Re: [PATCH] mm: Make swapoff interruptible when unusing mms/shmem

[Vineeth Remanan Pillai]

On Wed, Sep 30, 2026 at 7:17 PM Chris Down <chris@xxxxxxxxxxxxxx> wrote:
>
> try_to_unuse() only checks for a pending signal between mms, and
> shmem_unuse() doesn't check at all. That means that once swapoff gets to
> a process or a shmem file with a lot swapped out, nothing can interrupt
> it until every last page of it has been read back in.
>
> Just as one example of where this can concretely show up, freezing tasks
> for suspend or hibernation has to wait for swapoff to notice the
> freezer's fake signal, and gives up after freeze_timeout_msecs (20
> seconds by default).
>
> Here's a facetious example where one swaps out 2GiB of one process to a
> swap file on ext4, starts swapoff, and half a second later tries to
> freeze with pm_test=freezer. Writing to /sys/power/state then fails with
> EBUSY and this in dmesg:
>
>     Freezing user space processes failed after 20.003 seconds (1 tasks refusing to freeze, wq_busy=0):
>     task:swapoff         state:D stack:0     pid:3175  tgid:3175  ppid:2955   task_flags:0x400100 flags:0x00000419
>     Call trace:
>      [...]
>      io_schedule+0x44/0x70
>      folio_wait_bit_common+0x1ec/0x3d0
>      __folio_lock+0x24/0x40
>      unuse_pte_range+0x2d0/0x348
>      unuse_vma+0x158/0x248
>      unuse_mm+0xfc/0x150
>      try_to_unuse+0x104/0x3f8
>      __do_sys_swapoff+0x220/0x5d8
>      [...]
>
> The same goes for anything else that wants swapoff to stop, like an
> admin hitting ^C in a panic, of course.
>
> Prior to commit b56a2d8af914 ("mm: rid swapoff of quadratic complexity")
> try_to_unuse() was driven by find_next_to_unuse() which checks for a
> signal before every entry, so let's restore that behaviour.
>
Nice find, thanks for tracking this down and fixing it. The fix looks
good to me.

Acked-by: Vineeth Pillai (Google) <vineeth@xxxxxxxxxxxxxxx>

Thanks,
Vineeth