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

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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@xxxxxxxxxx/
> > 
> > Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Jay Shin <jaeshin@xxxxxxxxxx>
> > Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> > Cc: Waiman Long <longman@xxxxxxxxxx>
> > Cc: mkoutny@xxxxxxxx
> > Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20230609234249.1412858-1-ming.lei@xxxxxxxxxx
> > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> > ---
> > 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,
Ming




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