From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx109.postini.com [74.125.245.109]) by kanga.kvack.org (Postfix) with SMTP id 34E026B0070 for ; Thu, 22 Aug 2013 05:28:39 -0400 (EDT) Subject: Re: [PATCH] mm, fs: avoid page allocation beyond i_size on read From: Steven Whitehouse In-Reply-To: <20130821135821.fc8f5a2551a28c9ce9c4b049@linux-foundation.org> 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> Content-Type: text/plain; charset="UTF-8" Date: Thu, 22 Aug 2013 10:28:45 +0100 Message-ID: <1377163725.2720.18.camel@menhir> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: "Kirill A. Shutemov" , Dave Hansen , Jan Kara , Al Viro , NeilBrown , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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. > > > > 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. This should work for GFS2 though, and I did check OCFS2 and I think it should work for them too, Steve. -- 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