From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kirill A. Shutemov" Subject: Re: [PATCH] mm, fs: avoid page allocation beyond i_size on read Date: Thu, 22 Aug 2013 16:05:27 +0300 (EEST) Message-ID: <20130822130527.71C0AE0090@blue.fi.intel.com> References: <1377099441-2224-1-git-send-email-kirill.shutemov@linux.intel.com> <1377100012.2738.28.camel@menhir> <20130821160817.940D3E0090@blue.fi.intel.com> <1377103332.2738.37.camel@menhir> <20130821135821.fc8f5a2551a28c9ce9c4b049@linux-foundation.org> <1377163725.2720.18.camel@menhir> Content-Transfer-Encoding: 7bit Cc: Andrew Morton , "Kirill A. Shutemov" , Dave Hansen , Jan Kara , Al Viro , NeilBrown , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Steven Whitehouse Return-path: In-Reply-To: <1377163725.2720.18.camel@menhir> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org Steven Whitehouse wrote: > Hi, > > On Wed, 2013-08-21 at 13:58 -0700, Andrew Morton wrote: > > On Wed, 21 Aug 2013 17:42:12 +0100 Steven Whitehouse wrote: > > > > > > I don't think the change is harmful. The worst case scenario is race with > > > > write or truncate, but it's valid to return EOF in this case. > > > > > > > > What scenario do you have in mind? > > > > > > > > > > 1. File open on node A > > > 2. Someone updates it on node B by extending the file > > > 3. Someone reads the file on node A beyond end of original file size, > > > but within end of new file size as updated by node B. Without the patch > > > this works, with it, it will fail. The reason being the i_size would not > > > be up to date until after readpage(s) has been called. CC: +linux-fsdevel@ So in this case node A will see the file like it was never touched by node B. It's okay, if new i_size will eventually reach node A. Is ->readpage() the only way to get i_size updated on node A or it will be eventually updated without it? If it's the only way, we need add a explicit way to initiate i_size sync between nodes on read. Probably, distributed filesystems should provide own ->aio_read() which deal i_size as the filesystem need. > > > I think this is likely to be an issue for any distributed fs using > > > do_generic_file_read(), although it would certainly affect GFS2, since > > > the locking is done at page cache level, > > > > Boy, that's rather subtle. I'm surprised that the generic filemap.c > > stuff works at all in that sort of scenario. > > > > Can we put the i_size check down in the no_cached_page block? afaict > > that will solve the problem without breaking GFS2 and is more > > efficient? > > > > Well I think is even more subtle, since it relies on ->readpages > updating the file size, even if it has failed to actually read the > required pages :-) Having said that, we do rely on ->readpages updating > the inode size elsewhere in this function, as per the block comment > immediately following the page_ok label. That i_size recheck was invented to cover different use case: read vs. truncate race. Userspace should not see truncate-caused zeros in buffer. It's not to prevent file extending vs. read() race. This usually harmless: data is consistent. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org