Re: [PATCH v2 1/1] mm: reinstate unconditional writeback start in balance_dirty_pages()

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Mar 26, 2026 at 02:51:27PM -0700, Joanne Koong wrote:
> Commit 64dd89ae01f2 ("mm/block/fs: remove laptop_mode") removed this
> unconditional writeback start from balance_dirty_pages():
> 
>        if (unlikely(!writeback_in_progress(wb)))
> 	       wb_start_background_writeback(wb);
> 
> This logic needs to be reinstated to prevent performance regressions for
> strictlimited BDIs and memcg setups. The problem occurs because:
> 
> a) For strictlimited BDIs, throttling is calculated using per-wb
> thresholds. The per-wb threshold can be exceeded even when the global
> dirty threshold was not exceeded (nr_dirty < gdtc->bg_thresh)
> 
> b) For memcg-based throttling, memcg uses its own dirty count /
> thresholds and can trigger throttling even when the global threshold
> isn't exceeded
> 
> Without the unconditional writeback start, IO is throttled as it waits
> for dirty pages to be written back but there is no writeback running.
> This leads to severe stalls. On fuse, buffered write performance
> dropped from 1400 MiB/s to 2000 KiB/s.
> 
> Reinstate the unconditional writeback start so that writeback is
> guaranteed to be running whenever IO needs to be throttled.
> 
> Fixes: 64dd89ae01f2 ("mm/block/fs: remove laptop_mode")
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Thanks for the fix. This was a combination of the old comment and my
overly hasty read of the existing dirty > bg_thresh &&
!writeback_in_progress() block. The patch looks good to me.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>