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super: convert to plain hashtable

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Convert the rhltable to a plain hashtable under a spinlock. The walks
keep their place with a cursor entry in the chain. A claim is unhashed
the moment it's dropped and a walk that finds a superblock that isn't
born yet waits for it to be born or to die and looks again. Since
hashing can't fail anymore the entry for the superblock's own s_dev is
embedded in struct super_block and the preallocation in alloc_super()
goes away. This all simplifies things quite a lot.

It also fixes a btrfs bug where fs_bdev_freeze() ended up freezing a
btrfs filesystem through a stale member that open_ctree() had already
released while the freeze was waiting for SB_BORN. The resume doesn't
find a holder for that device anymore so the filesystem stays frozen
for good.

The walks return superblocks instead of table entries now and
user_get_super() hands out the passive reference the walk took.

Reproducer, a three device btrfs with a stale copy of a removed member.
dm-delay under the remaining members stretches the window between
btrfs_open_devices() and btrfs_free_extra_devids():

  truncate -s 1G a1 a2 b
  A1=$(losetup -f --show a1); A2=$(losetup -f --show a2); B=$(losetup -f --show b)
  mkfs.btrfs -f -d single -m single $A1 $A2 $B
  cp b b.old
  mount $A1 /mnt && btrfs device remove 3 /mnt && umount /mnt
  OLD=$(losetup -f --show b.old)
  dmsetup create d1 --table "0 $(blockdev --getsz $A1) delay $A1 0 200"
  dmsetup create d2 --table "0 $(blockdev --getsz $A2) delay $A2 0 200"
  dmsetup create old --table "0 $(blockdev --getsz $OLD) linear $OLD 0"
  mount -o device=/dev/mapper/d2,device=/dev/mapper/old /dev/mapper/d1 /mnt &
  sleep 1; dmsetup suspend old; wait
  dmsetup resume old
  touch /mnt/foo     # hangs until fsfreeze -u /mnt

That left btrfs frozen in 5 of 5 rounds with the same tables issued
through raw dm ioctls. The -o degraded variant needs the mount held
between the insertion and the bd_fsfreeze_count check in
fs_bdev_register() with a kprobe.

Fixes: cdb5146f8d ("fs: look up superblocks via the device table in fs_holder_ops")
Link: https://patch.msgid.link/20261012-work-super-dev_hash-v1-2-305cf71cd334@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-10-12 14:31:19 +02:00
parent 073d71c511
commit c84de57efe
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
5 changed files with 152 additions and 210 deletions
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2
block/bdev.c
View File

@ -1283,7 +1283,7 @@ int lookup_bdev(const char *pathname, dev_t *dev)
* registers major 0. Fake block device inodes (e.g. fuse with
* rootmode=S_IFBLK) can expose i_rdev == 0, and letting that
* propagate would confuse superblock lookup and trigger warnings
* in the device-to-superblock table (super_dev_register).
* in the device-to-superblock table (sget_fc).
*/
error = -ENODEV;
if (!inode->i_rdev)

1
fs/internal.h
View File

@ -137,7 +137,6 @@ extern int reconfigure_super(struct fs_context *);
extern bool super_trylock_shared(struct super_block *sb);
struct super_block *user_get_super(dev_t, bool excl);
void put_super(struct super_block *sb);
void __init super_dev_init(void);
extern bool mount_capable(struct fs_context *);
/*

2
fs/namespace.c
View File

@ -6492,8 +6492,6 @@ void __init mnt_init(void)
HASH_ZERO,
&mp_hash_shift, &mp_hash_mask, 0, 0);
super_dev_init();
kernfs_init();
err = sysfs_init();

