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

Jeff Moyer
2009-06-26T16:26:31Z

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