Re: [PATCH v8 1/2] mm/swap, PM: hibernate: fix swapoff race in uswsusp by pinning swap device

[Kairui Song <ryncsn@gmail.com>]

On Tue, Mar 24, 2026 at 01:08:21AM +0800, Youngjun Park wrote:
> Hibernation via uswsusp (/dev/snapshot ioctls) has a race window:
> after selecting the resume swap area but before user space is frozen,
> swapoff may run and invalidate the selected swap device.
> 
> Fix this by pinning the swap device with SWP_HIBERNATION while it is
> in use.  The pin is exclusive, which is sufficient since
> hibernate_acquire() already prevents concurrent hibernation sessions.
> 
> The kernel swsusp path (sysfs-based hibernate/resume) uses
> find_hibernation_swap_type() which is not affected by the pin.  It
> freezes user space before touching swap, so swapoff cannot race.
> 
> Introduce dedicated helpers:
> - pin_hibernation_swap_type(): Look up and pin the swap device.
>   Used by the uswsusp path.
> - find_hibernation_swap_type(): Lookup without pinning.
>   Used by the kernel swsusp path.
> - unpin_hibernation_swap_type(): Clear the hibernation pin.

Looks good to me, thanks!

Reviewed-by: Kairui Song <kasong@tencent.com>

Just one trivial nit picks below.

> +/**
> + * unpin_hibernation_swap_type - Unpin the swap device for hibernation
> + * @type: Swap type previously returned by pin_hibernation_swap_type()
> + *
> + * Clear the hibernation pin on the given swap device, allowing
> + * swapoff() to proceed normally.
> + *
> + * If @type does not refer to a valid swap device, this function
> + * does nothing.
> + */
> +void unpin_hibernation_swap_type(int type)
> +{
> +	struct swap_info_struct *si;
> +
> +	spin_lock(&swap_lock);
> +	si = swap_type_to_info(type);
> +	if (!si) {
> +		spin_unlock(&swap_lock);
> +		return;
> +	}
> +	si->flags &= ~SWP_HIBERNATION;

Will the code will be simpler if you just:

if (si)
	si->flags &= ~SWP_HIBERNATION;

Just personal taste, free feel to ignore.

And as you mentioned this is on top of swap table p3 so you based
it on mm-new - but isn't p3 already in mm-unstable? Maybe we can
have it there? Not sure how much conflict are there with PM.

The code and design looks OK.