Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756197Ab1DGQ5H (ORCPT ); Thu, 7 Apr 2011 12:57:07 -0400 Received: from cantor2.suse.de ([195.135.220.15]:54736 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755822Ab1DGQ5E (ORCPT ); Thu, 7 Apr 2011 12:57:04 -0400 Date: Thu, 7 Apr 2011 18:57:00 +0200 From: Jan Kara To: "Darrick J. Wong" 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: <20110407165700.GB7363@quack.suse.cz> References: <1298566775-sup-730@think> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110406232938.GF1110@tux1.beaverton.ibm.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5233 Lines: 118 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. Honza -- Jan Kara SUSE Labs, CR -- 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/