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

[Johannes Weiner]

On Wed, Nov 04, 2026 at 12:18:16AM +0800, Kairui Song via B4 Relay 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>

Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>