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Fri, 20 Mar 2020 18:11:42 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=p6kUQaVzyG/7aafQEj2oYoGKarn6agZvQF5qHrKX8Tc=; b=eyNztVgX76i16/jeYqqPpZu4sX sXc8vs25jMw4vVAYXjoA8DgM2uzIMN5yBWkGawjo5WX9KQRuVqhyOMcR+1OXjTrnmUOvKFLlajWTh C/zG1s50fN4WqpS6HehSEJni5oQ1WzUhIAStAK+kmIlg6EMycQupwu059cgWaJPn310ClNl6D4VPx 0e3/G4ggInF9NSQVBazYXx0s2qIlmusdCkBUh0+eyUkvYj7N4d9XynyqouwQZVf7LWrCI1BgqP8Cs UrxUi+eztNgKF6XQIzB1awOkum9npNrbgagQQwgGAqvgh6c/Ea6GWk8cbheHh79L9/ElbL5oi8/X6 G99RY7Ig==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jFM7I-0001VF-Jx; Fri, 20 Mar 2020 18:11:32 +0000 Date: Fri, 20 Mar 2020 11:11:32 -0700 From: Matthew Wilcox To: Eric Biggers Message-ID: <20200320181132.GD4971@bombadil.infradead.org> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> <20200320165828.GB851@sol.localdomain> <20200320173040.GB4971@bombadil.infradead.org> <20200320180017.GE851@sol.localdomain> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200320180017.GE851@sol.localdomain> X-Headers-End: 1jFM7Q-001bHA-97 Subject: Re: [f2fs-dev] [PATCH v9 12/25] mm: Move end_index check out of readahead loop X-BeenThere: linux-f2fs-devel@lists.sourceforge.net X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: cluster-devel@redhat.com, linux-mm@kvack.org, John Hubbard , linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org, William Kucharski , linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, ocfs2-devel@oss.oracle.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote: > On Fri, Mar 20, 2020 at 10:30:40AM -0700, Matthew Wilcox wrote: > > On Fri, Mar 20, 2020 at 09:58:28AM -0700, Eric Biggers wrote: > > > On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > > > > + /* Avoid wrapping to the beginning of the file */ > > > > + if (index + nr_to_read < index) > > > > + nr_to_read = ULONG_MAX - index + 1; > > > > + /* Don't read past the page containing the last byte of the file */ > > > > + if (index + nr_to_read >= end_index) > > > > + nr_to_read = end_index - index + 1; > > > > > > There seem to be a couple off-by-one errors here. Shouldn't it be: > > > > > > /* Avoid wrapping to the beginning of the file */ > > > if (index + nr_to_read < index) > > > nr_to_read = ULONG_MAX - index; > > > > I think it's right. Imagine that index is ULONG_MAX. We should read one > > page (the one at ULONG_MAX). That would be ULONG_MAX - ULONG_MAX + 1. > > > > > /* Don't read past the page containing the last byte of the file */ > > > if (index + nr_to_read > end_index) > > > nr_to_read = end_index - index + 1; > > > > > > I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1', so that > > > 'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0. > > > > > > Then 'index + nr_to_read > end_index' rather 'index + nr_to_read >= end_index', > > > since otherwise nr_to_read can be increased by 1 rather than decreased or stay > > > the same as expected. > > > > Ooh, I missed the overflow case here. It should be: > > > > + if (index + nr_to_read - 1 > end_index) > > + nr_to_read = end_index - index + 1; > > > > But then if someone passes index=0 and nr_to_read=0, this underflows and the > entire file gets read. nr_to_read == 0 doesn't make sense ... I thought we filtered that out earlier, but I can't find anywhere that does that right now. I'd rather return early from __do_page_cache_readahead() to fix that. > The page cache isn't actually supposed to contain a page at index ULONG_MAX, > since MAX_LFS_FILESIZE is at most ((loff_t)ULONG_MAX << PAGE_SHIFT), right? So > I don't think we need to worry about reading the page with index ULONG_MAX. > I.e. I think it's fine to limit nr_to_read to 'ULONG_MAX - index', if that makes > it easier to avoid an overflow or underflow in the next check. I think we can get a page at ULONG_MAX on 32-bit systems? I mean, we can buy hard drives which are larger than 16TiB these days: https://www.pcmag.com/news/seagate-will-ship-18tb-and-20tb-hard-drives-in-2020 (even ignoring RAID devices) _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel