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

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Sep 03, 2026 at 04:33:03PM +0800, Julian Sun 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.

Was this reproduced from an actual workload with adverse effects? If so, can
you please explain the workload and effects with concrete details?

> 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.

If you do this, tho, that means now cgroups that accumulated a lot of
metadata writes on ext4 and crunched for memory don't have a way to relieve
the pressure outside of periodic or other lucky flushes. ie. I have a hard
time judging whether this is net plus or not without learning more about how
this patch came to be.

Thanks.

-- 
tejun