linux-kernel - Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb

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Date:	Tue, 28 Jun 2011 18:56:41 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb_nr

Hi Dave:

Thanks for the response.

On Tue, Jun 28, 2011 at 5:54 PM, Dave Chinner <david@...morbit.com> wrote:
> On Tue, Jun 28, 2011 at 04:43:35PM -0700, Curt Wohlgemuth wrote:
>> Contrary to the comment block atop writeback_inodes_sb_nr(),
>> we *were* calling
>>
>>         wait_for_completion(&done);
>>
>> which should not be done, as this is not called for data
>> integrity sync.
>>
>> Signed-off-by: Curt Wohlgemuth <curtw@...gle.com>
>
> The comment says it does not wait for IO to be -completed-.
>
> The function as implemented waits for IO to be *submitted*.
>
> This provides the callers with same blocking semantics (i.e. request
> queue full) as if the caller submitted the IO themselves. The code
> that uses this function rely on this blocking to delay the next set
> of operations they do until after IO has been started, so removing
> the completion will change their behaviour significantly.

I don't quite understand this.  It's true that all IO done as a result
of calling wb_writeback() on this work item won't finish before the
completion takes place, but sending all those pages in flight *will*
take place.  And that's a lot of time.  To wait on this before we then
call sync_inodes_sb(), and do it all over again, seems odd at best.

Pre-2.6.35 kernels would start non-integrity sync writeback and
immediately return, which would seem like a reasonable "prefetch-y"
thing to do, considering it's going to be immediately followed by a
data integrity sync writeback operation.

The post 2.6.35 semantics are fine; but then I don't understand why we
do both a __sync_filesystem(0) followed by a __sync_filesystem(1) (in
the case of sync(2)).  It doesn't seem to be any safer or more correct
to me; why not just do the data integrity sync writeback and call it a
day?

Thanks,
Curt

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@...morbit.com
>
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