Re: [PATCH 6.3.y] blk-cgroup: Flush stats before releasing blkcg_gq

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jun 19, 2023 at 05:20:19PM +0800, Ming Lei wrote:
> On Mon, Jun 19, 2023 at 10:51:16AM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Jun 19, 2023 at 04:30:09PM +0800, Ming Lei wrote:
> > > As noted by Michal, the blkg_iostat_set's in the lockless list hold
> > > reference to blkg's to protect against their removal. Those blkg's
> > > hold reference to blkcg. When a cgroup is being destroyed,
> > > cgroup_rstat_flush() is only called at css_release_work_fn() which
> > > is called when the blkcg reference count reaches 0. This circular
> > > dependency will prevent blkcg and some blkgs from being freed after
> > > they are made offline.
> > > 
> > > It is less a problem if the cgroup to be destroyed also has other
> > > controllers like memory that will call cgroup_rstat_flush() which will
> > > clean up the reference count. If block is the only controller that uses
> > > rstat, these offline blkcg and blkgs may never be freed leaking more
> > > and more memory over time.
> > > 
> > > To prevent this potential memory leak:
> > > 
> > > - flush blkcg per-cpu stats list in __blkg_release(), when no new stat
> > > can be added
> > > 
> > > - add global blkg_stat_lock for covering concurrent parent blkg stat
> > > update
> > > 
> > > - don't grab bio->bi_blkg reference when adding the stats into blkcg's
> > > per-cpu stat list since all stats are guaranteed to be consumed before
> > > releasing blkg instance, and grabbing blkg reference for stats was the
> > > most fragile part of original patch
> > > 
> > > Based on Waiman's patch:
> > > 
> > > https://lore.kernel.org/linux-block/20221215033132.230023-3-longman@redhat.com/
> > > 
> > > Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()")
> > > Cc: stable@vger.kernel.org
> > > Reported-by: Jay Shin <jaeshin@redhat.com>
> > > Acked-by: Tejun Heo <tj@kernel.org>
> > > Cc: Waiman Long <longman@redhat.com>
> > > Cc: mkoutny@suse.com
> > > Cc: Yosry Ahmed <yosryahmed@google.com>
> > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > Link: https://lore.kernel.org/r/20230609234249.1412858-1-ming.lei@redhat.com
> > > Signed-off-by: Jens Axboe <axboe@kernel.dk>
> > > ---
> > > Context difference with linus tree: 2c275afeb61d ("block: make blkcg_punt_bio_submit
> > > optional") adds '#ifdef CONFIG_BLK_CGROUP_PUNT_BIO' in __blkg_release().
> > > 
> > > 
> > 
> > What is the git commit id of this change in Linus's tree?
> 
> 20cb1c2fb756 ("blk-cgroup: Flush stats before releasing blkcg_gq")

Thanks, now queued up.

greg k-h