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

[Andrew Morton]

On Thu, 1 Oct 2026 01:17:40 +0200 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).

(cc Rafael)

> 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
>      [...]

ugh.

> 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.

So things were all good before that change?

> Just as an example of the improvements, here's how long freezing takes
> in the same test while swapoff is happening on my computer:
> 
>                       before                  after
>     400MiB anon       8.925s                  0.028s
>     400MiB shmem      1.639s                  0.003s
>     2GiB anon         failed after 20.003s    0.011s
> 
> Fixes: b56a2d8af914 ("mm: rid swapoff of quadratic complexity")
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>

This sounds like a significant usability regression.  Should we
backport this?

otoh, it's been this way since 2019, so presumably nobody cares much?