From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f199.google.com (mail-pl1-f199.google.com [209.85.214.199]) by kanga.kvack.org (Postfix) with ESMTP id C2E268E0038 for ; Mon, 7 Jan 2019 17:33:22 -0500 (EST) Received: by mail-pl1-f199.google.com with SMTP id m13so944331pls.15 for ; Mon, 07 Jan 2019 14:33:22 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id y17si29755701pgh.353.2019.01.07.14.33.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 07 Jan 2019 14:33:21 -0800 (PST) Date: Mon, 7 Jan 2019 14:33:19 -0800 From: Andrew Morton Subject: Re: [PATCH] mm: Remove redundant test from find_get_pages_contig Message-Id: <20190107143319.c74593a70c86441b80e7cccc@linux-foundation.org> In-Reply-To: <20190107200224.13260-1-willy@infradead.org> References: <20190107200224.13260-1-willy@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Mon, 7 Jan 2019 12:02:24 -0800 Matthew Wilcox wrote: > After we establish a reference on the page, we check the pointer continues > to be in the correct position in i_pages. There's no need to check the > page->mapping or page->index afterwards; if those can change after we've > got the reference, they can change after we return the page to the caller. But that isn't what the comment says. > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -1837,16 +1837,6 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index, > if (unlikely(page != xas_reload(&xas))) > goto put_page; > > - /* > - * must check mapping and index after taking the ref. > - * otherwise we can get both false positives and false > - * negatives, which is just confusing to the caller. > - */ > - if (!page->mapping || page_to_pgoff(page) != xas.xa_index) { > - put_page(page); > - break; > - } The assertion here is that the page's state can alter before we take the ref but not afterwards. Which is contrary to your assertion that "they can change after we return the page to the caller". This: commit 9cbb4cb21b19fff46cf1174d0ed699ef710e641c Author: Nick Piggin AuthorDate: Thu Jan 13 15:45:51 2011 -0800 Commit: Linus Torvalds CommitDate: Thu Jan 13 17:32:32 2011 -0800 mm: find_get_pages_contig fixlet Testing ->mapping and ->index without a ref is not stable as the page may have been reused at this point. Signed-off-by: Nick Piggin Reviewed-by: Wu Fengguang Reviewed-by: Minchan Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds diff --git a/mm/filemap.c b/mm/filemap.c index ca389394fa2a..1a3dd5914726 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -837,9 +837,6 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index, if (radix_tree_deref_retry(page)) goto restart; - if (page->mapping == NULL || page->index != index) - break; - if (!page_cache_get_speculative(page)) goto repeat; @@ -849,6 +846,16 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index, goto repeat; } + /* + * must check mapping and index after taking the ref. + * otherwise we can get both false positives and false + * negatives, which is just confusing to the caller. + */ + if (page->mapping == NULL || page->index != index) { + page_cache_release(page); + break; + } + pages[ret] = page; ret++; index++;