On Tue, Jun 16, 2026 at 04:08:33PM +0200, Christian Brauner wrote:
> user_get_super() still finds the superblock for a device number by
> walking the global super_blocks list under sb_lock. Every superblock is
> registered in the device table under its s_dev since sget_fc() inserts
> it there, including superblocks on anonymous devices, so use the table
> instead.
>
> The refcount-pinning cursor helpers super_dev_{get,first,next}() only
> touch table state and do not depend on CONFIG_BLOCK, so drop the
> CONFIG_BLOCK guard around them: their new caller serves anonymous
> devices as well (ustat() on e.g. tmpfs) and is built without
> CONFIG_BLOCK. The guard falls in this patch rather than separately
> since without this caller the helpers would be unused without
> CONFIG_BLOCK.
>
> The pinned entry holds a passive reference on the superblock so
> super_lock() can be called directly; once the superblock is locked grab
> a passive reference for the caller before dropping the pin.
>
> The device table contains more than the old walk could find: a
> superblock is also registered for every additional device it claims
> (the xfs log and realtime devices, btrfs member devices, the ext4
> external journal, erofs blob devices). Don't filter those out:
> specifying any device a filesystem uses now resolves to that
> filesystem, so ustat() and quotactl() work on e.g. the xfs log device
> or a btrfs member device (the latter used to fail outright as btrfs
> superblocks carry an anonymous s_dev that never matches a member
> device). When several superblocks share a device (erofs blob devices)
> the first live superblock wins.
Does erofs have a means to find the other superblocks that share a
device given a notification coming in on one of them? As hch says, it
feels weird to have a lookup mechanism when there's also an upcall
mechanism.
<shrug> I've been on vacation for a while so maybe I missed that there's
another use for the bdev->sb lookup? There are 1600 more emails for me
to go through... :P
--D
>
> The cursor also keeps scanning past dying superblocks where the old
> walk gave up after the first s_dev match, so a mount racing with the
> unmount of the same device (or with the reuse of a recycled anonymous
> dev_t) finds the live superblock where the old walk could spuriously
> return NULL.
>
> This removes the last s_dev-keyed walk of the super_blocks list and
> takes ustat() and quotactl()'s block device lookup off sb_lock
> entirely.
>
> Signed-off-by: Christian Brauner (Amutable) <[email protected]>
> ---
> fs/super.c | 28 ++++++++--------------------
> 1 file changed, 8 insertions(+), 20 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index 2d0a07861bfc..93f24aea75c4 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -501,7 +501,6 @@ static int super_dev_register(struct super_block *sb)
> return err;
> }
>
> -#ifdef CONFIG_BLOCK
> static struct super_dev *super_dev_get(struct rhlist_head *pos)
> {
> struct super_dev *sb_dev;
> @@ -535,7 +534,6 @@ static struct super_dev *super_dev_next(struct super_dev
> *prev)
> super_dev_put(prev);
> return sb_dev;
> }
> -#endif
>
> static void kill_super_notify(struct super_block *sb)
> {
> @@ -1044,29 +1042,19 @@ EXPORT_SYMBOL(iterate_supers_type);
>
> struct super_block *user_get_super(dev_t dev, bool excl)
> {
> - struct super_block *sb;
> -
> - spin_lock(&sb_lock);
> - list_for_each_entry(sb, &super_blocks, s_list) {
> - bool locked;
> + struct super_dev *sb_dev;
>
> - if (sb->s_dev != dev)
> - continue;
> + for (sb_dev = super_dev_first(dev); sb_dev; sb_dev =
> super_dev_next(sb_dev)) {
> + struct super_block *sb = sb_dev->sd_sb;
>
> - if (!refcount_inc_not_zero(&sb->s_passive))
> + if (!super_lock(sb, excl))
> continue;
>
> - spin_unlock(&sb_lock);
> -
> - locked = super_lock(sb, excl);
> - if (locked)
> - return sb;
> -
> - put_super(sb);
> - spin_lock(&sb_lock);
> - break;
> + /* The pinned entry holds a passive reference, take our own. */
> + refcount_inc(&sb->s_passive);
> + super_dev_put(sb_dev);
> + return sb;
> }
> - spin_unlock(&sb_lock);
> return NULL;
> }
>
>
> --
> 2.47.3
>
>
