Re: [PATCH v4 1/3] mm, swap: speed up hibernation allocation and writeout

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

On Mon, 16 Feb 2026 22:58:02 +0800 Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org> wrote:

> From: Kairui Song <kasong@tencent.com>
> 
> Since commit 0ff67f990bd4 ("mm, swap: remove swap slot cache"),
> hibernation has been using the swap slot slow allocation path for
> simplification, which turns out might cause regression for some
> devices because the allocator now rotates clusters too often, leading to
> slower allocation and more random distribution of data.
> 
> Fast allocation is not complex, so implement hibernation support as
> well.
> 
> Test result with Samsung SSD 830 Series (SATA II, 3.0 Gbps) shows the
> performance is several times better [1]:
> 6.19:               324 seconds
> After this series:  35 seconds

Thanks.

I'll merge only [1/3] at this time, into mm-unstable at this time (I'll
move it to mm-unstable after resyncing mm.git with upstream).

We don't want the other two patches present during testing of this
backportable fix because doing so partially invalidates that testing -
[2/3] and[3/3] might accidentally fix issues which [1/3] added.  It happens,
occasionally.

> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1926,8 +1926,9 @@ void swap_put_entries_direct(swp_entry_t entry, int nr)
>  /* Allocate a slot for hibernation */
>  swp_entry_t swap_alloc_hibernation_slot(int type)
>  {
> -	struct swap_info_struct *si = swap_type_to_info(type);
> -	unsigned long offset;
> +	struct swap_info_struct *pcp_si, *si = swap_type_to_info(type);
> +	unsigned long pcp_offset, offset = SWAP_ENTRY_INVALID;
> +	struct swap_cluster_info *ci;
>  	swp_entry_t entry = {0};
>  
>  	if (!si)
> @@ -1937,11 +1938,21 @@ swp_entry_t swap_alloc_hibernation_slot(int type)
>  	if (get_swap_device_info(si)) {
>  		if (si->flags & SWP_WRITEOK) {
>  			/*
> -			 * Grab the local lock to be compliant
> -			 * with swap table allocation.
> +			 * Try the local cluster first if it matches the device. If
> +			 * not, try grab a new cluster and override local cluster.
>  			 */

nanonit, worrying about 80-cols is rather old fashioned but there's no
reason to overflow 80 in a block comment!

>  			local_lock(&percpu_swap_cluster.lock);
> -			offset = cluster_alloc_swap_entry(si, NULL);
> +			pcp_si = this_cpu_read(percpu_swap_cluster.si[0]);
> +			pcp_offset = this_cpu_read(percpu_swap_cluster.offset[0]);
> +			if (pcp_si == si && pcp_offset) {
> +				ci = swap_cluster_lock(si, pcp_offset);
> +				if (cluster_is_usable(ci, 0))
> +					offset = alloc_swap_scan_cluster(si, ci, NULL, pcp_offset);
> +				else
> +					swap_cluster_unlock(ci);
> +			}
> +			if (!offset)
> +				offset = cluster_alloc_swap_entry(si, NULL);
>  			local_unlock(&percpu_swap_cluster.lock);
>  			if (offset)
>  				entry = swp_entry(si->type, offset);