Re: [PATCH] mm/swap: submit the last readahead batch before unplugging

[Joshua Hahn]

On Thu,  1 Oct 2026 10:57:12 +0200 Alexandre Ghiti <alex@xxxxxxxx> wrote:

> 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.

Hi Alex,

I read through the code and this looks very reasonable to me.

Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>

> Fixes: dda8fb68b590 ("mm/swap: also use struct swap_iocb for block I/O")

My only question is whether we need the fixes tag, it seems like
dda8fb68b590 is still in mm-stable and not merged to linus yet.

Andrew, what do we usually do in these cases? I am wondering if it will
just become folded into the original commit or if this should still be
standalone.

Thank you both! Have a great day : -)
Joshua

> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
>  mm/swap_state.c | 4 ++--
>  1 file 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);
> -- 
> 2.53.0-Meta