Re: [PATCH v2 3/4] mm/swap: account synchronous swap reads as memory stall

[Nhat Pham]

On Tue, Nov 3, 2026 at 9:18 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
>
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
>
> Swap-ins from SWP_SYNCHRONOUS_IO devices such as zram are no longer
> counted in PSI memory pressure or in the swapin and thrashing delays of
> delay accounting.  Refaults of the workingset from zram, the main input
> to userspace OOM handlers like lmkd and systemd-oomd on zram systems,
> become invisible in /proc/pressure/memory.
>
> swap_read_folio() opens the accounting window with
> psi_memstall_enter() and delayacct_swapin_start() and, per its comment,
> means to count the submission time.  For synchronous devices the
> submission is the entire read: swap_bdev_submit_read() does
> submit_bio_wait() and completes the folio.  Since reads are batched in
> a swap_iocb, swap_read_folio() only queues the folio with
> swap_add_folio(), and the bio is submitted later by swap_read_submit()
> in the caller, e.g. swapin_sync(), after the window has been closed.
> By then the folio is uptodate and unlocked, so the fault does not wait
> on the folio lock either and the read time is not counted anywhere.
>
> Fix by submitting synchronous reads from within swap_read_folio(), as
> was done before the batching, rather than from the caller.  This gives
> up batching of readahead for synchronous devices, which do their I/O
> inline anyway, in exchange for keeping the accounting simple and in one
> place.
>
> Fixes: dda8fb68b590 ("mm/swap: also use struct swap_iocb for block I/O")
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/page_io.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 65681eed9c80..83ed0246ebfc 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -500,6 +500,13 @@ void swap_read_folio(struct swap_io_ctx *ctx, struct folio *folio)
>         zswap_folio_swapin(folio);
>         swap_add_folio(ctx, folio, READ);
>
> +       /*
> +        * Synchronous devices do the whole read at submission time, submit
> +        * it here so that it is accounted as memory stall and delay.
> +        */
> +       if (synchronous)
> +               swap_read_submit(ctx);
> +

Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>