From mboxrd@z Thu Jan 1 00:00:00 1970 From: Theodore Ts'o Subject: Re: [PATCH 1/2] vfs: Fix data corruption when blocksize < pagesize for mmaped data Date: Wed, 1 Oct 2014 22:06:19 -0400 Message-ID: <20141002020619.GA16766@thunk.org> References: <1411648916-16773-1-git-send-email-jack@suse.cz> <1411648916-16773-2-git-send-email-jack@suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, xfs@oss.sgi.com, Dave Chinner To: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <1411648916-16773-2-git-send-email-jack@suse.cz> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Thu, Sep 25, 2014 at 02:41:55PM +0200, Jan Kara wrote: > ->page_mkwrite() is used by filesystems to allocate blocks under a page > which is becoming writeably mmapped in some process' address space. This > allows a filesystem to return a page fault if there is not enough space > available, user exceeds quota or similar problem happens, rather than > silently discarding data later when writepage is called. > > However VFS fails to call ->page_mkwrite() in all the cases where > filesystems need it when blocksize < pagesize. For example when > blocksize = 1024, pagesize = 4096 the following is problematic: > ftruncate(fd, 0); > pwrite(fd, buf, 1024, 0); > map = mmap(NULL, 1024, PROT_WRITE, MAP_SHARED, fd, 0); > map[0] = 'a'; ----> page_mkwrite() for index 0 is called > ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */ > mremap(map, 1024, 10000, 0); > map[4095] = 'a'; ----> no page_mkwrite() called > > At the moment ->page_mkwrite() is called, filesystem can allocate only > one block for the page because i_size == 1024. Otherwise it would create > blocks beyond i_size which is generally undesirable. But later at > ->writepage() time, we also need to store data at offset 4095 but we > don't have block allocated for it. > > This patch introduces a helper function filesystems can use to have > ->page_mkwrite() called at all the necessary moments. > > Signed-off-by: Jan Kara Applied to the ext4 tree, thanks. - Ted