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

[Youngjun Park <youngjun.park@lge.com>]

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