Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757751Ab1CSAII (ORCPT ); Fri, 18 Mar 2011 20:08:08 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:40485 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757648Ab1CSAIA (ORCPT ); Fri, 18 Mar 2011 20:08:00 -0400 Date: Fri, 18 Mar 2011 17:07:55 -0700 From: "Darrick J. Wong" To: Dave Chinner Cc: Chris Mason , Jan Kara , 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: <20110319000755.GD1110@tux1.beaverton.ibm.com> Reply-To: djwong@us.ibm.com References: <20110222020022.GH32261@tux1.beaverton.ibm.com> <20110223202446.GG4020@noexit> <1298493173-sup-8301@think> <20110224164758.GH23042@quack.suse.cz> <1298566775-sup-730@think> <20110224182732.GV27190@tux1.beaverton.ibm.com> <1298897186-sup-9394@think> <20110304210724.GF27190@tux1.beaverton.ibm.com> <20110308045626.GD1956@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110308045626.GD1956@dastard> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9110 Lines: 194 On Tue, Mar 08, 2011 at 03:56:26PM +1100, Dave Chinner wrote: > On Fri, Mar 04, 2011 at 01:07:24PM -0800, Darrick J. Wong wrote: > > On Mon, Feb 28, 2011 at 07:54:05AM -0500, Chris Mason wrote: > > > Excerpts from Darrick J. Wong's message of 2011-02-24 13:27:32 -0500: > > > > On Thu, Feb 24, 2011 at 12:37:53PM -0500, Chris Mason wrote: > > > > > Excerpts from Jan Kara's message of 2011-02-24 11:47:58 -0500: > > > > > > On Wed 23-02-11 15:35:11, Chris Mason wrote: > > > > > > > Excerpts from Joel Becker's message of 2011-02-23 15:24:47 -0500: > > > > > > > > On Tue, Feb 22, 2011 at 11:45:44AM -0500, Martin K. Petersen wrote: > > > > > > > > > Also, DIX is only the tip of the iceberg. Many other impending > > > > > > > > > technologies feature checksums and require pages to be stable during I/O > > > > > > > > > due to checksumming, encryption and so on. > > > > > > > > > > > > > > > > > > The VM is already trying to do the right thing. We just need the > > > > > > > > > relevant filesystems to catch up. > > > > > > > > > > > > > > > > ocfs2 handles stable metadata for its checksums when feeding > > > > > > > > things to the journal. If we're doing pagecache-based I/O, is the > > > > > > > > pagecache going to help here for data? > > > > > > > > > > > > > > Data is much easier than metadata. All you really need is to wait on > > > > > > > writeback in file_write, wait on writeback in page_mkwrite, and make > > > > > > > sure you don't free blocks back to the allocator that are actively under > > > > > > > IO. > > > > > > > > > > > > > > I expect the hard part to be jbd and metadata in ext34. > > > > > > But JBD already has to do data copy if a buffer is going to be modified > > > > > > before/while it is written to the journal. So we should alredy do all that > > > > > > is needed for metadata. I don't say there aren't any bugs as they could be > > > > > > triggered only by crashing at the wrong moment and observing fs corruption. > > > > > > But most of the work should be there... > > > > > > > > > > Most of it is there, but there are always little bits and pieces. The > > > > > ext4 journal csumming code was one semi-recent example where we found > > > > > metadata changing in flight. > > > > > > > > > > A big part of testing this is getting some way to detect the bugs > > > > > without dif/dix. With btrfs I have patches to do set_memory_ro on > > > > > pages once I've don the crc, hopefully we can generalize that idea or > > > > > some up with something smarter. > > > > > > > > Right now I'm faking it with modprobe scsi_debug ato=1 guard=1 dif=3 dix=199. > > > > > > > > Hm, would you mind sharing those patches? I've been working on a second patch > > > > to do the wait-on-writeback per everyone's suggestions, but I still see the > > > > occasional corruption error as soon as I enable the mmap write case and covet > > > > some more debugging tools. It does seem to be working for the pure pwrite() > > > > case. :) > > > > > > Here's an ext4 version of the debugging patch. It's a few years old but > > > it'll give you the idea. This only covers metadata pages. > > > > > > Looks like I hacked the btrfs version up and didn't keep the original, > > > I'll have to rework it, I was trying to use it for the big