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Date:	Wed, 7 Mar 2012 16:05:10 -0800
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"Theodore Ts'o" <tytso@....edu>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if
	!CONFIG_BLKDEV_INTEGRITY

On Wed, Mar 07, 2012 at 05:54:11PM -0600, Eric Sandeen wrote:
> On 3/7/12 5:40 PM, Theodore Ts'o wrote:
> > We've recently discovered a workload at Google where the page
> > stablization patches (specifically commit 0e499890c1f: ext4: wait for
> > writeback to complete while making pages writable) resulted in a
> > **major** performance regression.  As in, kernel threads that were
> > writing to log files were getting hit by up to 2 seconds stalls, which
> > very badly hurt a particular application.  Reverting this commit fixed
> > the performance regression.
> > 
> > The main reason for the page stablizatoin patches was for DIF/DIX
> > support, right?   So I'm wondering if we should just disable the calls
> > to wait_on_page_writeback if CONFIG_BLKDEV_INTEGRITY is not defined.
> > i.e., something like this.
> 
> Can you devise a non-secret testcase that demonstrates this?

It sure would be nice if the block device could know if it requires stable
writeback, and the fs could figure that out.... though iirc there was more to
my patchset than just these two wait_on_page_writeback() calls.

--D
> 
> Thanks,
> -Eric
> 
> > What do people think?  I have a feeling this is going to be very
> > controversial....
> > 
> > 					- Ted
> > 
> > ext4: Disable page stablization if DIF/DIX not enabled
> > 
> > Requiring processes which are writing to files which are under writeback
> > until the writeback is complete can result in massive performance hits.
> > This is especially true if writes are being throttled due to I/O cgroup
> > limits and the application is running on an especially busy server.
> > 
> > If CONFIG_BLKDEV_INTEGRITY is not enabled, disable page stablization,
> > since that's the main case where this is needed, and page stablization
> > can be very painful.
> > 
> > Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> > 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 1a30db7..d25c60f 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -2333,7 +2333,9 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> >  		ret = -EAGAIN;
> >  		goto out_unlock;
> >  	}
> > +#ifdef CONFIG_BLKDEV_INTEGRITY
> >  	wait_on_page_writeback(page);
> > +#endif
> >  	return 0;
> >  out_unlock:
> >  	unlock_page(page);
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 5f8081c..01f86c5 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4638,8 +4638,10 @@ 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)) {
> > +#ifdef CONFIG_BLKDEV_INTEGRITY
> >  			/* Wait so that we don't change page under IO */
> >  			wait_on_page_writeback(page);
> > +#endif
> >  			ret = VM_FAULT_LOCKED;
> >  			goto out;
> >  		}
> > --
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> 
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