Re: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain

[Matthew Wilcox]

On Thu, Apr 23, 2026 at 09:43:07AM -0700, JP Kobryn (Meta) wrote:
> Of all observable lruvec lock contention in our fleet, we find that ~24%
> occurs when dead folios are present in lru_add batches at drain time. This
> is wasteful in the sense that the folio is added to the LRU just to be
> immediately removed via folios_put_refs(), incurring two unnecessary lock
> acquisitions.

Well, this is a lovely patch with no obvious downsides.  Nicely done.

> Eliminate this overhead by preemptively cleaning up dead folios before they
> make it into the LRU. Use folio_ref_freeze() to filter folios whose only
> remaining refcount is the batch ref. When dead folios are found, move them
> off the add batch and onto a temporary batch to be freed.
> 
> During A/B testing on one of our prod instagram workloads (high-frequency
> short-lived requests), the patch intercepted almost all dead folios before
> they entered the LRU. Data collected using the mm_lru_insertion tracepoint
> shows the effectiveness of the patch:
> 
> Per-host LRU add averages at 95% CPU load
> (60 hosts each side, 3 x 60s intervals)
> 
>             dead folios/min  total folios/min   dead %
> unpatched:        1,297,785        19,341,986  6.7097%
> patched:                 14        19,039,996  0.0001%
> 
> Within this workload, we save ~2.6M lock acquisitions per minute per host
> as a result.
> 
> System-wide memory stats improved on the patched side also at 95% CPU load:
>  - direct reclaim scanning reduced 7%
>  - allocation stalls reduced 5.2%
>  - compaction stalls reduced 12.3%
>  - page frees reduced 4.9%
> 
> No regressions were observed in requests served per second or request tail
> latency (p99). Both metrics showed directional improvement at higher CPU
> utilization (comparing 85% to 95%).
> 
> Signed-off-by: JP Kobryn (Meta) <jp.kobryn@xxxxxxxxx>
> ---
>  mm/swap.c | 36 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 5cc44f0de9877..71607b0ce3d18 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -160,13 +160,36 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
>  	int i;
>  	struct lruvec *lruvec = NULL;
>  	unsigned long flags = 0;
> +	struct folio_batch free_fbatch;
> +	bool is_lru_add = (move_fn == lru_add);
> +
> +	/*
> +	 * If we're adding to the LRU, preemptively filter dead folios. Use
> +	 * this dedicated folio batch for temp storage and deferred cleanup.
> +	 */
> +	if (is_lru_add)
> +		folio_batch_init(&free_fbatch);
>  
>  	for (i = 0; i < folio_batch_count(fbatch); i++) {
>  		struct folio *folio = fbatch->folios[i];
>  
>  		/* block memcg migration while the folio moves between lru */
> -		if (move_fn != lru_add && !folio_test_clear_lru(folio))
> +		if (!is_lru_add && !folio_test_clear_lru(folio))
> +			continue;
> +
> +		/*
> +		 * Filter dead folios by moving them from the add batch to the temp
> +		 * batch for freeing after this loop.
> +		 *
> +		 * Since the folio may be part of a huge page, unqueue from
> +		 * deferred split list to avoid a dangling list entry.
> +		 */
> +		if (is_lru_add && folio_ref_freeze(folio, 1)) {
> +			folio_unqueue_deferred_split(folio);

Would it be better to do this outside the lru lock; it's just that we
don't have a convenient batched version to do it?  It seems like
there are a few places that could use a batched version in vmscan.c and
swap.c.  Not that I think we should hold up this patch to investigate
that micro-optimisation!  Just something you couldlook at as a
follow-up.

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>