Hi everyone,
It was reported in [1] that blk_get_rl() cleanup patch was causing a
null pointer dereference. After some back and forth debugging with
Valdis, it turns out I wasn't properly handling association with
recursive calls to make_request().
Another issue was identified with the blk_get_rl() update as it is
possible under certain circumstances that a blkg cannot be allocated
when called in blk_get_rl(). This could result in the blkcg_root being
returned. However, the blkcg_root is a special case where all blkgs
share the request_queue's request_list.
The original series can be found at [2].
[1] https://lore.kernel.org/lkml/[email protected]/
[2] https://lore.kernel.org/lkml/[email protected]/
This patchset contains the following 2 patches:
0001-blkcg-fix-edge-case-for-blk_get_rl-under-memory-pres.patch
0002-blkcg-reassociate-bios-when-make_request-is-called-r.patch
0001 addresses an edge case where a blkg cannot be created and can
possibly return a blkg associated with the blkcg_root. 0002 fixes the
stale association when make_request() is called recursively.
This patchset is on top of axboe#for-4.20/block bbc152825afc.
diffstats below:
Dennis Zhou (2):
blkcg: fix edge case for blk_get_rl() under memory pressure
blkcg: reassociate bios when make_request() is called recursively
block/bio.c | 20 ++++++++++++++++++++
block/blk-core.c | 1 +
include/linux/bio.h | 3 +++
include/linux/blk-cgroup.h | 2 +-
4 files changed, 25 insertions(+), 1 deletion(-)
Thanks,
Dennis
When submitting a bio, multiple recursive calls to make_request() may
occur. This causes the initial associate done in blkcg_bio_issue_check()
to be incorrect and reference the prior request_queue. This introduces
a helper to do reassociation when make_request() is recursively called.
Fixes: a7b39b4e961c ("blkcg: always associate a bio with a blkg")
Reported-by: Valdis Kletnieks <[email protected]>
Signed-off-by: Dennis Zhou <[email protected]>
Tested-by: Valdis Kletnieks <[email protected]>
---
block/bio.c | 20 ++++++++++++++++++++
block/blk-core.c | 1 +
include/linux/bio.h | 3 +++
3 files changed, 24 insertions(+)
diff --git a/block/bio.c b/block/bio.c
index 17a8b0aa7050..bbfeb4ee2892 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2083,6 +2083,26 @@ int bio_associate_create_blkg(struct request_queue *q, struct bio *bio)
return ret;
}
+/**
+ * bio_reassociate_blkg - reassociate a bio with a blkg from q
+ * @q: request_queue where bio is going
+ * @bio: target bio
+ *
+ * When submitting a bio, multiple recursive calls to make_request() may occur.
+ * This causes the initial associate done in blkcg_bio_issue_check() to be
+ * incorrect and reference the prior request_queue. This performs reassociation
+ * when this situation happens.
+ */
+int bio_reassociate_blkg(struct request_queue *q, struct bio *bio)
+{
+ if (bio->bi_blkg) {
+ blkg_put(bio->bi_blkg);
+ bio->bi_blkg = NULL;
+ }
+
+ return bio_associate_create_blkg(q, bio);
+}
+
/**
* bio_disassociate_task - undo bio_associate_current()
* @bio: target bio
diff --git a/block/blk-core.c b/block/blk-core.c
index cdfabc5646da..3ed60723e242 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2433,6 +2433,7 @@ blk_qc_t generic_make_request(struct bio *bio)
if (q)
blk_queue_exit(q);
q = bio->bi_disk->queue;
+ bio_reassociate_blkg(q, bio);
flags = 0;
if (bio->bi_opf & REQ_NOWAIT)
flags = BLK_MQ_REQ_NOWAIT;
diff --git a/include/linux/bio.h b/include/linux/bio.h
index f447b0ebb288..b47c7f716731 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -514,6 +514,7 @@ int bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg);
int bio_associate_blkg_from_css(struct bio *bio,
struct cgroup_subsys_state *css);
int bio_associate_create_blkg(struct request_queue *q, struct bio *bio);
+int bio_reassociate_blkg(struct request_queue *q, struct bio *bio);
void bio_disassociate_task(struct bio *bio);
void bio_clone_blkg_association(struct bio *dst, struct bio *src);
#else /* CONFIG_BLK_CGROUP */
@@ -522,6 +523,8 @@ static inline int bio_associate_blkg_from_css(struct bio *bio,
{ return 0; }
static inline int bio_associate_create_blkg(struct request_queue *q,
struct bio *bio) { return 0; }
+static inline int bio_reassociate_blkg(struct request_queue *q, struct bio *bio)
+{ return 0; }
static inline void bio_disassociate_task(struct bio *bio) { }
static inline void bio_clone_blkg_association(struct bio *dst,
struct bio *src) { }
--
2.17.1
It is possible for blkg creation to fail when in blk_get_rl(). In this
situation, the fallback logic returns the nearest created blkg. There is
however special handling for the request_list for the root blkcg. This
fixes the missing edge case from the earlier series changing
blk_get_rl().
Fixes: e2b0989954ae ("blkcg: cleanup and make blk_get_rl use blkg_lookup_create")
Signed-off-by: Dennis Zhou <[email protected]>
---
include/linux/blk-cgroup.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index b7fd08013de2..1e76ceebeb5d 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -597,7 +597,7 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
if (unlikely(!blkg))
blkg = __blkg_lookup_create(blkcg, q);
- if (!blkg_tryget(blkg))
+ if (blkg->blkcg == &blkcg_root || !blkg_tryget(blkg))
goto rl_use_root;
rcu_read_unlock();
--
2.17.1
On 10/20/18 12:56 PM, Dennis Zhou wrote:
> Hi everyone,
>
> It was reported in [1] that blk_get_rl() cleanup patch was causing a
> null pointer dereference. After some back and forth debugging with
> Valdis, it turns out I wasn't properly handling association with
> recursive calls to make_request().
>
> Another issue was identified with the blk_get_rl() update as it is
> possible under certain circumstances that a blkg cannot be allocated
> when called in blk_get_rl(). This could result in the blkcg_root being
> returned. However, the blkcg_root is a special case where all blkgs
> share the request_queue's request_list.
>
> The original series can be found at [2].
>
> [1] https://lore.kernel.org/lkml/[email protected]/
> [2] https://lore.kernel.org/lkml/[email protected]/
>
> This patchset contains the following 2 patches:
> 0001-blkcg-fix-edge-case-for-blk_get_rl-under-memory-pres.patch
> 0002-blkcg-reassociate-bios-when-make_request-is-called-r.patch
>
> 0001 addresses an edge case where a blkg cannot be created and can
> possibly return a blkg associated with the blkcg_root. 0002 fixes the
> stale association when make_request() is called recursively.
Thanks Dennis, applied.
--
Jens Axboe