linux-kernel - Re: [PATCH v2 2/3] mm, swap: remove fragment clusters counter

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Message-ID: <CAF8kJuMxnMP67GT9poH2eopOSJaXcnior47rEQGipmDCPuc5eQ@mail.gmail.com>
Date: Wed, 6 Aug 2025 22:34:19 -0700
From: Chris Li <chrisl@...nel.org>
To: Kairui Song <kasong@...cent.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Nhat Pham <nphamcs@...il.com>, 
	Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, 
	"Huang, Ying" <ying.huang@...ux.alibaba.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mm, swap: remove fragment clusters counter

Acked-by: Chris Li <chrisl@...nel.org>

Chris

On Wed, Aug 6, 2025 at 9:18 AM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> It was used for calculating the iteration number when the swap allocator
> wants to scan the whole fragment list. Now the allocator only scans one
> fragment cluster at a time, so no one uses this counter anymore.
>
> Remove it as a cleanup; the performance change is marginal:
>
> Build linux kernel using 10G ZRAM, make -j96, defconfig with 2G cgroup
> memory limit, on top of tmpfs, 64kB mTHP enabled:
>
> Before:  sys time: 6278.45s
> After:   sys time: 6176.34s
>
> Change to 8G ZRAM:
>
> Before:  sys time: 5572.85s
> After:   sys time: 5531.49s
>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  include/linux/swap.h | 1 -
>  mm/swapfile.c        | 7 -------
>  2 files changed, 8 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2fe6ed2cc3fd..a060d102e0d1 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -310,7 +310,6 @@ struct swap_info_struct {
>                                         /* list of cluster that contains at least one free slot */
>         struct list_head frag_clusters[SWAP_NR_ORDERS];
>                                         /* list of cluster that are fragmented or contented */
> -       atomic_long_t frag_cluster_nr[SWAP_NR_ORDERS];
>         unsigned int pages;             /* total of usable pages of swap */
>         atomic_long_t inuse_pages;      /* number of those currently in use */
>         struct swap_sequential_cluster *global_cluster; /* Use one global cluster for rotating device */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 1f1110e37f68..5fdb3cb2b8b7 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -470,11 +470,6 @@ static void move_cluster(struct swap_info_struct *si,
>         else
>                 list_move_tail(&ci->list, list);
>         spin_unlock(&si->lock);
> -
> -       if (ci->flags == CLUSTER_FLAG_FRAG)
> -               atomic_long_dec(&si->frag_cluster_nr[ci->order]);
> -       else if (new_flags == CLUSTER_FLAG_FRAG)
> -               atomic_long_inc(&si->frag_cluster_nr[ci->order]);
>         ci->flags = new_flags;
>  }
>
> @@ -965,7 +960,6 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>                  * allocation, but reclaim may drop si->lock and race with another user.
>                  */
>                 while ((ci = isolate_lock_cluster(si, &si->frag_clusters[o]))) {
> -                       atomic_long_dec(&si->frag_cluster_nr[o]);
>                         found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
>                                                         0, usage);
>                         if (found)
> @@ -3217,7 +3211,6 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
>         for (i = 0; i < SWAP_NR_ORDERS; i++) {
>                 INIT_LIST_HEAD(&si->nonfull_clusters[i]);
>                 INIT_LIST_HEAD(&si->frag_clusters[i]);
> -               atomic_long_set(&si->frag_cluster_nr[i], 0);
>         }
>
>         /*
> --
> 2.50.1
>
>