Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754704Ab1DKRkT (ORCPT ); Mon, 11 Apr 2011 13:40:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:11724 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752224Ab1DKRkR (ORCPT ); Mon, 11 Apr 2011 13:40:17 -0400 Date: Mon, 11 Apr 2011 12:42:29 -0400 From: Jeff Layton To: djwong@us.ibm.com Cc: Jan Kara , Chris Mason , Dave Chinner , Joel Becker , "Martin K. Petersen" , Jens Axboe , linux-kernel , linux-fsdevel , Mingming Cao , linux-scsi Subject: Re: [RFC] block integrity: Fix write after checksum calculation problem Message-ID: <20110411124229.47bc28f6@corrin.poochiereds.net> In-Reply-To: <20110408203135.GH1110@tux1.beaverton.ibm.com> References: <20110224182732.GV27190@tux1.beaverton.ibm.com> <1298897186-sup-9394@think> <20110304210724.GF27190@tux1.beaverton.ibm.com> <20110308045626.GD1956@dastard> <20110319000755.GD1110@tux1.beaverton.ibm.com> <20110321140451.GA7153@quack.suse.cz> <1300716666-sup-2087@think> <20110321164305.GC7153@quack.suse.cz> <20110406232938.GF1110@tux1.beaverton.ibm.com> <20110407165700.GB7363@quack.suse.cz> <20110408203135.GH1110@tux1.beaverton.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8332 Lines: 201 On Fri, 8 Apr 2011 13:31:35 -0700 "Darrick J. Wong" wrote: > On Thu, Apr 07, 2011 at 06:57:00PM +0200, Jan Kara wrote: > > On Wed 06-04-11 16:29:38, Darrick J. Wong wrote: > > > On Mon, Mar 21, 2011 at 05:43:05PM +0100, Jan Kara wrote: > > > > On Mon 21-03-11 10:24:41, Chris Mason wrote: > > > > > Excerpts from Jan Kara's message of 2011-03-21 10:04:51 -0400: > > > > > > On Fri 18-03-11 17:07:55, Darrick J. Wong wrote: > > > > > > > > > Ok, here's what I have so far. I took everyone's suggestions of where to add > > > > > > > > > calls to wait_on_page_writeback, which seems to handle the multiple-write case > > > > > > > > > adequately. Unfortunately, it is still possible to generate checksum errors by > > > > > > > > > scribbling furiously on a mmap'd region, even after adding the writeback wait > > > > > > > > > in the ext4 writepage function. Oddly, I couldn't break btrfs with mmap by > > > > > > > > > removing its wait_for_page_writeback call, so I suspect there's a bit more > > > > > > > > > going on in btrfs than I've been able to figure out. > > > > > > > > > > > > > > I wonder, is it possible for this to happen: > > > > > > > > > > > > > > 1. Thread A mmaps a page and tries to write to it. ext4_page_mkwrite executes, > > > > > > > but there's no ongoing writeback, so it returns without delay. > > > > > > > 2. Thread A starts writing furiously to the page. > > > > > > > 3. Thread B runs fsync() or something that results in the page being > > > > > > > checksummed and scheduled for writeout. > > > > > > > 4. Thread A continues to write furiously(!) on that same page before the > > > > > > > controller finishes the DMA transfer. > > > > > > > 5. Disk gets the page, which now doesn't match its checksum, and *boom* > > > > > > What happens on writepage (see mm/page-writeback.c:write_cache_pages()) > > > > > > is: > > > > > > lock_page(page) > > > > > > ... > > > > > > clear_page_dirty_for_io() - removes PageDirty, marks page as read-only in > > > > > > PTE > > > > > > ... > > > > > > set_page_writeback() (happens e.g. in __block_write_full_page() called > > > > > > from filesystem's writepage implementation). > > > > > > unlock_page(page) > > > > > > > > > > > > So if you compute the checksum after set_page_writeback() is done in the > > > > > > writepage() implementation (you cannot use __block_write_full_page() in > > > > > > that case) > > > > I should add that if you are computing the checksum in the block layer > > > > once the bio is submitted, you obviously are computing it after the page is > > > > marked as writeback. So that should be fine... > > > > > > > > > > and you call wait_on_page_writeback() in ext4_page_mkwrite() > > > > > > under page lock, you should be safe. If you do all this and still see > > > > > > errors, something is broken I'd say... > > > > > > > > > > Looking at the ext4_page_mkwrite, it does this: > > > > > > > > > > lock the page > > > > > check for holes > > > > > unlock the page > > > > > if (no_holes) > > > > > return; > > > > > > > > > > write_begin/write_end > > > > > return > > > > > > > > > > So, to have page_mkwrite work, you need to wait for writeback with the > > > > > page locked in both the no holes case and after the > > > > > write_begin/write_end. write_begin will