Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932565Ab1CDVHh (ORCPT ); Fri, 4 Mar 2011 16:07:37 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:43817 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932452Ab1CDVH2 (ORCPT ); Fri, 4 Mar 2011 16:07:28 -0500 Date: Fri, 4 Mar 2011 13:07:24 -0800 From: "Darrick J. Wong" To: Chris Mason Cc: 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: <20110304210724.GF27190@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1298897186-sup-9394@think> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) X-Content-Scanned: Fidelis XPS MAILER Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6700 Lines: 163 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. 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; + } out: return ret; diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 9f7f9e4..2364704 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2730,12 +2730,15 @@ static int ext4_writepage(struct page *page, struct inode *inode = page->mapping->host; trace_ext4_writepage(inode, page); +lock_page(page); size = i_size_read(inode); if (page->index == size >> PAGE_CACHE_SHIFT) len = size & ~PAGE_CACHE_MASK; else len = PAGE_CACHE_SIZE; +wait_on_page_writeback(page); + /* * If the page does not have buffers (for whatever reason), * try to create them using __block_write_begin. If this 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); static ssize_t generic_perform_write(struct file *file, -- 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/