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

[Jeff Moyer <jmoyer@redhat.com>]

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?

Cheers,
Jeff