linux-kernel - Re: PATCH 3/6 - direct-io: do not merge logically non-contiguous requests

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Message-ID: <20100811140841.GD9349@dhcp231-156.rdu.redhat.com>
Date:	Wed, 11 Aug 2010 10:08:41 -0400
From:	Josef Bacik <josef@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Josef Bacik <josef@...hat.com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: PATCH 3/6 - direct-io: do not merge logically non-contiguous
	requests

On Wed, Aug 11, 2010 at 09:27:45AM -0400, Jeff Moyer wrote:
> Josef Bacik <josef@...hat.com> writes:
> 
> > So say blocksize of 4k, we do dio to 12k, the first time around
> > dio->block_in_file is 0, we set dio->cur_page, and move on to the next page, and
> > bio->block_in_file is set to 1.  We find that dio->cur_page is set, so we do
> > dio_send_cur_page().  Since !dio->bio we create a new bio, and set
> > dio->logical_offset_in_bio to 0, since thats the offset of dio->cur_page.  Then
> > we setup the next cur_page as the page for logical block 1, and
> > dio->block_in_file gets bumped to 2.  We map the next block and come into
> > dio_send_cur_page() again.  At this point cur_offset would be 8192...and shit I
> > just realized what was wrong.  If you change
> >
> > loff_t cur_offset = dio->block_in_file << dio->blkbits;
> >
> > to
> >
> > loff_t cur_offset = dio->cur_page_fs_offset << dio->blkbits;
> 
> Sorry, I wasn't very clear in my description, but you figured it out.
> ;-)  Of course, cur_page_fs_offset is already in bytes, so that left
> shift is not necessary.
>

Ah right, sorry.
 
> > That should fix the problem.  Sorry guys, I screwed that up.  I'll look at this
> > again tomorrow after I've had my 2 hours of sleep and see if this all still
> > makes sense, but I think the above should fixe the performance thing.  As for
> > the dio->boundary thing, dio_bio_submit() sets dio->boundary to 0, so the same
> > bio won't be submitted twice.
> 
> While I don't doubt that you are right, I will sleep better at night if
> we do an else if.  (To be fair, this ambiguity was not introduced by
> you).
> 
> I've tested this patch, added printk's and watched blktrace to verify
> that we don't split up I/Os.  So long as no one objects, I'll post this
> for inclusion in a new thread.
> 
> Thanks for looking into it, Josef.
>

No problem, thanks for making me look at it again.  You can add

Acked-by: Josef Bacik <josef@...hat.com>

Thanks,

Josef 
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