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Shutemov" To: Qian Cai Cc: Matthew Wilcox , akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault() Message-ID: <20200210212149.u6zlpi2jefi2vkfg@box> References: <1581354029-20154-1-git-send-email-cai@lca.pw> <20200210172511.GL8731@bombadil.infradead.org> <20200210180546.vt7yhdjav5oinij7@box> <1581364697.7365.45.camel@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1581364697.7365.45.camel@lca.pw> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Feb 10, 2020 at 02:58:17PM -0500, Qian Cai wrote: > On Mon, 2020-02-10 at 21:05 +0300, Kirill A. Shutemov wrote: > > On Mon, Feb 10, 2020 at 09:25:11AM -0800, Matthew Wilcox wrote: > > > On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote: > > > > @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf, > > > > if (page->index >= max_idx) > > > > goto unlock; > > > > > > > > - if (file->f_ra.mmap_miss > 0) > > > > + if (data_race(file->f_ra.mmap_miss > 0)) > > > > file->f_ra.mmap_miss--; > > > > > > How is this safe? Two threads can each see 1, and then both decrement the > > > in-memory copy, causing it to end up at -1. > > > > Right, it is bogus. > > > > Below is my completely untested attempt on fix this. It still allows > > races, but they will only lead to missed accounting, but not underflow. > > Looks good to me. Do you plan to send out an official patch? Feel free to submit it. After testing. -- Kirill A. Shutemov