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    Subject[PATCH 5.9 086/133] blk-cgroup: Pre-allocate tree node on blkg_conf_prep
    Date
    From: Gabriel Krisman Bertazi <krisman@collabora.com>

    [ Upstream commit f255c19b3ab46d3cad3b1b2e1036f4c926cb1d0c ]

    Similarly to commit 457e490f2b741 ("blkcg: allocate struct blkcg_gq
    outside request queue spinlock"), blkg_create can also trigger
    occasional -ENOMEM failures at the radix insertion because any
    allocation inside blkg_create has to be non-blocking, making it more
    likely to fail. This causes trouble for userspace tools trying to
    configure io weights who need to deal with this condition.

    This patch reduces the occurrence of -ENOMEMs on this path by preloading
    the radix tree element on a GFP_KERNEL context, such that we guarantee
    the later non-blocking insertion won't fail.

    A similar solution exists in blkcg_init_queue for the same situation.

    Acked-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    block/blk-cgroup.c | 14 ++++++++++++--
    1 file changed, 12 insertions(+), 2 deletions(-)

    diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
    index 7b2df042220d4..c85fbb666e40a 100644
    --- a/block/blk-cgroup.c
    +++ b/block/blk-cgroup.c
    @@ -648,6 +648,12 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
    goto fail;
    }

    + if (radix_tree_preload(GFP_KERNEL)) {
    + blkg_free(new_blkg);
    + ret = -ENOMEM;
    + goto fail;
    + }
    +
    rcu_read_lock();
    spin_lock_irq(&q->queue_lock);

    @@ -655,7 +661,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
    if (IS_ERR(blkg)) {
    ret = PTR_ERR(blkg);
    blkg_free(new_blkg);
    - goto fail_unlock;
    + goto fail_preloaded;
    }

    if (blkg) {
    @@ -664,10 +670,12 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
    blkg = blkg_create(pos, q, new_blkg);
    if (IS_ERR(blkg)) {
    ret = PTR_ERR(blkg);
    - goto fail_unlock;
    + goto fail_preloaded;
    }
    }

    + radix_tree_preload_end();
    +
    if (pos == blkcg)
    goto success;
    }
    @@ -677,6 +685,8 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
    ctx->body = input;
    return 0;

    +fail_preloaded:
    + radix_tree_preload_end();
    fail_unlock:
    spin_unlock_irq(&q->queue_lock);
    rcu_read_unlock();
    --
    2.27.0


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