Re: [PATCH] mm: filemap: move lruvec accounting outside the xarray lock

[Shakeel Butt]

On Wed, Sep 16, 2026 at 05:51:22AM -0700, Usama Arif wrote:
> __filemap_add_folio() inserts a folio and updates mapping->nrpages
> while holding mapping->i_pages.xa_lock with interrupts disabled. The
> XArray insertion and nrpages update require the lock, but the lruvec
> statistic updates do not. With CONFIG_MEMCG, those calls also update
> per-CPU memcg and lruvec counters and notify cgroup rstat, extending the
> critical section.
> 
> Move the lruvec accounting after a successful XArray insertion and
> after xas_unlock_irq(). The page-cache references pin the folio, while
> the folio lock keeps folio->mapping stable and prevents removal until
> accounting is complete. This moves one lruvec update for ordinary folios
> and a second for PMD-mappable folios out of the serialized section.
> 
> In a 30-second system-wide perf lock contention -ab capture on a
> production host, the hottest caller-stack record attributed to
> __filemap_add_folio() had 20,867 contentions and 557.930 ms total wait.
> That was 14% of the 3.998 seconds of aggregate lock wait in the
> capture. Moving lruvec accuting outside of critical section should
> help optimize it.
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>