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    SubjectRe: [PATCH 2/2] cfq-iosched: get rid of the need for __GFP_FAIL in cfq_find_alloc_queue()
    On Mon, Jun 29 2009, Jeff Moyer wrote:
    > Jens Axboe <jens.axboe@oracle.com> writes:
    >
    > > 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.
    >
    > OK, that looks better. Are you reposting the series, then?

    Yeah, I'll fold the last two patches together and repost.

    --
    Jens Axboe



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