Re: [External] [PATCH v4 3/3] fs: cache the string generated by reading /proc/filesystems

[yunhui cui]

Hi Mateusz,

On Sat, May 30, 2026 at 1:27 AM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> It is being read surprisingly often (e.g., by mkdir, ls and even sed!).
>
> This is lock-protected pointer chasing over a linked list to pay for
> sprintf for every fs (32 on my boxen).
>
> Instead cache the result.
>
> While here make the file as permanent to avoid spurious ref trips in
> procfs.
>
> Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
> ---
>  fs/filesystems.c | 153 ++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 151 insertions(+), 2 deletions(-)
>
> diff --git a/fs/filesystems.c b/fs/filesystems.c
> index 7976366d4197..673a03b5f32b 100644
> --- a/fs/filesystems.c
> +++ b/fs/filesystems.c
> @@ -31,6 +31,36 @@
>  static HLIST_HEAD(file_systems);
>  static DEFINE_SPINLOCK(file_systems_lock);
>
> +#ifdef CONFIG_PROC_FS
> +/*
> + * Cache a stringified version of the filesystem list.
> + *
> + * The fs list gets queried a lot by userspace because of libselinux, including
> + * rather surprising programs (would you guess *sed* is on the list?). In order
> + * to reduce the overhead we cache the resulting string, which normally hangs
> + * around below 512 bytes in size.
> + *
> + * As the list almost never changes, its creation is not particularly optimized
> + * to keep things simple.
> + *
> + * We sort it out on read in order to not introduce a failure point for fs
> + * registration (in principle we may be unable to alloc memory for the list).
> + */
> +struct file_systems_string {
> +       struct rcu_head rcu;
> +       unsigned long gen;
> +       size_t len;
> +       char string[];
> +};
> +
> +static unsigned long file_systems_gen;
> +static struct file_systems_string __read_mostly __rcu *file_systems_string;
> +
> +static void invalidate_filesystems_string(void);
> +#else
> +static inline void invalidate_filesystems_string(void) { }
> +#endif
> +
>  /* WARNING: This can be used only if we _already_ own a reference */
>  struct file_system_type *get_filesystem(struct file_system_type *fs)
>  {
> @@ -80,6 +110,7 @@ int register_filesystem(struct file_system_type *fs)
>         if (find_filesystem(fs->name, strlen(fs->name)))
>                 return -EBUSY;
>         hlist_add_tail_rcu(&fs->list, &file_systems);
> +       invalidate_filesystems_string();
>         return 0;
>  }
>  EXPORT_SYMBOL(register_filesystem);
> @@ -101,6 +132,7 @@ int unregister_filesystem(struct file_system_type *fs)
>                 if (hlist_unhashed(&fs->list))
>                         return -EINVAL;
>                 hlist_del_init_rcu(&fs->list);
> +               invalidate_filesystems_string();
>         }
>         synchronize_rcu();
>         return 0;
> @@ -209,7 +241,100 @@ int __init list_bdev_fs_names(char *buf, size_t size)
>  }
>
>  #ifdef CONFIG_PROC_FS
> -static int filesystems_proc_show(struct seq_file *m, void *v)
> +static void invalidate_filesystems_string(void)
> +{
> +       struct file_systems_string *old;
> +
> +       lockdep_assert_held_write(&file_systems_lock);
> +       file_systems_gen++;
> +       old = rcu_replace_pointer(file_systems_string, NULL,
> +                          lockdep_is_held(&file_systems_lock));
> +       if (old)
> +               kfree_rcu(old, rcu);
> +}
> +
> +static __cold noinline int regen_filesystems_string(void)
> +{
> +       struct file_system_type *p;
> +       struct file_systems_string *old, *new;
> +       size_t newlen, usedlen;
> +       unsigned long gen;
> +
> +retry:
> +       newlen = 0;
> +
> +       /* pre-calc space for each fs */
> +       spin_lock(&file_systems_lock);
> +       gen = file_systems_gen;
> +       hlist_for_each_entry_rcu(p, &file_systems, list) {
> +               if (!(p->fs_flags & FS_REQUIRES_DEV))
> +                       newlen += strlen("nodev");
> +               newlen += strlen("\t") + strlen(p->name) + strlen("\n");
> +       }
> +       spin_unlock(&file_systems_lock);
> +
> +       new = kmalloc(offsetof(struct file_systems_string, string) + newlen + 1,
> +                     GFP_KERNEL);
> +       if (!new)
> +               return -ENOMEM;
> +
> +       new->gen = gen;
> +       new->len = newlen;
> +       new->string[newlen] = '\0';
> +
> +       spin_lock(&file_systems_lock);
> +       old = file_systems_string;
> +
> +       /*
> +        * Did someone beat us to it?
> +        */
> +       if (old && old->gen == file_systems_gen) {
> +               spin_unlock(&file_systems_lock);