346
fs/super.c
View File

@ -25,7 +25,7 @@
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/memcontrol.h>
#include <linux/rhashtable.h>
#include <linux/hashtable.h>
#include <linux/mount.h>
#include <linux/security.h>
#include <linux/writeback.h> /* for the emergency remount stuff */
@ -274,8 +274,6 @@ static unsigned long super_cache_count(struct shrinker *shrink,
return total_objects;
}
static struct super_dev *super_dev_alloc(dev_t dev, struct super_block *sb);
static void destroy_super_work(struct work_struct *work)
{
struct super_block *s = container_of(work, struct super_block,
@ -283,8 +281,6 @@ static void destroy_super_work(struct work_struct *work)
fsnotify_sb_free(s);
security_sb_free(s);
put_user_ns(s->s_user_ns);
/* Only an unregistered entry is still owned by the superblock. */
kfree(s->s_super_dev);
kfree(s->s_subtype);
for (int i = 0; i < SB_FREEZE_LEVELS; i++)
percpu_free_rwsem(&s->s_writers.rw_sem[i]);
@ -398,9 +394,6 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
goto fail;
if (list_lru_init_memcg(&s->s_inode_lru, s->s_shrink))
goto fail;
s->s_super_dev = super_dev_alloc(0, s);
if (!s->s_super_dev)
goto fail;
s->s_min_writeback_pages = MIN_WRITEBACK_PAGES;
return s;
@ -435,110 +428,92 @@ void put_super(struct super_block *s)
}
}
struct super_dev {
dev_t sd_dev;
struct super_block *sd_sb;
refcount_t sd_ref;
struct rhlist_head sd_node;
struct rcu_head sd_rcu;
};
/*
* Superblocks hashed by the devices they claim: s_dev from sget_fc() to
* kill_super_notify() and every block device opened through
* fs_bdev_file_open_by_{dev,path}(). Walks keep their place with a cursor, an
* entry without a superblock.
*/
#define SUPER_DEV_HASH_BITS 8
static DEFINE_HASHTABLE(super_dev_hash, SUPER_DEV_HASH_BITS);
static DEFINE_SPINLOCK(super_dev_lock);
static struct rhltable super_dev_table;
static const struct rhashtable_params super_dev_params = {
.key_len = sizeof(dev_t),
.key_offset = offsetof(struct super_dev, sd_dev),
.head_offset = offsetof(struct super_dev, sd_node),
};
static struct super_dev *super_dev_alloc(dev_t dev, struct super_block *sb)
static void super_dev_add(struct super_dev *sd, struct super_block *sb, dev_t dev)
{
struct super_dev *fsd;
lockdep_assert_held(&super_dev_lock);
fsd = kzalloc_obj(*fsd);
if (!fsd)
return NULL;
fsd->sd_dev = dev;
fsd->sd_sb = sb;
refcount_set(&fsd->sd_ref, 1);
return fsd;
sd->sd_sb = sb;
sd->sd_dev = dev;
sd->sd_claims = 1;
refcount_inc(&sb->s_passive);
hash_add(super_dev_hash, &sd->sd_hash, dev);
}
static void super_dev_put(struct super_dev *fsd)
/* Drop a claim, the last one unhashes the entry. */
static void super_dev_put(struct super_dev *sd)
{
/* Unlink only once unpinned, so a cursor never resumes from a removed node. */
if (fsd && refcount_dec_and_test(&fsd->sd_ref)) {
rhltable_remove(&super_dev_table, &fsd->sd_node, super_dev_params);
put_super(fsd->sd_sb);
kfree_rcu(fsd, sd_rcu);
struct super_block *sb = sd->sd_sb;
scoped_guard(spinlock, &super_dev_lock) {
if (--sd->sd_claims)
return;
hash_del(&sd->sd_hash);
}
if (sd != &sb->s_super_dev)
kfree(sd);
put_super(sb);
}
/*
* Return the superblock of the next claim behind @cursor with a passive
