From: Matthew Wilcox <email@example.com> To: firstname.lastname@example.org Cc: "Matthew Wilcox" <email@example.com>, "Andrew Morton" <firstname.lastname@example.org>, "Kirill A . Shutemov" <email@example.com>, "Christoph Lameter" <firstname.lastname@example.org>, "Lai Jiangshan" <email@example.com>, "Pekka Enberg" <firstname.lastname@example.org>, "Vlastimil Babka" <email@example.com>, "Dave Hansen" <firstname.lastname@example.org>, "Jérôme Glisse" <email@example.com> Subject: [PATCH v5 11/17] mm: Improve struct page documentation Date: Fri, 4 May 2018 11:33:12 -0700 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> From: Matthew Wilcox <firstname.lastname@example.org> Rewrite the documentation to describe what you can use in struct page rather than what you can't. Signed-off-by: Matthew Wilcox <email@example.com> Reviewed-by: Randy Dunlap <firstname.lastname@example.org> --- include/linux/mm_types.h | 40 +++++++++++++++++++--------------------- 1 file changed, 19 insertions(+), 21 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index cf3bbee8c9a1..90a6dbeeef11 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -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 -- 2.17.0
next prev parent reply other threads:[~2018-05-04 18:33 UTC|newest] Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-05-04 18:33 [PATCH v5 00/17] Rearrange struct page Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 01/17] s390: Use _refcount for pgtables Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 02/17] mm: Split page_type out from _mapcount Matthew Wilcox 2018-05-14 14:33 ` Vlastimil Babka 2018-05-04 18:33 ` [PATCH v5 03/17] mm: Mark pages in use for page tables Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 04/17] mm: Switch s_mem and slab_cache in struct page Matthew Wilcox 2018-05-07 14:58 ` Christopher Lameter 2018-05-14 14:37 ` Vlastimil Babka 2018-05-04 18:33 ` [PATCH v5 05/17] mm: Move 'private' union within " Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 06/17] mm: Move _refcount out of struct page union Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 07/17] mm: Combine first three unions in struct page Matthew Wilcox 2018-05-14 14:42 ` Vlastimil Babka 2018-05-04 18:33 ` [PATCH v5 08/17] mm: Use page->deferred_list Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 09/17] mm: Move lru union within struct page Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 10/17] mm: Combine LRU and main union in " Matthew Wilcox 2018-05-04 18:33 ` Matthew Wilcox [this message] 2018-05-15 9:22 ` [PATCH v5 11/17] mm: Improve struct page documentation Vlastimil Babka 2018-05-04 18:33 ` [PATCH v5 12/17] mm: Add pt_mm to struct page Matthew Wilcox 2018-05-15 9:27 ` Vlastimil Babka 2018-05-15 12:23 ` Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 13/17] mm: Add hmm_data " Matthew Wilcox 2018-05-15 9:32 ` Vlastimil Babka 2018-05-15 12:40 ` Vlastimil Babka 2018-05-04 18:33 ` [PATCH v5 14/17] slab,slub: Remove rcu_head size checks Matthew Wilcox 2018-05-15 11:30 ` Vlastimil Babka 2018-05-04 18:33 ` [PATCH v5 15/17] slub: Remove kmem_cache->reserved Matthew Wilcox 2018-05-04 18:33 ` [PATCH v5 16/17] slub: Remove 'reserved' file from sysfs Matthew Wilcox 2018-05-07 15:01 ` Christopher Lameter 2018-05-04 18:33 ` [PATCH v5 17/17] mm: Distinguish VMalloc pages Matthew Wilcox 2018-05-04 20:52 ` [PATCH v5 00/17] Rearrange struct page Andrew Morton 2018-05-04 21:25 ` Matthew Wilcox
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