mm/swap_state: remove unnecessary lru_add_drain() from readahead

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
1aa43598c0 ("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.

Link: https://lore.kernel.org/20260608143242.2869392-1-usama.arif@linux.dev
Link: https://lore.kernel.org/all/dca2824e8e88e826c6b260a831d79089b5b9c79d.camel@surriel.com/T/#u [1]
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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,