From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f70.google.com (mail-pl0-f70.google.com [209.85.160.70]) by kanga.kvack.org (Postfix) with ESMTP id 69E0B6B0271 for ; Wed, 20 Dec 2017 10:56:04 -0500 (EST) Received: by mail-pl0-f70.google.com with SMTP id d3so9702894plj.22 for ; Wed, 20 Dec 2017 07:56:04 -0800 (PST) Received: from bombadil.infradead.org (bombadil.infradead.org. [65.50.211.133]) by mx.google.com with ESMTPS id k192si12003825pgc.650.2017.12.20.07.56.03 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 20 Dec 2017 07:56:03 -0800 (PST) From: Matthew Wilcox Subject: [PATCH v2 7/8] mm: Document how to use struct page Date: Wed, 20 Dec 2017 07:55:51 -0800 Message-Id: <20171220155552.15884-8-willy@infradead.org> In-Reply-To: <20171220155552.15884-1-willy@infradead.org> References: <20171220155552.15884-1-willy@infradead.org> Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org Cc: akpm@linuxfoundation.org, Matthew Wilcox From: Matthew Wilcox 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 Acked-by: Kirill A. Shutemov Reviewed-by: Randy Dunlap Acked-by: Michal Hocko --- include/linux/mm_types.h | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 1a3ba1f1605d..ef35ea524c26 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 -- 2.15.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org