    It looks like linux-next still carries the older version of this patch and
    misses the unlock here:

      if (old && old->gen == file_systems_gen) {
              kfree(new);
              return 0;
      }

    The May 29 version has:
      spin_unlock(&file_systems_lock);

    before kfree(new), but that seems to be lost in linux-next.
    Could you please check?

> +               kfree(new);
> +               return 0;
> +       }
> +
> +       /*
> +        * Did the list change in the meantime?
> +        */
> +       if (gen != file_systems_gen) {
> +               spin_unlock(&file_systems_lock);
> +               kfree(new);
> +               goto retry;
> +       }
> +
> +       /*
> +        * Populate the string.
> +        *
> +        * We know we have just enough space because we calculated the right
> +        * size the previous time we had the lock and confirmed the list has
> +        * not changed after reacquiring it.
> +        */
> +       usedlen = 0;
> +       hlist_for_each_entry_rcu(p, &file_systems, list) {
> +               usedlen += sprintf(&new->string[usedlen], "%s\t%s\n",
> +                                  (p->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
> +                                  p->name);
> +       }
> +
> +       if (WARN_ON_ONCE(new->len != strlen(new->string))) {
> +               /*
> +                * Should never happen of course, keep this in case someone changes string
> +                * generation above and messes it up.
> +                */
> +               spin_unlock(&file_systems_lock);
> +               kfree(new);
> +               return -EINVAL;
> +       }
> +
> +       rcu_assign_pointer(file_systems_string, new);
> +       spin_unlock(&file_systems_lock);
> +       if (old)
> +               kfree_rcu(old, rcu);
> +       return 0;
> +}
> +
> +static __cold noinline int filesystems_proc_show_fallback(struct seq_file *m, void *v)
>  {
>         struct file_system_type *p;
>
> @@ -222,9 +347,33 @@ static int filesystems_proc_show(struct seq_file *m, void *v)
>         return 0;
>  }
>
> +static int filesystems_proc_show(struct seq_file *m, void *v)
> +{
> +       struct file_systems_string *fss;
> +
> +       for (;;) {
> +               scoped_guard(rcu) {
> +                       fss = rcu_dereference(file_systems_string);
> +                       if (likely(fss)) {
> +                               seq_write(m, fss->string, fss->len);
> +                               return 0;
> +                       }
> +               }
> +
> +               int err = regen_filesystems_string();
> +               if (unlikely(err))
> +                       return filesystems_proc_show_fallback(m, v);
> +       }
> +}
> +
>  static int __init proc_filesystems_init(void)
>  {
> -       proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
> +       struct proc_dir_entry *pde;
> +
> +       pde = proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
> +       if (!pde)
> +               return -ENOMEM;
> +       proc_make_permanent(pde);
>         return 0;
>  }
>  module_init(proc_filesystems_init);
> --
> 2.48.1
>
>

Thanks,
Yunhui