Message-ID: <20110406232938.GF1110@tux1.beaverton.ibm.com>
Date:	Wed, 6 Apr 2011 16:29:38 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	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 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.

On the odd chance that jbd2 really can provide stable pages during writeback, I
am now rerunning the test with no patches and data=journal, while noting that
(a) DIO mode doesn't work with data=journal and (b) the first write failure
will probably cause the journal to abort == game over.  When that's done I'll
give the wait-for-writeback patches a whirl with 2.6.39-rc.

Enjoy the warm SF weather!

--D
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