <mawilcox@microsoft.com> | 2018-01-31 16:19:06 -0800 |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2018-01-31 17:18:38 -0800 |
commit | be50015d7eec0e96b312468291d8209c1cc49908 (patch) | |
tree | d2f83adc7cc05ad041793d3e883b457d4894ee50 /include/linux | |
parent | 036e7aa49fb29e0b49b99a56fa5611d4a5b99fb1 (diff) |
mm: document how to use struct page
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>
Diffstat (limited to 'include/linux')
-rw-r--r-- | include/linux/mm_types.h | 24 |
1 files changed, 23 insertions, 1 deletions
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 3e7e99784656..3f1fae8fb140 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -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 |