Re: [PATCH 1/3] direct-io: add a hook for the fs to provide its ownbio merging check function

[Miao Xie]

On 	wed, 17 Nov 2010 11:55:28 -0500, Chris Mason wrote:
> Excerpts from Josef Bacik's message of 2010-11-17 07:50:11 -0500:
> > On Wed, Nov 17, 2010 at 06:11:03PM +0800, Miao Xie wrote:
> > > Hi, Josef
> > >
> > > On wed, 17 Nov 2010 04:37:21 -0500, Josef Bacik wrote:
> > > > > Heh so I was going to fix this after the hole punching stuff.  The fact is btrfs
> > > > > maps everything that is ok to do in one IO via get_blocks().  So all we need to
> > > > > do is add another DIO_ flag to tell us to treat each get_blocks() call as
> > > > > discrete.  I wanted to use buffer_boundary for this, but I think it's too
> > > > > drastic of a change for people who already use buffer_boundary();
> > > > >
> > > > > What happens today is that say we map 4k, we do submit_page_section, but if this
> > > > > is our first bit of IO we just set dio->cur_page and such and then loop again.
> > > > > Say there is 4k-hole-4k, we do the next mapping and set buffer_boundary again,
> > > > > and come into submit_page_section and because cur_page is set, we do
> > > > > dio_send_cur_page.  Because there is no dio->bio we setup a new bio, but when we
> > > > > do that we clear dio->boundary, and leave the bio all setup.  So the next time
> > > > > we loop around the tail 4k gets added to our previously setup bio and boom we
> > > > > hit this problem with btrfs.
> > > > >
> > > > > If we can add a DIO_GET_BLOCKS_DISCRETE or some other such non-sense then we can
> > > > > easily kill all the logical offset code I had and just make some simple changes
> > > > > to make the DIO stuff work for us.  All we do is in get_more_blocks we do
> > > > >
> > > > > if ((dio->flags&   DIO_GET_BLOCKS_DISCRETE)&&   dio->bio)
> > > > >      dio_submit_bio(dio);
> > > >
> > > > Right after I went to bed I realized this should be
> > > >
> > > > if (dio->flags&   DIO_GET_BLOCKS_DISCRETE) {
> > > >      if (dio->cur_page) {
> > > >          dio_send_cur_page(dio);
> > > >          page_cache_release(dio->cur_page);
> > > >          dio->cur_page = NULL;
> > > >      }
> > > >
> > > >      if (dio->bio)
> > > >          dio_submit_bio(dio);
> > > >    }
> > >
> > > As far as I know, get_block() can not make sure the IO doesn't span the chunks or
> > > stripes. Maybe we can do this check in get_blocks(). In this way, we needn't change
> > > vfs.
> >
> > Right thats the idea, if we can't span chunks/stripes we should be doing that
> > limiting in our get_blocks call and that way we don't have to screw with the
> > generic direct io stuff too much.  Thanks,
>
> In this case we're adding complexity to the O_DIRECT mapping code, when
> we really should be adding it to the btrfs submit bio hook.  It can
> easily break up the bio into smaller units, which will leave us with a
> smaller number of get_blocks calls overall.
>
> I'm working that out now.

Do you mean you are fixing this bug now?

Thanks
Miao
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