corruption I > > > fixed recently and made a bunch of changes. > > > > > > For data if mmap is giving you trouble you need to wait on writeback in > > > page_mkwrite, with the page locked. fs/btrfs/inode.c has our > > > page_mkwrite, which uses wait_on_page_writeback() and also the btrfs > > > ordered write code. But for the other filesystems, waiting on writeback > > > should be enough. > > > > 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* After letting the stress tool run for a few days, I can say fairly confidently that the write() case doesn't seem to fail regardless of the O_DIRECT setting. However, with writes to mmap regions, failures happen about once every 20-40 minutes, even with O_DIRECT set. To me this suggests some sort of race condition that we seem to win except once every 20 minutes. I then thought, if page_mkwrite contains a wait_on_page_writeback, then perhaps there's something that I could do just prior to calculating the DIF checksum that would cause any subsequent write attempts to be shuffled back into page_mkwrite. I tried the set_memory_ro thing again, though that led to some recursive lock errors and I noticed that those functions only seem to exist in arch/x86/. Next I tried directly mucking with PTEs, in addition to feeling messy, only seemed to corrupt memory. :) Is there a "correct" way to take a writeable page and make it so that any process trying to write to it ends up hitting the page fault handler where we can then wait for writeback? Or perhaps I am simply barking up the wrong tree? (Just FYI I took the old copy-everything-to-bounce-buffers patch that few people liked for a second spin, and the errors did not surface regardless of what combination of write/mmap and directio/bufferedio I told it to use.) --D > > > > The set_memory_ro debugging trick didn't ferret out any write paths that I > > didn't catch... though it did have the effect of causing occasional fsync() > > deadlocks. I suppose I could sprinkle in a few more of those write calls to > > see what happens. > > > > Either way, I'm emailing to ask everyone's advice since I've run out of ideas. > > Or: Did I miss something? > > > > Thanks all for the feedback so far! > > > > -- > > fs: Wait for page writeback when rewrite detected > > > > Signed-off-by: Darrick J. Wong > > --- > > > > fs/buffer.c | 4 +++- > > fs/ext4/inode.c | 3 +++ > > mm/filemap.c | 15 +++++++++++++-- > > 3 files changed, 19 insertions(+), 3 deletions(-) > > > > diff --git a/fs/buffer.c b/fs/buffer.c > > index 2219a76..39e934c 100644 > > --- a/fs/buffer.c > > +++ b/fs/buffer.c > > @@ -2379,8 +2379,10 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf, > > ret = VM_FAULT_OOM; > > else /* -ENOSPC, -EIO, etc */ > > ret = VM_FAULT_SIGBUS; > > - } else > > + } else { > > + wait_on_page_writeback(page); > > ret = VM_FAULT_LOCKED; > > + } > > I think this needs to wait before the __block_write_begin() call, > not after it. i.e. wait before the page is mapped, not afterwards. > > .... > > diff --git a/mm/filemap.c b/mm/filemap.c > > index 83a45d3..f201d80 100644 > > --- a/mm/filemap.c > > +++ b/mm/filemap.c > > @@ -2217,8 +2217,8 @@ EXPORT_SYMBOL(generic_file_direct_write); > > * Find or create a page at the given pagecache position. Return the locked > > * page. This function is specifically for buffered writes. > > */ > > -struct page *grab_cache_page_write_begin(struct address_space *mapping, > > - pgoff_t index, unsigned flags) > > +struct page *__grab_cache_page_write_begin(struct address_space *mapping, > > + pgoff_t index, unsigned flags) > > { > > int status; > > struct page *page; > > @@ -2243,6 +2243,17 @@ repeat: > > } > > return page; > > } > > +struct page *grab_cache_page_write_begin(struct address_space *mapping, > > + pgoff_t index, unsigned flags) > > +{ > > + struct page *p; > > + > > + p = __grab_cache_page_write_begin(mapping, index, flags); > > + if (p) > > + wait_on_page_writeback(p); > > + > > + return p; > > +} > > EXPORT_SYMBOL(grab_cache_page_write_begin); > > Not much point in add in a wrapper when nothing else calls > __grab_cache_page_write_begin(), which should also be static.... > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- 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/