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

[Mateusz Guzik]

On Tue, Jun 9, 2026 at 1:31 PM yunhui cui <cuiyunhui@xxxxxxxxxxxxx> wrote:
>
> 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?
>

That's my bad. I got a response by Christian I failed to address, I'm
going to do it shortly.

> > +               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