[PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Jan Kara]

commit 22b106e5355d6e7a9c3b5cb5ed4ef22ae585ea94 upstream.

Commit d92c370a16cb ("block: really clone the block cgroup in
bio_clone_blkg_association") changed bio_clone_blkg_association() to
just clone bio->bi_blkg reference from source to destination bio. This
is however wrong if the source and destination bios are against
different block devices because struct blkcg_gq is different for each
bdev-blkcg pair. This will result in IOs being accounted (and throttled
as a result) multiple times against the same device (src bdev) while
throttling of the other device (dst bdev) is ignored. In case of BFQ the
inconsistency can even result in crashes in bfq_bic_update_cgroup().
Fix the problem by looking up correct blkcg_gq for the cloned bio.

Reported-by: Logan Gunthorpe <logang@deltatee.com>
Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
Fixes: d92c370a16cb ("block: really clone the block cgroup in bio_clone_blkg_association")
CC: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220602081242.7731-1-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-cgroup.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

This patch should be a backport for 5.15, 5.17, and 5.18 stable branches.

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 07a2524e6efd..ce5858dadca5 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1886,12 +1886,8 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
  */
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
-	if (src->bi_blkg) {
-		if (dst->bi_blkg)
-			blkg_put(dst->bi_blkg);
-		blkg_get(src->bi_blkg);
-		dst->bi_blkg = src->bi_blkg;
-	}
+	if (src->bi_blkg)
+		bio_associate_blkg_from_css(dst, &bio_blkcg(src)->css);
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
 
-- 
2.35.3

Re: [PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Greg KH]

On Wed, Jun 08, 2022 at 01:45:28PM +0200, Jan Kara wrote:
> commit 22b106e5355d6e7a9c3b5cb5ed4ef22ae585ea94 upstream.
> 
> Commit d92c370a16cb ("block: really clone the block cgroup in
> bio_clone_blkg_association") changed bio_clone_blkg_association() to
> just clone bio->bi_blkg reference from source to destination bio. This
> is however wrong if the source and destination bios are against
> different block devices because struct blkcg_gq is different for each
> bdev-blkcg pair. This will result in IOs being accounted (and throttled
> as a result) multiple times against the same device (src bdev) while
> throttling of the other device (dst bdev) is ignored. In case of BFQ the
> inconsistency can even result in crashes in bfq_bic_update_cgroup().
> Fix the problem by looking up correct blkcg_gq for the cloned bio.
> 
> Reported-by: Logan Gunthorpe <logang@deltatee.com>
> Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
> Fixes: d92c370a16cb ("block: really clone the block cgroup in bio_clone_blkg_association")
> CC: stable@vger.kernel.org
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Jan Kara <jack@suse.cz>
> Link: https://lore.kernel.org/r/20220602081242.7731-1-jack@suse.cz
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  block/blk-cgroup.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> This patch should be a backport for 5.15, 5.17, and 5.18 stable branches.

Now queued up, thanks.

greg k-h

Re: [PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Jan Kara]

On Wed 01-06-22 10:42:32, Christoph Hellwig wrote:
> On Wed, Jun 01, 2022 at 06:34:05PM +0200, Jan Kara wrote:
> > --- a/block/blk-cgroup.c
> > +++ b/block/blk-cgroup.c
> > @@ -1975,10 +1975,9 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
> >  void bio_clone_blkg_association(struct bio *dst, struct bio *src)
> >  {
> >  	if (src->bi_blkg) {
> > +		rcu_read_lock();
> > +		bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
> > +		rcu_read_unlock();
> 
> What do we even need the rcu critical section here?

Good question. I've just blindly copied it from bio_associate_blkg() but
bio_blkcg_css(src) is safe without RCU (we hold object references for all
the dereferences) and bio_associate_blkg_from_css() takes RCU lock in
blkg_tryget_closest() which is the only place where it needs it. So no, we
don't need the RCU lock there. Thanks for noticing. I'll send V2 shortly
with your change and the added tags.

> Otherwise looks good:
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Thanks for review!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Donald Buczek]

On 01.06.22 18:34, Jan Kara wrote:
> Commit d92c370a16cb ("block: really clone the block cgroup in
> bio_clone_blkg_association") changed bio_clone_blkg_association() to
> just clone bio->bi_blkg reference from source to destination bio. This
> is however wrong if the source and destination bios are against
> different block devices because struct blkcg_gq is different for each
> bdev-blkcg pair. This will result in IOs being accounted (and throttled
> as a result) multiple times against the same device (src bdev) while
> throttling of the other device (dst bdev) is ignored. In case of BFQ the
> inconsistency can even result in crashes in bfq_bic_update_cgroup().
> Fix the problem by looking up correct blkcg_gq for the cloned bio.
> 
> Reported-by: Logan Gunthorpe <logang@deltatee.com>
> Reported-by: Donald Buczek <buczek@molgen.mpg.de>
> Fixes: d92c370a16cb ("block: really clone the block cgroup in bio_clone_blkg_association")
> CC: stable@vger.kernel.org
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>   block/blk-cgroup.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 40161a3f68d0..ecb4eaff6817 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1975,10 +1975,9 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
>   void bio_clone_blkg_association(struct bio *dst, struct bio *src)
>   {
>   	if (src->bi_blkg) {
> -		if (dst->bi_blkg)
> -			blkg_put(dst->bi_blkg);
> -		blkg_get(src->bi_blkg);
> -		dst->bi_blkg = src->bi_blkg;
> +		rcu_read_lock();
> +		bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
> +		rcu_read_unlock();
>   	}
>   }
>   EXPORT_SYMBOL_GPL(bio_clone_blkg_association);


Great. Fixed the problem for me. Thanks to you, also to Logan.

Tested-By: Donald Buczek <buczek@molgen.mpg.de>



-- 
Donald Buczek
buczek@molgen.mpg.de
Tel: +49 30 8413 1433

Re: [PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Christoph Hellwig]

On Wed, Jun 01, 2022 at 06:34:05PM +0200, Jan Kara wrote:
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1975,10 +1975,9 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
>  void bio_clone_blkg_association(struct bio *dst, struct bio *src)
>  {
>  	if (src->bi_blkg) {
> +		rcu_read_lock();
> +		bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
> +		rcu_read_unlock();

What do we even need the rcu critical section here?

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCH] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Jan Kara]

Commit d92c370a16cb ("block: really clone the block cgroup in
bio_clone_blkg_association") changed bio_clone_blkg_association() to
just clone bio->bi_blkg reference from source to destination bio. This
is however wrong if the source and destination bios are against
different block devices because struct blkcg_gq is different for each
bdev-blkcg pair. This will result in IOs being accounted (and throttled
as a result) multiple times against the same device (src bdev) while
throttling of the other device (dst bdev) is ignored. In case of BFQ the
inconsistency can even result in crashes in bfq_bic_update_cgroup().
Fix the problem by looking up correct blkcg_gq for the cloned bio.

Reported-by: Logan Gunthorpe <logang@deltatee.com>
Reported-by: Donald Buczek <buczek@molgen.mpg.de>
Fixes: d92c370a16cb ("block: really clone the block cgroup in bio_clone_blkg_association")
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
---
 block/blk-cgroup.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 40161a3f68d0..ecb4eaff6817 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1975,10 +1975,9 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
 	if (src->bi_blkg) {
-		if (dst->bi_blkg)
-			blkg_put(dst->bi_blkg);
-		blkg_get(src->bi_blkg);
-		dst->bi_blkg = src->bi_blkg;
+		rcu_read_lock();
+		bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
+		rcu_read_unlock();
 	}
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
-- 
2.35.3