[dm-devel] [RFC 1/2] block: implement an unprep function corresponding directly to prep

[Mike Snitzer]

Hi,

On Wed, Jun 30 2010 at  9:47pm -0400,
FUJITA Tomonori <fujita.tomonori at lab.ntt.co.jp> wrote:

> On Wed, 30 Jun 2010 12:01:04 -0500
> James Bottomley <James.Bottomley at suse.de> wrote:
> 
> > So this is more directly what I'm thinking.  It gives us an exactly
> > correct place to hang the discard allocation in SCSI.  The next patch
> > shows a potential implementation in sd.
> 
> Yeah, making the prep_rq_fn API symmetrical makes sense lots.
> 
>  
> > I think it should avoid all the leaks people have been seeing trying to
> > move the discard allocation/free into scsi.  I also think it should
> > facilitate sending discard through SCSI as a REQ_TYPE_FS.
> 
> I think so. If I can figure out why qemu scsi driver is broken, the
> job is done.

When I combine this patch 1/2 and patch 2/2 with Christoph's discard
payload rework (a1d949f5f448) already staged in for-2.6.36 I'm finding
that: sd_unprep_fn's __free_page causes the system to hard hang/crash.
I need to get serial connected and/or kdump configured to see if I can
catch what is happening.

But if I comment out sd_unprep_fn's __free_page() from James' patch 2/2
I don't see a crash.  Seems some other code is altering the request
before James' new sd_unprep_fn hhook?

Are James' patches meant to be dependent on your REQ_TYPE_FS conversion
work?  I wouldn't think so.. but I haven't traced the call chains close
enough to know.
 
> I'll see how this patchset works and update my FS discard patchset on
> the top of this.

I look forward to your results!

Thanks,
Mike