mm/swap: submit the last readahead batch before unplugging

Since block swap I/O is batched in struct swap_iocb, swap_add_folio() only
appends a folio to the pending batch when its slot directly follows the
previous one, and submits the pending batch otherwise.  Batches submitted
inside the readahead loop go through the plug, where the block layer can
still merge them, but the final batch is submitted after blk_finish_plug()
and always becomes a separate request.

So submit the final batch before finishing the plug.

Link: https://lore.kernel.org/20261001085730.2029773-1-alex@ghiti.fr
Fixes: dda8fb68b590 ("mm/swap: also use struct swap_iocb for block I/O")
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Youngjun Park <youngjun.park@lge.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 2475ba29126d..ebef568cd62e 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -906,8 +906,8 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
 			continue;
 		folio_put(folio);
 	}
-	blk_finish_plug(&plug);
 	swap_read_submit(&ctx);
+	blk_finish_plug(&plug);
 skip:
 	return swap_cache_read_folio_sync(entry, gfp_mask, mpol, ilx);
 }
@@ -1019,8 +1019,8 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 	}
 	if (pte)
 		pte_unmap(pte);
-	blk_finish_plug(&plug);
 	swap_read_submit(&ctx);
+	blk_finish_plug(&plug);
 skip:
 	/* The folio was likely read above, so no need for plugging here */
 	return swap_cache_read_folio_sync(targ_entry, gfp_mask, mpol, targ_ilx);