From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Subject: 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@xxxxxxxxxxxxx Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/mm_types.h | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) diff -puN include/linux/mm_types.h~mm-document-how-to-use-struct-page include/linux/mm_types.h --- a/include/linux/mm_types.h~mm-document-how-to-use-struct-page +++ a/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 _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
