Re: [PATCH v2] writeback, memcg: skip foreign dirty tracking for bdev inodes

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  3 Sep 2026 16:33:03 +0800 Julian Sun <sunjunchao@bytedance.com> wrote:

> Foreign dirty flushing handles cases where a dirtied folio's memcg and
> the memcg owning its inode wb differ.
> 
> wbc_detach_inode() notes that "concurrent write sharing of an inode is
> expected to be very rare". This expectation does not hold for bdev inodes.
> A bdev inode and mapping are shared by buffered users of the block device,
> while the inode has a single wb owner. On ext4, buffer-cache folios used
> by metadata and journal I/O can therefore be charged to different memcgs
> while sharing the same mapping. Normal bdev activity can thus produce
> frequent folio/wb ownership mismatches.
> 
> This was reproduced on cgroup v2 with the memory and I/O controllers
> enabled, using ext4 on a loop device. jbd2 repeatedly generated
> track_foreign_dirty events for bdev folios charged to unrelated memcgs.
> When one of those memcgs entered dirty throttling, the resulting record led
> to a flush_foreign event and queued writeback with reason=foreign_flush for
> the bdev inode's root wb.
> 
> Skip foreign dirty tracking when the folio mapping's host inode is on the
> blockdev pseudo superblock. This prevents bdev-originated records from
> triggering later foreign flushes.
> 
> The tradeoff is that dirty throttling in the folio's memcg no longer uses
> bdev dirtiness to queue writeback on the bdev inode's owner wb. Those
> folios remain subject to normal writeback. This is preferable because a
> single bdev wb owner does not identify which cgroup is responsible for a
> mapping shared by buffered bdev users. Flushing it can write back data
> unrelated to the throttled memcg. Dirty accounting, normal writeback, and
> foreign dirty tracking for non-bdev inodes remain unchanged.

When fixing something, please always describe the userspace-visible
runtime effects.

I'm thinking probably "unnecessary I/O"?  If so, has this been
quantified/measured?  If not, can the effects be guessed about?

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3898,6 +3898,15 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
>  	u64 oldest_at = now;
>  	int oldest = -1;
>  	int i;
> +	struct address_space *mapping = folio_mapping(folio);
> +	struct inode *bdev_inode = mapping ? mapping->host : NULL;
> +
> +	/*
> +	 * Bdev inodes are usually shared by many memcgs so foreign
> +	 * tracking leads to frequent flushes which is counterproductive.
> +	 */
> +	if (bdev_inode && sb_is_blkdev_sb(bdev_inode->i_sb))
> +		return;
>  
>  	trace_track_foreign_dirty(folio, wb);
>  
> -- 
> 2.39.5