[PATCH v4 0/3] mm/swap, PM: hibernate: fix swapoff race and optimize swap

Youngjun Park
2026-03-17T18:13:28Z

Currently, in the uswsusp path, only the swap type value is retrieved at
lookup time without holding a reference. If swapoff races after the type
is acquired, subsequent slot allocations operate on a stale swap device.

Additionally, grabbing and releasing the swap device reference on every
slot allocation is inefficient across the entire hibernation swap path.

This patch series addresses these issues:
- Patch 1: Fixes the swapoff race in uswsusp by holding the swap device
reference from the point the swap device is looked up.
- Patch 2: Removes the overhead of per-slot reference counting in alloc/free
paths and cleans up the redundant SWP_WRITEOK check.
- Patch 3: Fixes a spurious WARNING in the uswsusp GFP mask restore path.
(Founded during uswsusp test)

Links:
RFC v1: https://lore.kernel.org/linux-mm/20260305202413.1888499-1-usama.arif@linux.dev/T/#m3693d45180f14f441b6951984f4b4bfd90ec0c9d
RFC v2: https://lore.kernel.org/linux-mm/20260306024608.1720991-1-youngjun.park@lge.com/
RFC v3: https://lore.kernel.org/linux-mm/20260312112511.3596781-1-youngjun.park@lge.com/

Testing:
- Hibernate/resume via sysfs
(echo reboot > /sys/power/disk && echo disk > /sys/power/state)
- Hibernate with suspend via sysfs
(echo suspend > /sys/power/disk && echo disk > /sys/power/state)
- Hibernate/resume via uswsusp (suspend-utils s2disk/resume on QEMU)
- Verified swap I/O works correctly after resume.
- Verified swapoff succeeds after snapshot resume completes.
- Verified pm_restore_gfp_mask() WARNING no longer triggers (Patch 3).
- swapoff during active uswsusp session:
- Verified swapoff blocks while uswsusp holds swap reference.
- Verified swapoff can be cancelled by signal (e.g. Ctrl+C).
- Verified swapoff succeeds after uswsusp process terminates.

Changelog:
rfc v3 -> v4:
- Introduced get/find/put_hibernation_swap_type() helpers per Kairui's
feedback. find_ for lookup-only, get/put for reference management.
- Switched to swap_type_to_info() and added type < 0 check per
Kairui's suggestion.
- Fixed get_hibernation_swap_type() return when ref == false (Reviewd by Kairui)
- Made swapoff wait interruptible to prevent hang when uswsusp
holds a swap reference.
- Fixed spurious WARN_ON in pm_restore_gfp_mask() by introducing
pm_restore_gfp_mask_safe() (Patch 3).
- Updated commit messages and added comments for clarity.
- Rebased onto latest mm-new tree.

Note: Kairui suggested adding WARN on NULL in put_hibernation_swap_type(),
but kept silent return instead, as type can legitimately be -1 when
snapshot_open() fails to find a matching swap device. swap_type_to_info()
returns NULL for type < 0, so the cleanup path stays simple.

rfc v2 -> rfc v3:
- Split into 2 patches per Chris Li's feedback.
- Simplified by not holding reference in normal hibernation path
per Chris Li's suggestion.
- Removed redundant SWP_WRITEOK check.
- Rebased onto f543926f9d0c3f6dfb354adfe7fbaeedd1277c6b.

rfc v1 -> rfc v2:
- Squashed into single patch per Usama Arif's feedback.

Youngjun Park (3):
mm/swap, PM: hibernate: fix swapoff race in uswsusp by getting swap
reference
mm/swap: remove redundant swap device reference in alloc/free
PM: hibernate: fix spurious GFP mask WARNING in uswsusp path

include/linux/suspend.h | 1 +
include/linux/swap.h | 4 +-
kernel/power/main.c | 7 ++
kernel/power/swap.c | 2 +-
kernel/power/user.c | 19 +++--
mm/swapfile.c | 154 +++++++++++++++++++++++++++-------------
6 files changed, 132 insertions(+), 55 deletions(-)

--

base-commit: 7d47a508dfdc335c107fb00b4d9ef46488281a52
2.34.1