Re: [rfc][patch] remove racy sync_page?

[Jens Axboe]

On Mon, May 29 2006, Andrew Morton wrote:
> On Tue, 30 May 2006 12:54:53 +1000
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> 
> > Andrew Morton wrote:
> > 
> > >On Tue, 30 May 2006 10:08:06 +1000
> > >Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> > >
> > >
> > >>Which is what I want to know. I don't exactly have an interesting
> > >>disk setup.
> > >>
> > >
> > >You don't need one - just a single disk should show up such problems.  I
> > >forget which workloads though.  Perhaps just a linear read (readahead
> > >queues the I/O but doesn't unplug, subsequent lock_page() sulks).
> > >
> > 
> > I guess so. Is plugging still needed now that the IO layer should
> > get larger requests? Disabling it might result in a small initial
> > request (although even that may be good for pipelining)...
> 
> Mysterious question, that.  A few years ago I think Jens tried pulling
> unplugging out, but some devices still want it (magneto-optical
> storage iirc).  And I think we did try removing it, and it caused
> hurt.

I did, back when we had problems due to the blk_plug_lock being a global
one. I first wanted to investigate if plugging still made a difference,
otherwise we could've just ripped it out back than and the problem would
be solved. But it did get us about a 10% boost on normal SCSI drives
(don't think I tested MO drives at all), so it was fixed up.

> > sync_page wants to get either the current mapping, or a NULL one.
> > The sync_page methods must then be able to handle running into a
> > NULL mapping.
> > 
> > With splice, the mapping can change, so you can have the wrong
> > sync_page callback run against the page.
> 
> Oh.

Maybe I'm being dense, but I don't see a problem there. You _should_
call the new mapping sync page if it has been migrated.

> > >>Well yes, writing to a page would be the main reason to set it dirty.
> > >>Is splice broken as well? I'm not sure that it always has a ref on the
> > >>inode when stealing a page.
> > >>
> > >
> > >Whereabouts?
> > >
> > 
> > The ->pin() calls in pipe_to_file and pipe_to_sendpage?
> 
> One for Jens...

splice never incs/decs any inode related reference counts, so if it
needs to then yes it's broken. Any references to kernel code that deals
with that?

-- 
Jens Axboe

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