Re: [PATCH] PM: hibernate: Freeze kernel threads after image preallocation

[Mario Limonciello <superm1@kernel.org>]

On 9/20/26 09:35, Florian Schmaus via B4 Relay wrote:
> From: Florian Schmaus <flo@geekplace.eu>
> 
> Commit 783c81098445 ("PM: hibernate: call preallocate_image() after freeze
> prepare") moved hibernate_preallocate_memory() after dpm_prepare() so
> that device drivers have the opportunity to release pinned/unswappable
> memory during their ->prepare() callback before memory is preallocated
> for the snapshot image.
> 
> However, that commit also placed hibernate_preallocate_memory() after
> freeze_kernel_threads(). While it was assumed during review that swap
> I/O submitted via submit_bio() is synchronous and would not depend on
> frozen kernel threads, this does not hold in practice. Calling
> hibernate_preallocate_memory() with kernel threads frozen leads to
> intermittent deadlocks during hibernation.
> 
> Inside hibernate_preallocate_memory(), shrink_all_memory() is invoked with
> .may_writepage = 1 and .may_swap = 1 to aggressively reclaim and swap out
> pages. Any writeback or swap I/O that relies on freezable kernel threads,
> block device helpers, or WQ_FREEZABLE workqueues (such as those in storage
> drivers, device mapper, or filesystems) deadlocks waiting on tasks that
> are stuck in the refrigerator.
> 
> Fix this by reordering hibernation_snapshot():
> 1. Call dpm_prepare(PMSG_FREEZE) first, allowing device drivers to release
>     pinned resources while kernel threads are still active.
> 2. Call hibernate_preallocate_memory() second, performing page reclaim and
>     swapout while storage layers, workqueues, and kernel threads are alive.
> 3. Call freeze_kernel_threads() third, only after all memory preallocation
>     and swap I/O have completed.
> 
> Additionally, restore the call to swsusp_free() in the cleanup path so
> that preallocated image memory is properly freed if freeze_kernel_threads()
> fails or if TEST_FREEZER is enabled.
> 
> Fixes: 783c81098445 ("PM: hibernate: call preallocate_image() after freeze prepare")
> Signed-off-by: Florian Schmaus <flo@geekplace.eu>
> ---
> During the review of commit 783c81098445 ("PM: hibernate: call
> preallocate_image() after freeze prepare") [1], concerns were raised
> regarding whether memory reclaim and swap I/O could deadlock if kernel
> threads were already frozen.
> 
> At the time, it was thought that pageout to swap would not depend on
> frozen threads. However, on Linux 7.2, I ran into reliable issues with
> suspend-to-disk hanging during hibernation. Reordering the sequence so
> that kernel threads are frozen after image preallocation (as done in
> this patch) fixes the issue for me.
> 
> This patch restores the ordering where kernel threads are frozen only after
> memory preallocation and swap I/O have completed, while keeping the
> benefit of calling dpm_prepare() beforehand so drivers can release pinned
> pages.
> 
> [1] https://patch.msgid.link/20260403-hibernation-fixes-v3-1-31bc9fa3ba2d@collabora.com
> ---
>   kernel/power/hibernate.c | 26 ++++++++++++++------------
>   1 file changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
> index d2479c69d71a..c13f68ab7f6e 100644
> --- a/kernel/power/hibernate.c
> +++ b/kernel/power/hibernate.c
> @@ -408,9 +408,18 @@ int hibernation_snapshot(int platform_mode)
>   	if (error)
>   		goto Close;
>   
> +	error = dpm_prepare(PMSG_FREEZE);
> +	if (error)
> +		goto Complete;
> +
> +	/* Preallocate image memory before freezing kernel threads and shutting down devices. */
> +	error = hibernate_preallocate_memory();
> +	if (error)
> +		goto Complete;
> +
>   	error = freeze_kernel_threads();
>   	if (error)
> -		goto Close;
> +		goto Cleanup;
>   
>   	if (hibernation_test(TEST_FREEZER)) {
>   
> @@ -422,15 +431,6 @@ int hibernation_snapshot(int platform_mode)
>   		goto Thaw;
>   	}
>   
> -	error = dpm_prepare(PMSG_FREEZE);
> -	if (error)
> -		goto Complete;
> -
> -	/* Preallocate image memory before shutting down devices. */
> -	error = hibernate_preallocate_memory();
> -	if (error)
> -		goto Complete;
> -
>   	console_suspend_all();
>   	pm_restrict_gfp_mask();
>   
> @@ -464,10 +464,12 @@ int hibernation_snapshot(int platform_mode)
>   	platform_end(platform_mode);
>   	return error;
>   
> - Complete:
> -	dpm_complete(PMSG_RECOVER);
>    Thaw:
>   	thaw_kernel_threads();
> + Cleanup:
> +	swsusp_free();
> + Complete:
> +	dpm_complete(PMSG_RECOVER);
>   	goto Close;
>   }
>   
> 
> ---
> base-commit: 40288c9206c17eb66a603262e06a58d300d0f279
> change-id: 20260915-fix-hibernation-aad94ce17506
> 
> Best regards,
> --
> Florian Schmaus <flo@geekplace.eu>

Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>