mm/swap, PM: hibernate: fix swapoff race and optimize swap

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 pinning the swap device
  from the point it is looked up until the session completes.
- 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.
  (Found during uswsusp test)

Links:
RFC v1: https://lore.kernel.org/linux-mm/20260305202413.1888499-1...
RFC v2: https://lore.kernel.org/linux-mm/20260306024608.1720991-1...
RFC v3: https://lore.kernel.org/linux-mm/20260312112511.3596781-1...
v4: https://lore.kernel.org/linux-mm/abv+rjgyArqZ2uym@yjaykim...
v5: https://lore.kernel.org/linux-mm/ab0YEn+Fd41q6LM7@yjaykim...

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).
  - Verified SNAPSHOT_FREEZE followed by snapshot_release() does not
    trigger pm_restore_gfp_mask() WARNING (Patch 3).
- swapoff during active uswsusp session:
  - Verified swapoff returns -EBUSY while swap device is pinned (Patch 1).
  - Verified swapoff succeeds after uswsusp process terminates.

Changelog:
v5 -> v6:
 - Replaced get/put reference approach with SWP_HIBERNATION
   pinning to prevent swapoff, per Kairui's feedback.  Renamed helpers
   from get/find/put_hibernation_swap_type() to
   pin/find/unpin_hibernation_swap_type().
 - Removed swapoff waiting on hibernation reference.
   swapoff now returns -EBUSY immediately when the swap device is
   pinned.
 - Updated function comments per Kairui's review.
 - Updated commit message.
 - Fixed pm_restore_gfp_mask_safe() to use saved_gfp_count
   check instead of saved_gfp_mask, and hold system_transition_mutex
   for the check.  Addressed AI review finding that SNAPSHOT_FREEZE
   followed by snapshot_release() could still trigger the WARNING.

v4 -> v5:
 - Rebased onto v7.0-rc4 (Rafael J. Wysocki comment)
 - No functional changes. rebase conflict fix.

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 (Reviewed 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.

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 pinning swap
    device
  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    |   5 +-
 kernel/power/main.c     |  18 ++++
 kernel/power/swap.c     |   2 +-
 kernel/power/user.c     |  21 +++--
 mm/swapfile.c           | 178 +++++++++++++++++++++++++++++++---------
 6 files changed, 178 insertions(+), 47 deletions(-)


base-commit: f338e77383789c0cae23ca3d48adcc5e9e137e3c
-- 
2.34.1