Re: [PATCH] mm/swap_state: remove unnecessary lru_add_drain() from readahead

[Kairui Song]

On Mon, Jun 8, 2026 at 10:45 PM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>
> swap_cluster_readahead() and swap_vma_readahead() end the readahead
> loop with an explicit lru_add_drain() call. That drain is a leftover
> from 2.6.12 era code and serves no functional purpose for the callers:
>
> - do_swap_page() ignores LRU residency for the readahead folios;
>   it only needs the target folio it called swapin_readahead() for,
>   and if the write-fault path needs the target folio on the LRU to count
>   references accurately, it runs its own lru_add_drain() at the
>   wp_can_reuse_anon_folio() and do_swap_page() sites.
>
> - shmem_swapin_cluster() immediately locks the  returned folio, waits
>   for writeback, then operates on it - LRU residency of either the target
>   or the readahead folios is irrelevant.
>`
> - try_to_unuse() likewise locks the folio and calls unuse_pte() without
>   depending on LRU presence.
>
> Folios newly added to the swap cache by the readahead loop sit in
> the per-CPU LRU folio_batch and will be drained naturally as the
> batch fills (FOLIO_BATCH_SIZE),by the next reclaim/compaction
> lru_add_drain_all() and so on.  The unconditional drain only
> synchronously flushes a partial batch and forces contention on
> lruvec_lock.
>
> On a 176-CPU production host running a memory-pressured workload, this
> path was observed to call folio_batch_move_lru() from
> swap_cluster_readahead() ~28K/min, a very large source of LRU lock
> traffic.
>
> This is a direct continuation of the cleanup started in commit
> 1aa43598c03b ("mm: remove unnecessary calls to lru_add_drain") which
> removed the equivalent drain from free_pages_and_swap_cache() with
> the same rationale. A detailed reasoning for this is present in [1].
>
> Remove both drains.
>
> [1] https://lore.kernel.org/all/dca2824e8e88e826c6b260a831d79089b5b9c79d.camel@xxxxxxxxxxx/T/#u
>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---
>  mm/swap_state.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 9c3a5cf99778..6fd6e3415b71 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -836,7 +836,6 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>         }
>         blk_finish_plug(&plug);
>         swap_read_unplug(splug);
> -       lru_add_drain();        /* Push any new pages onto the LRU now */
>  skip:
>         /* The page was likely read above, so no need for plugging here */
>         return swap_cache_read_folio(entry, gfp_mask, mpol, ilx, NULL, false);
> @@ -951,7 +950,6 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
>                 pte_unmap(pte);
>         blk_finish_plug(&plug);
>         swap_read_unplug(splug);
> -       lru_add_drain();
>  skip:
>         /* The folio was likely read above, so no need for plugging here */
>         folio = swap_cache_read_folio(targ_entry, gfp_mask, mpol, targ_ilx,
> --
> 2.52.0

Thanks!

Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>