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    Subject[PATCH 3.12 035/175] cfq-iosched: handle failure of cfq group allocation
    Date
    From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    commit 69abaffec7d47a083739b79e3066cb3730eba72e upstream.

    Cfq_lookup_create_cfqg() allocates struct blkcg_gq using GFP_ATOMIC.
    In cfq_find_alloc_queue() possible allocation failure is not handled.
    As a result kernel oopses on NULL pointer dereference when
    cfq_link_cfqq_cfqg() calls cfqg_get() for NULL pointer.

    Bug was introduced in v3.5 in commit cd1604fab4f9 ("blkcg: factor
    out blkio_group creation"). Prior to that commit cfq group lookup
    had returned pointer to root group as fallback.

    This patch handles this error using existing fallback oom_cfqq.

    Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Acked-by: Tejun Heo <tj@kernel.org>
    Acked-by: Vivek Goyal <vgoyal@redhat.com>
    Fixes: cd1604fab4f9 ("blkcg: factor out blkio_group creation")
    Signed-off-by: Jens Axboe <axboe@fb.com>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    block/cfq-iosched.c | 7 ++++++-
    1 file changed, 6 insertions(+), 1 deletion(-)

    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index 06c2bab69756..396bff25226b 100644
    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -3575,6 +3575,11 @@ retry:

    blkcg = bio_blkcg(bio);
    cfqg = cfq_lookup_create_cfqg(cfqd, blkcg);
    + if (!cfqg) {
    + cfqq = &cfqd->oom_cfqq;
    + goto out;
    + }
    +
    cfqq = cic_to_cfqq(cic, is_sync);

    /*
    @@ -3611,7 +3616,7 @@ retry:
    } else
    cfqq = &cfqd->oom_cfqq;
    }
    -
    +out:
    if (new_cfqq)
    kmem_cache_free(cfq_pool, new_cfqq);

    --
    2.3.0


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