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Sun, 22 Mar 2020 16:28:34 +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=+q5SE8PabkJ4E2ARJpyvT2voAuJ2mKWWauYSLk4KvP8=; b=DMIkZSYbAXVYZnU8ct+6eTALIM IIwMq32XvglVnA0tcHkjB7yxlMiata26hBz45kwB5a05tIb02c9RE5ySYhs3Ho1G+lFVSgzlzVVDc DUMiK8p2sT6gtwlUuoh5Nc7LPQ6R2kVmwSly/JqhfRE2B1JKygrsPQSdgxP2bs4+VdMciVAE86kHt faPtljJno8DX46yFjGcZZwU56Jt23D6buy8qiu9daFTCr8dE5hV5cU6dlBPdz0Fx7mooeF/M7/3E9 cV1R/zopL7FbuerKPz2Tn81i5E79BF6SUI1praSaJFyVmD8DjvZ9DxsxJvgden2Wq6jHCv3KoNE8F cMFMFWAw==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jG3SU-0000CF-PX; Sun, 22 Mar 2020 16:28:18 +0000 Date: Sun, 22 Mar 2020 09:28:18 -0700 From: Matthew Wilcox To: Eric Biggers Message-ID: <20200322162818.GG4971@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> <20200320181132.GD4971@bombadil.infradead.org> <20200320182452.GF851@sol.localdomain> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200320182452.GF851@sol.localdomain> X-Headers-End: 1jG3Sg-00CpWF-Tk 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 11:24:52AM -0700, Eric Biggers wrote: > On Fri, Mar 20, 2020 at 11:11:32AM -0700, Matthew Wilcox wrote: > > On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote: > > > 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) > > The max file size is ((loff_t)ULONG_MAX << PAGE_SHIFT) which means the maximum > page *index* is ULONG_MAX - 1, not ULONG_MAX. I see where we set that for _files_ ... I can't find anywhere that we prevent i_size getting that big for block devices. Maybe I'm missing something. > Anyway, I think we may be making this much too complicated. How about just: > > pgoff_t i_nrpages = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE); > > if (index >= i_nrpages) > return; > /* Don't read past the end of the file */ > nr_to_read = min(nr_to_read, i_nrpages - index); > > That's 2 branches instead of 4. (Note that assigning to i_nrpages can't > overflow, since the max number of pages is ULONG_MAX not ULONG_MAX + 1.) I like where you're going with this. Just to be on the safe side, I'd prefer to do this: @@ -266,11 +266,8 @@ void __do_page_cache_readahead(struct address_space *mapping, end_index = (isize - 1) >> PAGE_SHIFT; if (index > end_index) return; - /* 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) + if (nr_to_read > end_index - index) nr_to_read = end_index - index + 1; page_cache_readahead_unbounded(mapping, file, index, nr_to_read, end_index - index + 1 could only overflow if end_index is ULONG_MAX and index is 0. But if end_index is ULONG_MAX and index is 0, then nr_to_read is necessarily <= ULONG_MAX, so the condition is false. And if nr_to_read is 0, then the condition is also false, so it won't increase nr_to_read from 0 to 1. It might assign x to nr_to_read when nr_to_read is already x, but that's harmless. Thanks! _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel