Re: [PATCH v3 01/12] mm, swap: protect si->swap_file properly and use as a mount indicator

[Chris Li]

Acked-by: Chris Li <chrisl@xxxxxxxxxx>

Chris

On Tue, Feb 17, 2026 at 12:06 PM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> /proc/swaps uses si->swap_map as the indicator to check if the swap
> device is mounted. swap_map will be removed soon, so change it to use
> si->swap_file instead because:
>
> - si->swap_file is exactly the only dynamic content that /proc/swaps is
>   interested in. Previously, it was checking si->swap_map just to ensure
>   si->swap_file is available. si->swap_map is set under mutex
>   protection, and after si->swap_file is set, so having si->swap_map set
>   guarantees si->swap_file is set.
>
> - Checking si->flags doesn't work here. SWP_WRITEOK is cleared during
>   swapoff, but /proc/swaps is supposed to show the device under swapoff
>   too to report the swapoff progress. And SWP_USED is set even if the
>   device hasn't been properly set up.
>
> We can have another flag, but the easier way is to just check
> si->swap_file directly. So protect si->swap_file setting with mutext,
> and set si->swap_file only when the swap device is truly enabled.
>
> /proc/swaps only interested in si->swap_file and a few static data
> reading. Only si->swap_file needs protection. Reading other static
> fields is always fine.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/swapfile.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 32e0e7545ab8..25dfe992538d 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -110,6 +110,7 @@ struct swap_info_struct *swap_info[MAX_SWAPFILES];
>
>  static struct kmem_cache *swap_table_cachep;
>
> +/* Protects si->swap_file for /proc/swaps usage */
>  static DEFINE_MUTEX(swapon_mutex);
>
>  static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
> @@ -2532,7 +2533,8 @@ static void drain_mmlist(void)
>  /*
>   * Free all of a swapdev's extent information
>   */
> -static void destroy_swap_extents(struct swap_info_struct *sis)
> +static void destroy_swap_extents(struct swap_info_struct *sis,
> +                                struct file *swap_file)
>  {
>         while (!RB_EMPTY_ROOT(&sis->swap_extent_root)) {
>                 struct rb_node *rb = sis->swap_extent_root.rb_node;
> @@ -2543,7 +2545,6 @@ static void destroy_swap_extents(struct swap_info_struct *sis)
>         }
>
>         if (sis->flags & SWP_ACTIVATED) {
> -               struct file *swap_file = sis->swap_file;
>                 struct address_space *mapping = swap_file->f_mapping;
>
>                 sis->flags &= ~SWP_ACTIVATED;
> @@ -2626,9 +2627,9 @@ EXPORT_SYMBOL_GPL(add_swap_extent);
>   * Typically it is in the 1-4 megabyte range.  So we can have hundreds of
>   * extents in the rbtree. - akpm.
>   */
> -static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
> +static int setup_swap_extents(struct swap_info_struct *sis,
> +                             struct file *swap_file, sector_t *span)
>  {
> -       struct file *swap_file = sis->swap_file;
>         struct address_space *mapping = swap_file->f_mapping;
>         struct inode *inode = mapping->host;
>         int ret;
> @@ -2646,7 +2647,7 @@ static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
>                 sis->flags |= SWP_ACTIVATED;
>                 if ((sis->flags & SWP_FS_OPS) &&
>                     sio_pool_init() != 0) {
> -                       destroy_swap_extents(sis);
> +                       destroy_swap_extents(sis, swap_file);
>                         return -ENOMEM;
>                 }
>                 return ret;
> @@ -2862,7 +2863,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>         flush_work(&p->reclaim_work);
>         flush_percpu_swap_cluster(p);
>
> -       destroy_swap_extents(p);
> +       destroy_swap_extents(p, p->swap_file);
>         if (p->flags & SWP_CONTINUED)
>                 free_swap_count_continuations(p);
>
> @@ -2952,7 +2953,7 @@ static void *swap_start(struct seq_file *swap, loff_t *pos)
>                 return SEQ_START_TOKEN;
>
>         for (type = 0; (si = swap_type_to_info(type)); type++) {
> -               if (!(si->flags & SWP_USED) || !si->swap_map)
> +               if (!(si->swap_file))
>                         continue;
>                 if (!--l)
>                         return si;
> @@ -2973,7 +2974,7 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
>
>         ++(*pos);
>         for (; (si = swap_type_to_info(type)); type++) {
> -               if (!(si->flags & SWP_USED) || !si->swap_map)
> +               if (!(si->swap_file))
>                         continue;
>                 return si;
>         }
> @@ -3390,7 +3391,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>                 goto bad_swap;
>         }
>
> -       si->swap_file = swap_file;
>         mapping = swap_file->f_mapping;
>         dentry = swap_file->f_path.dentry;
>         inode = mapping->host;
> @@ -3440,7 +3440,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>
>         si->max = maxpages;
>         si->pages = maxpages - 1;
> -       nr_extents = setup_swap_extents(si, &span);
> +       nr_extents = setup_swap_extents(si, swap_file, &span);
>         if (nr_extents < 0) {
>                 error = nr_extents;
>                 goto bad_swap_unlock_inode;
> @@ -3549,6 +3549,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>         prio = DEF_SWAP_PRIO;
>         if (swap_flags & SWAP_FLAG_PREFER)
>                 prio = swap_flags & SWAP_FLAG_PRIO_MASK;
> +
> +       si->swap_file = swap_file;
>         enable_swap_info(si, prio, swap_map, cluster_info, zeromap);
>
>         pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s\n",
> @@ -3573,10 +3575,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>         kfree(si->global_cluster);
>         si->global_cluster = NULL;
>         inode = NULL;
> -       destroy_swap_extents(si);
> +       destroy_swap_extents(si, swap_file);
>         swap_cgroup_swapoff(si->type);
>         spin_lock(&swap_lock);
> -       si->swap_file = NULL;
>         si->flags = 0;
>         spin_unlock(&swap_lock);
>         vfree(swap_map);
>
> --
> 2.52.0
>
>