Re: [PATCH] mm/swap, PM: hibernate: atomically replace hibernation pin

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 30 Apr 2026 20:56:51 +0100 David Carlier <devnexen@gmail.com> wrote:

> snapshot_set_swap_area() unpins the previously selected swap device
> and pins the new one in two separate swap_lock critical sections.
> In the gap between them, swapoff() observes SWP_HIBERNATION cleared,
> bypasses the guard, and tears down the device, reopening the race
> the SWP_HIBERNATION pin was meant to close. The window is reachable
> on any SNAPSHOT_SET_SWAP_AREA call after the snapshot device is
> opened for hibernation, and on any retry after the resume path's
> first selection.
> 
> Add repin_hibernation_swap_type(), which looks up the new device,
> clears the old SWP_HIBERNATION flag and sets the new one under a
> single swap_lock acquisition. The same-device case is short-
> circuited so userspace can re-select the same swap area without
> tripping WARN_ON_ONCE and -EBUSY. Switch snapshot_set_swap_area()
> to the new helper.
> 
> A failed lookup now preserves the previous pin instead of dropping
> it, so a bad SNAPSHOT_SET_SWAP_AREA leaves the prior selection
> intact. The open and release paths keep using
> pin_hibernation_swap_type() and unpin_hibernation_swap_type().
> 
> The race was identified during AI-assisted review of the
> SWP_HIBERNATION pinning series.

Thanks, I'll queue this for testing.  Can the swap maintainers please
do a detailed review at some point?