dirty the page, so someone can > > > > > wander in and start the IO while we are still in page_mkwrite. > > > > Oh right, that's a good point. > > > > > > > > > This is untested and uncompiled, but it should > > > > > do the trick. > > > > > > > > > > Jan, did you get rid of all the buffer head based writeback for > > > > > data=ordered in ext4? That's my only other idea, that someone is doing > > > > > writeback directly without taking the page lock. > > > > Yes, ext4 shouldn't do any buffer based writeback. > > > > > > > > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > > > > > index 9f7f9e4..8a75e12 100644 > > > > > --- a/fs/ext4/inode.c > > > > > +++ b/fs/ext4/inode.c > > > > > @@ -5880,6 +5880,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf) > > > > > if (page_has_buffers(page)) { > > > > > if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL, > > > > > ext4_bh_unmapped)) { > > > > > + wait_on_page_writeback(page); > > > > > unlock_page(page); > > > > > goto out_unlock; > > > > > } > > > > > @@ -5901,6 +5902,16 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf) > > > > > if (ret < 0) > > > > > goto out_unlock; > > > > > ret = 0; > > > > > + > > > > > + /* > > > > > + * write_begin/end might have created a dirty page and someone > > > > > + * could wander in and start the IO. Make sure that hasn't > > > > > + * happened > > > > > + */ > > > > > + lock_page(page); > > > > > + wait_on_page_writeback(page); > > > > > + unlock_page(page); > > > > > + > > > > > out_unlock: > > > > > if (ret) > > > > > ret = VM_FAULT_SIGBUS; > > > > > > > > > This looks good AFAICT. > > > > > > I gave this a spin a couple of weeks ago (and accidentally left the test > > > machines running for a full week!) From what I can tell, with all the various > > > wait_for_page_writeback stuff-ins, we've cut the frequency of writeback errors > > > down to about 7-8 per day. Not bad, but not fixed. > > Ugh, strange. Can you post the full patch you are currently using? I've > > already lost track of all the proposed changes... Thanks. > > Yes, me too. Attached is the giant patch I've been working on. > > --D > > fs: Wait for page writeback when rewrite detected, and mark pages ro during writeback > > Signed-off-by: Darrick J. Wong > --- > diff --git a/fs/buffer.c b/fs/buffer.c > index a08bb8e..dd429fe 100644 > --- a/fs/buffer.c > +++ b/fs/buffer.c > @@ -2357,6 +2357,8 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf, > else > end = PAGE_CACHE_SIZE; > > + WARN_ON(!PageLocked(page)); > + wait_on_page_writeback(page); > ret = __block_write_begin(page, 0, end, get_block); > if (!ret) > ret = block_commit_write(page, 0, end); > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 1a86282..57cd028 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -2666,6 +2666,8 @@ static int ext4_writepage(struct page *page, > */ > goto redirty_page; > } > + WARN_ON(!PageLocked(page)); > + wait_on_page_writeback(page); > if (commit_write) > /* now mark the buffer_heads as dirty and uptodate */ > block_commit_write(page, 0, len); > @@ -2778,7 +2780,8 @@ static int write_cache_pages_da(struct address_space *mapping, > } > > lock_page(page); > - > + if (PageWriteback(page)) > + wait_on_page_writeback(page); > /* > * If the page is no longer dirty, or its > * mapping no longer corresponds to inode we > @@ -5803,12 +5806,14 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf) > if (PageMappedToDisk(page)) > goto out_unlock; > > + lock_page(page); > + /* this one seems to handle mmap */ > + wait_on_page_writeback(page); > if (page->index == size >> PAGE_CACHE_SHIFT) > len = size & ~PAGE_CACHE_MASK; > else > len = PAGE_CACHE_SIZE; > > - lock_page(page); > /* > * return if we have all the buffers mapped. This avoid > * the need to call write_begin/write_end which does a > @@ -5839,6 +5844,15 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf) > if (ret < 0) > goto out_unlock; > ret = 0; > + > + /* > + * write_begin/end might have created a dirty page and someone > + * could wander in and start the IO. Make sure that hasn't > + * happened. > + */ > + lock_page(page); > + wait_on_page_writeback(page); > + unlock_page(page); nit: The callers of page_mkwrite always lock the page afterward if you return from page_mkwrite with it unlocked. If you plan to take page lock anyway, it's probably slightly more efficient not to unlock it and instead return VM_FAULT_LOCKED. -- Jeff Layton -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/