From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Be really explicit about what bits / bytes are reserved for users that want to store extra information about the pages they allocate. Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> --- include/linux/mm_types.h | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 1a3ba1f1605d..a517d210f177 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -31,7 +31,28 @@ 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 + * will read and writes some of the fields in struct page to lock the page, + * then check 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 word 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. + * + * 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 + * three 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 -- 2.15.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