* reference and put @prev. The mount of a superblock that isn't born yet may
* still drop its claim, so wait for it to be born or to die and look again.
*/
static struct super_block *super_dev_next(struct super_dev *cursor,
struct super_block *prev)
{
struct super_block *sb;
struct super_dev *sd;
if (prev)
put_super(prev);
for (;;) {
scoped_guard(spinlock, &super_dev_lock) {
if (!hash_hashed(&cursor->sd_hash))
hash_add(super_dev_hash, &cursor->sd_hash, cursor->sd_dev);
sd = cursor;
hlist_for_each_entry_continue(sd, sd_hash) {
if (sd->sd_sb && sd->sd_dev == cursor->sd_dev)
break;
}
if (!sd) {
hash_del(&cursor->sd_hash);
return NULL;
}
sb = sd->sd_sb;
refcount_inc(&sb->s_passive);
if (super_flags(sb, SB_BORN | SB_DYING)) {
hash_del(&cursor->sd_hash);
hlist_add_behind(&cursor->sd_hash, &sd->sd_hash);
return sb;
}
}
wait_var_event(&sb->s_flags, super_flags(sb, SB_BORN | SB_DYING));
put_super(sb);
}
}
void __init super_dev_init(void)
/* Unhash the cursor of a walk that stops early. */
static void super_dev_walk_end(struct super_dev *cursor)
{
if (rhltable_init(&super_dev_table, &super_dev_params))
panic("VFS: Cannot initialise super_dev_table\n");
guard(spinlock)(&super_dev_lock);
hash_del(&cursor->sd_hash);
}
static int super_dev_insert(struct super_dev *fsd)
{
int err;
err = rhltable_insert(&super_dev_table, &fsd->sd_node, super_dev_params);
if (!err)
refcount_inc(&fsd->sd_sb->s_passive);
return err;
}
/* Register @sb under @sb->s_dev as the final fallible act of a set callback. */
static int super_dev_register(struct super_block *sb)
{
struct super_dev *fsd = sb->s_super_dev;
int err;
lockdep_assert_held(&sb_lock);
VFS_WARN_ON_ONCE(!sb->s_dev);
VFS_WARN_ON_ONCE(!fsd || fsd->sd_dev);
fsd->sd_dev = sb->s_dev;
err = super_dev_insert(fsd);
if (err)
fsd->sd_dev = 0;
return err;
}
static struct super_dev *super_dev_get(struct rhlist_head *pos)
{
struct super_dev *sb_dev;
for (; pos; pos = rcu_dereference_all(pos->next)) {
sb_dev = container_of(pos, struct super_dev, sd_node);
if (refcount_inc_not_zero(&sb_dev->sd_ref))
return sb_dev;
}
return NULL;
}
static struct super_dev *super_dev_first(dev_t dev)
{
struct super_dev *sb_dev;
rcu_read_lock();
sb_dev = super_dev_get(rhltable_lookup(&super_dev_table, &dev, super_dev_params));
rcu_read_unlock();
return sb_dev;
}
static struct super_dev *super_dev_next(struct super_dev *prev)
{
struct super_dev *sb_dev;
rcu_read_lock();
sb_dev = super_dev_get(rcu_dereference_all(prev->sd_node.next));
rcu_read_unlock();
super_dev_put(prev);
return sb_dev;
}
#define for_each_dev_super(sb, cursor) \
for (sb = super_dev_next(cursor, NULL); sb; sb = super_dev_next(cursor, sb))
static void kill_super_notify(struct super_block *sb)
{
@ -548,11 +523,8 @@ static void kill_super_notify(struct super_block *sb)
if (sb->s_flags & SB_DEAD)
return;
/* Drop sget_fc()'s claim; a never-registered entry stays with the sb. */
if (sb->s_super_dev->sd_dev) {
super_dev_put(sb->s_super_dev);
sb->s_super_dev = NULL;
}
/* Drop sget_fc()'s claim on s_dev. */
super_dev_put(&sb->s_super_dev);
/*
* Let concurrent mounts know that this thing is really dead.
@ -880,13 +852,12 @@ struct super_block *sget_fc(struct fs_context *fc,
s->s_fs_info = fc->s_fs_info;
err = set(s, fc);
if (err) {
VFS_WARN_ON_ONCE(s->s_super_dev->sd_dev);
s->s_fs_info = NULL;
spin_unlock(&sb_lock);
destroy_unused_super(s);
return ERR_PTR(err);
}
VFS_WARN_ON_ONCE(!s->s_super_dev->sd_dev);
VFS_WARN_ON_ONCE(!s->s_dev);
fc->s_fs_info = NULL;
s->s_type = fc->fs_type;
s->s_iflags |= fc->s_iflags;
@ -899,6 +870,8 @@ struct super_block *sget_fc(struct fs_context *fc,
list_add_tail(&s->s_list, &super_blocks);
hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
spin_unlock(&sb_lock);
scoped_guard(spinlock, &super_dev_lock)
super_dev_add(&s->s_super_dev, s, s->s_dev);
get_filesystem(s->s_type);
shrinker_register(s->s_shrink);
return s;
@ -1042,18 +1015,14 @@ EXPORT_SYMBOL(iterate_supers_type);
struct super_block *user_get_super(dev_t dev, bool excl)
{
struct super_dev *sb_dev;
struct super_dev cursor = { .sd_dev = dev };
struct super_block *sb;
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 (!super_lock(sb, excl))
continue;
/* The pinned entry holds a passive reference, take our own. */
refcount_inc(&sb->s_passive);
super_dev_put(sb_dev);
return sb;
for_each_dev_super(sb, &cursor) {
if (super_lock(sb, excl)) {
super_dev_walk_end(&cursor);
return sb;
}
}
return NULL;
}
@ -1336,16 +1305,7 @@ EXPORT_SYMBOL(free_anon_bdev);
int set_anon_super(struct super_block *s, void *data)
{
int error;
error = get_anon_bdev(&s->s_dev);
if (error)
return error;
error = super_dev_register(s);
if (error)
free_anon_bdev(s->s_dev);
return error;
return get_anon_bdev(&s->s_dev);
}
EXPORT_SYMBOL(set_anon_super);
@ -1442,7 +1402,7 @@ EXPORT_SYMBOL(get_tree_keyed);
static int set_bdev_super(struct super_block *s, void *data)
{
s->s_dev = *(dev_t *)data;
return super_dev_register(s);
return 0;
}
static int super_s_dev_set(struct super_block *s, struct fs_context *fc)
@ -1502,14 +1462,12 @@ static int fs_super_thaw(struct super_block *sb)
static void fs_bdev_mark_dead(struct block_device *bdev, bool surprise)
{
struct super_dev *sb_dev;
dev_t dev = bdev->bd_dev;
struct super_dev cursor = { .sd_dev = bdev->bd_dev };
struct super_block *sb;
mutex_unlock(&bdev->bd_holder_lock);
for (sb_dev = super_dev_first(dev); sb_dev; sb_dev = super_dev_next(sb_dev)) {
struct super_block *sb = sb_dev->sd_sb;
for_each_dev_super(sb, &cursor) {
if (!super_lock_shared(sb))
continue;
if (sb->s_root && (sb->s_flags & SB_ACTIVE)) {
@ -1529,14 +1487,12 @@ static void fs_bdev_mark_dead(struct block_device *bdev, bool surprise)
static void fs_bdev_sync(struct block_device *bdev)
{
struct super_dev *sb_dev;
dev_t dev = bdev->bd_dev;
struct super_dev cursor = { .sd_dev = bdev->bd_dev };
struct super_block *sb;
mutex_unlock(&bdev->bd_holder_lock);
for (sb_dev = super_dev_first(dev); sb_dev; sb_dev = super_dev_next(sb_dev)) {
struct super_block *sb = sb_dev->sd_sb;
for_each_dev_super(sb, &cursor) {
if (!super_lock_shared(sb))
continue;
if (sb->s_root && (sb->s_flags & SB_ACTIVE))
@ -1559,8 +1515,8 @@ static void fs_bdev_sync(struct block_device *bdev)
*/
static int fs_bdev_freeze(struct block_device *bdev)
{
dev_t dev = bdev->bd_dev;
struct super_dev *sb_dev;
struct super_dev cursor = { .sd_dev = bdev->bd_dev };
