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@surriel.com/T/#u Signed-off-by: Usama Arif --- 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