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Date:	Wed, 07 Mar 2012 17:54:11 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
CC:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV_INTEGRITY

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?

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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