Date:	Tue, 8 Mar 2011 15:56:26 +1100
From:	Dave Chinner <david@...morbit.com>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	Chris Mason <chris.mason@...cle.com>, Jan Kara <jack@...e.cz>,
	Joel Becker <jlbec@...lplan.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Mingming Cao <mcao@...ibm.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [RFC] block integrity: Fix write after checksum calculation
 problem

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.
> 
> 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 <djwong@...ibm.com>
> ---
> 
>  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@...morbit.com
--
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