From: Jan Kara Subject: Re: [PATCH] direct-io: fix stale data exposure from concurrent buffered read Date: Wed, 11 May 2016 18:57:15 +0200 Message-ID: <20160511165715.GH14744@quack2.suse.cz> References: <1462033162-21837-1-git-send-email-guaneryu@gmail.com> <20160506104622.GB10350@eguan.usersys.redhat.com> <20160507030400.GC10350@eguan.usersys.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eryu Guan , Jan Kara , Theodore T'so , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org To: Jeff Moyer Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon 09-05-16 09:55:26, Jeff Moyer wrote: > Eryu Guan writes: > > > On Fri, May 06, 2016 at 10:13:39AM -0400, Jeff Moyer wrote: > >> Eryu Guan writes: > >> > >> > On Thu, May 05, 2016 at 03:39:29PM -0400, Jeff Moyer wrote: > >> >> I think this code operates on blocks for a reason: we're trying to > >> >> determine if we'll trigger block allocation, right? For example, > >> >> consider a sparse file with i_size of 2k, and a write to offset 2k into > >> >> the file, with a file system block size of 4k. Should that have create > >> >> set or not? > >> > > >> > Thanks for pointing this out! I think 'create' should be 0 in this case, > >> > my test failed in this case, with both 4.6-rc6 stock kernel and my > >> > patched kernel. > >> > > >> > I'm testing an updated patch now, hopefully it's doing the right thing. > >> > It's basiclly something like: > >> > > >> > if (offset < i_size) > >> > create = 0; > >> > else if ((block_in_file >> blkfactor) == (i_size >> (blkbits + blkfactor)) && > >> > (i_size & ((1 << (blkbits + blkfactor)) - 1))) > >> > create = 0; > >> > >> I think that can be simplified to a single check; something like: > >> > >> if (block_in_file < total_blocks_in_file) > >> create = 0; > > > > I may miss something, but this doesn't seem right to me. Still take your > > example, on a 4k block size & 512 sector size filesystem > > ... where blocks are in file system block size units. So: > > if (fs_block_in_file < total_fs_blocks_in_file) Agreed. The test should be: if (fs_startblk < (i_size_read(dio->inode) >> (sdio->blkbits + sdio->blkfactor))) Sorry for not having a look earlier. Honza -- Jan Kara SUSE Labs, CR