From 740128dd1a3f2bd81a80314d079b6b03c39ba3af Mon Sep 17 00:00:00 2001 From: Chris Down Date: Mon, 26 Oct 2026 22:31:03 +0800 Subject: [PATCH] mm: make swapoff interruptible when unusing mms/shmem 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 Link: https://lore.kernel.org/at9kJwdGkHPsWwgG@chrisdown.name Fixes: b56a2d8af914 ("mm: rid swapoff of quadratic complexity") Signed-off-by: Chris Down Signed-off-by: Andrew Morton Acked-by: Vineeth Pillai (Google) Reviewed-by: Baolin Wang Reviewed-by: Kairui Song Cc: Hugh Dickins Cc: Chris Li Cc: Nhat Pham Cc: Baoquan He Cc: Barry Song Cc: Youngjun Park Cc: Ying Huang Cc: Kelley Nielsen Cc: Rafael J. Wysocki Cc: --- 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 af21a89abb0f..3c0c7edd2020 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; } success: