Re: [PATCH 2/2] cfq-iosched: get rid of the need for __GFP_FAIL in cfq_find_alloc_queue()

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Jun 26 2009, Jeff Moyer wrote:
> Jens Axboe <jens.axboe@oracle.com> writes:
> 
> > Setup an emergency fallback cfqq that we allocate at IO scheduler init
> > time. If the slab allocation fails in cfq_find_alloc_queue(), we'll just
> > punt IO to that cfqq instead. This ensures that cfq_find_alloc_queue()
> > never fails without having to ensure free memory.
> >
> > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> > ---
> >  block/cfq-iosched.c |  124 +++++++++++++++++++++++++++-----------------------
> >  1 files changed, 67 insertions(+), 57 deletions(-)
> >
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index c760ae7..91e7e0b 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > +	/*
> > +	 * Fallback dummy cfqq for extreme OOM conditions
> > +	 */
> > +	struct cfq_queue oom_cfqq;
> 
> OK, so you're embedding a cfqq into the cfqd.  That's 136 bytes, so I
> guess that's not too bad.
> 
> > +	/*
> > +	 * Our fallback cfqq if cfq_find_alloc_queue() runs into OOM issues.
> > +	 * Grab a permanent reference to it, so that the normal code flow
> > +	 * will not attempt to free it.
> > +	 */
> > +	cfq_init_cfqq(cfqd, &cfqd->oom_cfqq, 1, 0);
> > +	atomic_inc(&cfqd->oom_cfqq.ref);
> > +
> 
> I guess this is so we never try to free it, good.  ;)
> 
> One issue I have with this patch is that, if a task happens to run into
> this condition, there is no way out.  It will always have the oom_cfqq
> as it's cfqq.  Can't we fix that if we recover from the OOM condition?

Yeah, I fixed that about an hour after posting the patches. See:

http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=0370bc158cb1d5faa4b8a38c0de3211f0fd5bd64

I didn't post the 3/2 patch though.

-- 
Jens Axboe