struct super_block *sb;
unsigned int count = 0;
int error = 0, err;
@ -1568,13 +1524,13 @@ static int fs_bdev_freeze(struct block_device *bdev)
mutex_unlock(&bdev->bd_holder_lock);
for (sb_dev = super_dev_first(dev); sb_dev; sb_dev = super_dev_next(sb_dev)) {
if (!get_active_super(sb_dev->sd_sb))
for_each_dev_super(sb, &cursor) {
if (!get_active_super(sb))
continue;
err = fs_super_freeze(sb_dev->sd_sb);
err = fs_super_freeze(sb);
if (err && !error)
error = err;
deactivate_super(sb_dev->sd_sb);
deactivate_super(sb);
count++;
}
@ -1604,8 +1560,8 @@ static int fs_bdev_freeze(struct block_device *bdev)
*/
static int fs_bdev_thaw(struct block_device *bdev)
{
dev_t dev = bdev->bd_dev;
struct super_dev *sb_dev;
struct super_dev cursor = { .sd_dev = bdev->bd_dev };
struct super_block *sb;
unsigned int count = 0;
int error = 0, err;
@ -1613,13 +1569,13 @@ static int fs_bdev_thaw(struct block_device *bdev)
mutex_unlock(&bdev->bd_holder_lock);
for (sb_dev = super_dev_first(dev); sb_dev; sb_dev = super_dev_next(sb_dev)) {
if (!get_active_super(sb_dev->sd_sb))
for_each_dev_super(sb, &cursor) {
if (!get_active_super(sb))
continue;
err = fs_super_thaw(sb_dev->sd_sb);
err = fs_super_thaw(sb);
if (err && !error)
error = err;
deactivate_super(sb_dev->sd_sb);
deactivate_super(sb);
count++;
}
@ -1636,55 +1592,39 @@ static const struct blk_holder_ops fs_holder_ops = {
.thaw = fs_bdev_thaw,
};
static struct super_dev *super_dev_lookup(dev_t dev, struct super_block *sb)
static struct super_dev *super_dev_find(dev_t dev, const struct super_block *sb)
{
struct super_dev *it;
struct rhlist_head *list, *pos;
struct super_dev *sd;
RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious super_dev_lookup() usage");
VFS_WARN_ON_ONCE(!dev);
VFS_WARN_ON_ONCE(!sb);
lockdep_assert_held(&super_dev_lock);
list = rhltable_lookup(&super_dev_table, &dev, super_dev_params);
rhl_for_each_entry_rcu(it, pos, list, sd_node) {
if (it->sd_sb == sb)
return it;
hash_for_each_possible(super_dev_hash, sd, sd_hash, dev) {
if (sd->sd_dev == dev && sd->sd_sb == sb)
return sd;
}
return NULL;
}
static int fs_bdev_register(struct file *bdev_file, struct super_block *sb)
static int fs_bdev_register(struct block_device *bdev, struct super_block *sb)
{
struct super_dev *sb_dev __free(kfree) = NULL;
dev_t dev = file_bdev(bdev_file)->bd_dev;
int err;
struct super_dev *new __free(kfree) = NULL;
struct super_dev *sd;
scoped_guard(rcu) {
sb_dev = super_dev_lookup(dev, sb);
if (sb_dev && refcount_inc_not_zero(&sb_dev->sd_ref)) {
retain_and_null_ptr(sb_dev);
return 0;
}
new = kzalloc_obj(*new);
guard(spinlock)(&super_dev_lock);
sd = super_dev_find(bdev->bd_dev, sb);
if (sd) {
sd->sd_claims++;
return 0;
}
sb_dev = super_dev_alloc(dev, sb);
if (!sb_dev)
/* bdev_freeze() raises the count before its walk takes the lock. */
if (atomic_read(&bdev->bd_fsfreeze_count) > 0)
return -EBUSY;
if (!new)
return -ENOMEM;
err = super_dev_insert(sb_dev);
if (err)
return err;
/* Publish the entry before reading the count; pairs with bdev_freeze(). */
smp_mb();
if (atomic_read(&file_bdev(bdev_file)->bd_fsfreeze_count) > 0) {
err = -EBUSY;
super_dev_put(sb_dev);
}
retain_and_null_ptr(sb_dev);
