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

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

On Thu, Mar 19, 2026 at 02:33:30PM +0100, Rafael J. Wysocki wrote:
> On Tue, Mar 17, 2026 at 7:13 PM Youngjun Park <youngjun.park@lge.com> wrote:
> >
> > 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(-)
> >
> > --
> 
> Which kernel version does this series apply to?
> 
> It doesn't apply for me on top of 7.0-rc4, so please consider rebasing.

It is based on mm-new 
I will soon rebase on the top of 7.0-rc4 with build & test.

Best regards,
Youngjun Park