Re: [PATCH v2] mm: Make swapoff interruptible when unusing mms/shmem

[Kairui Song]

On Mon, Oct 26, 2026 at 10:35 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.
>
> 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")
> Cc: stable@xxxxxxxxxxxxxxx
> Acked-by: Vineeth Pillai (Google) <vineeth@xxxxxxxxxxxxxxx>
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
> ---
> Changes in v2:
> - Return -ERESTARTSYS at both new signal checks and at the existing
>   try_to_unuse() signal exit (Baolin).
> - Request stable backporting with Cc: stable@xxxxxxxxxxxxxxx (Andrew).
>
>  mm/shmem.c    | 4 ++++
>  mm/swapfile.c | 4 +++-
>  2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index fadf23f9e21e..13e0786436cd 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1742,6 +1742,10 @@ static int shmem_unuse_inode(struct inode *inode, unsigned int type)
>                 if (ret < 0)
>                         break;
>
> +               if (signal_pending(current)) {
> +                       ret = -ERESTARTSYS;
> +                       break;
> +               }
>                 start = indices[folio_batch_count(&fbatch) - 1];
>         } while (true);
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 254ce86fa923..0f72a5b23a3a 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2689,6 +2689,8 @@ static inline int unuse_pmd_range(struct vm_area_struct *vma, pud_t *pud,
>         pmd = pmd_offset(pud, addr);
>         do {
>                 cond_resched();
> +               if (signal_pending(current))
> +                       return -ERESTARTSYS;
>                 next = pmd_addr_end(addr, end);
>                 ret = unuse_pte_range(vma, pmd, addr, next, type);
>                 if (ret)
> @@ -2922,7 +2924,7 @@ static int try_to_unuse(unsigned int type)
>         if (swap_usage_in_pages(si)) {
>                 if (!signal_pending(current))
>                         goto retry;
> -               return -EINTR;
> +               return -ERESTARTSYS;
>         }

Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>