[PATCH v2 0/2] Fixes to blkg_conf_prep

[PATCH v2 0/2] Fixes to blkg_conf_prep

[Gabriel Krisman Bertazi]

Hi Tejun,

In addition to addressing your comments the previously submitted fix to
pre-allocate the radix node on blkg_conf_prep, this series fixes a small
memleak on the same function.

Gabriel Krisman Bertazi (2):
  blk-cgroup: Fix memleak on error path
  blk-cgroup: Pre-allocate tree node on blkg_conf_prep

 block/blk-cgroup.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

-- 
2.28.0

[PATCH v2 1/2] blk-cgroup: Fix memleak on error path

[Gabriel Krisman Bertazi]

If new_blkg allocation raced with blk_policy change and
blkg_lookup_check fails, new_blkg is leaked.

Signed-off-by: Gabriel Krisman Bertazi <krisman-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org>
---
 block/blk-cgroup.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f9b55614d67d..f9389b7cf823 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -663,6 +663,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		blkg = blkg_lookup_check(pos, pol, q);
 		if (IS_ERR(blkg)) {
 			ret = PTR_ERR(blkg);
+			blkg_free(new_blkg);
 			goto fail_unlock;
 		}
 
-- 
2.28.0

[PATCH v2 2/2] blk-cgroup: Pre-allocate tree node on blkg_conf_prep

[Gabriel Krisman Bertazi]

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.

Signed-off-by: Gabriel Krisman Bertazi <krisman-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org>

---
Changes since v1:
 - Fail function if preload failed. (Tejun Heo)
---
 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 f9389b7cf823..c68bdf58c9a6 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -657,6 +657,12 @@ 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);
 
@@ -664,7 +670,7 @@ 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) {
@@ -673,10 +679,12 @@ 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;
 	}
@@ -686,6 +694,8 @@ 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();
-- 
2.28.0

Re: [PATCH v2 1/2] blk-cgroup: Fix memleak on error path

[Tejun Heo]

On Thu, Oct 22, 2020 at 04:58:41PM -0400, Gabriel Krisman Bertazi wrote:
> If new_blkg allocation raced with blk_policy change and
> blkg_lookup_check fails, new_blkg is leaked.
> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org>

Acked-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>

Thanks.

-- 
tejun

Re: [PATCH v2 2/2] blk-cgroup: Pre-allocate tree node on blkg_conf_prep

[Tejun Heo]

On Thu, Oct 22, 2020 at 04:58:42PM -0400, Gabriel Krisman Bertazi wrote:
> 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.
> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org>

Acked-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>

Thanks.

-- 
tejun

Re: [PATCH v2 0/2] Fixes to blkg_conf_prep

[Jens Axboe]

On 10/22/20 2:58 PM, Gabriel Krisman Bertazi wrote:
> Hi Tejun,
> 
> In addition to addressing your comments the previously submitted fix to
> pre-allocate the radix node on blkg_conf_prep, this series fixes a small
> memleak on the same function.

Applied, thanks.

-- 
Jens Axboe

Re: [PATCH v2 1/2] blk-cgroup: Fix memleak on error path

[Gabriel Krisman Bertazi]

Gabriel Krisman Bertazi <krisman-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org> writes:

> If new_blkg allocation raced with blk_policy change and
> blkg_lookup_check fails, new_blkg is leaked.

hm, sorry for the duplicate, the first attempt my script tripped on the
cover letter for some reason. Please disregard this one.

-- 
Gabriel Krisman Bertazi

[PATCH v2 1/2] blk-cgroup: Fix memleak on error path

[Gabriel Krisman Bertazi]

If new_blkg allocation raced with blk_policy change and
blkg_lookup_check fails, new_blkg is leaked.

Signed-off-by: Gabriel Krisman Bertazi <krisman-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org>
---
 block/blk-cgroup.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f9b55614d67d..f9389b7cf823 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -663,6 +663,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		blkg = blkg_lookup_check(pos, pol, q);
 		if (IS_ERR(blkg)) {
 			ret = PTR_ERR(blkg);
+			blkg_free(new_blkg);
 			goto fail_unlock;
 		}
 
-- 
2.28.0