From 97b4a671987123009724dfbdf188ec6e122573d4 Mon Sep 17 00:00:00 2001 From: Matthew Wilcox Date: Thu, 7 Jun 2018 17:08:53 -0700 Subject: mm: improve struct page documentation MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Randy Dunlap Acked-by: Vlastimil Babka Cc: Christoph Lameter Cc: Dave Hansen Cc: Jérôme Glisse Cc: "Kirill A . Shutemov" Cc: Lai Jiangshan Cc: Martin Schwidefsky Cc: Pekka Enberg Cc: Andrey Ryabinin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mm_types.h | 40 +++++++++++++++++++--------------------- 1 file changed, 19 insertions(+), 21 deletions(-) (limited to 'include/linux/mm_types.h') diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index dfe6d648e055..9b45760cc797 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 -- cgit v1.2.3-59-g8ed1b