return err;
super_dev_add(no_free_ptr(new), sb, bdev->bd_dev);
return 0;
}
/**
@ -1711,7 +1651,7 @@ struct file *fs_bdev_file_open_by_dev(dev_t dev, blk_mode_t mode, void *holder,
if (IS_ERR(bdev_file))
return bdev_file;
err = fs_bdev_register(bdev_file, sb);
err = fs_bdev_register(file_bdev(bdev_file), sb);
if (err) {
bdev_fput(bdev_file);
return ERR_PTR(err);
@ -1745,7 +1685,7 @@ struct file *fs_bdev_file_open_by_path(const char *path, blk_mode_t mode,
if (IS_ERR(bdev_file))
return bdev_file;
err = fs_bdev_register(bdev_file, sb);
err = fs_bdev_register(file_bdev(bdev_file), sb);
if (err) {
bdev_fput(bdev_file);
return ERR_PTR(err);
@ -1760,21 +1700,20 @@ EXPORT_SYMBOL_GPL(fs_bdev_file_open_by_path);
* @sb: superblock the device was claimed for
*
* The inverse of fs_bdev_register(): drop one claim on the {dev, @sb} entry
* (the last claim unregisters it; a pinning cursor defers the actual unlink)
* without closing the device. A caller that must act on the still-open device
* between unregistering and closing - e.g. re-allow freezing one denied for a
* membership change - pairs this with bdev_fput(). fs_bdev_file_release() is
* the common unregister-and-close.
* (the last claim unregisters it) without closing the device. A caller that
* must act on the still-open device between unregistering and closing - e.g.
* re-allow freezing one denied for a membership change - pairs this with
* bdev_fput(). fs_bdev_file_release() is the common unregister-and-close.
*/
void fs_bdev_unregister(struct file *bdev_file, struct super_block *sb)
{
dev_t dev = file_bdev(bdev_file)->bd_dev;
struct super_dev *sb_dev;
struct super_dev *sd;
rcu_read_lock();
sb_dev = super_dev_lookup(dev, sb);
rcu_read_unlock();
super_dev_put(sb_dev);
/* The claim we're dropping keeps the entry hashed. */
scoped_guard(spinlock, &super_dev_lock)
sd = super_dev_find(file_bdev(bdev_file)->bd_dev, sb);
if (sd)
super_dev_put(sd);
}
EXPORT_SYMBOL_GPL(fs_bdev_unregister);
@ -1817,8 +1756,7 @@ int setup_bdev_super(struct super_block *sb, int sb_flags,
return -EACCES;
}
/* The sget_fc() entry is already published; pairs with bdev_freeze(). */
smp_mb();
/* sget_fc() hashed @sb, super_dev_lock orders that against bdev_freeze(). */
if (atomic_read(&bdev->bd_fsfreeze_count) > 0) {
if (fc)
warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);

11
include/linux/fs/super_types.h
View File

@ -30,7 +30,6 @@ struct mount;
struct mtd_info;
struct quotactl_ops;
struct shrinker;
struct super_dev;
struct unicode_map;
struct user_namespace;
struct workqueue_struct;
@ -132,10 +131,18 @@ struct super_operations {
void (*report_error)(const struct fserror_event *event);
};
/* A superblock's claim on a device, see fs/super.c. */
struct super_dev {
struct hlist_node sd_hash;
struct super_block *sd_sb;
dev_t sd_dev;
unsigned int sd_claims;
};
struct super_block {
struct list_head s_list; /* Keep this first */
dev_t s_dev; /* search index; _not_ kdev_t */
struct super_dev *s_super_dev; /* sget_fc()'s device table claim */
struct super_dev s_super_dev; /* claim on s_dev */
unsigned char s_blocksize_bits;
unsigned long s_blocksize;
loff_t s_maxbytes; /* Max file size */