[PATCH 1/3] fs: distinguish between user initiated freeze and kernel initiated freeze

Darrick J. Wong
2023-06-16T01:48:40Z

From: Darrick J. Wong <djwong@kernel.org>

Userspace can freeze a filesystem using the FIFREEZE ioctl or by
suspending the block device; this state persists until userspace thaws
the filesystem with the FITHAW ioctl or resuming the block device.
Since commit 18e9e5104fcd ("Introduce freeze_super and thaw_super for
the fsfreeze ioctl") we only allow the first freeze command to succeed.

The kernel may decide that it is necessary to freeze a filesystem for
its own internal purposes, such as suspends in progress, filesystem fsck
activities, or quiescing a device prior to removal. Userspace thaw
commands must never break a kernel freeze, and kernel thaw commands
shouldn't undo userspace's freeze command.

Introduce a couple of freeze holder flags and wire it into the
sb_writers state. One kernel and one userspace freeze are allowed to
coexist at the same time; the filesystem will not thaw until both are
lifted.

I wonder if the f2fs/gfs2 code should be using a kernel freeze here, but
for now we'll use FREEZE_HOLDER_USERSPACE to preserve existing
behaviors.

Cc: mcgrof@kernel.org
Cc: jack@suse.cz
Cc: hch@infradead.org
Cc: ruansy.fnst@fujitsu.com
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
Documentation/filesystems/vfs.rst | 6 ++-
block/bdev.c | 8 ++--
fs/f2fs/gc.c | 4 +-
fs/gfs2/glops.c | 2 -
fs/gfs2/super.c | 6 +--
fs/gfs2/sys.c | 4 +-
fs/gfs2/util.c | 2 -
fs/ioctl.c | 8 ++--
fs/super.c | 79 +++++++++++++++++++++++++++++++++----
include/linux/fs.h | 15 +++++--
10 files changed, 101 insertions(+), 33 deletions(-)

diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 769be5230210..6f7e971edb2d 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -260,9 +260,11 @@ filesystem. The following members are defined:
void (*evict_inode) (struct inode *);
void (*put_super) (struct super_block *);
int (*sync_fs)(struct super_block *sb, int wait);
- int (*freeze_super) (struct super_block *);
+ int (*freeze_super) (struct super_block *sb,
+ enum freeze_holder who);
int (*freeze_fs) (struct super_block *);
- int (*thaw_super) (struct super_block *);
+ int (*thaw_super) (struct super_block *sb,
+ enum freeze_wholder who);
int (*unfreeze_fs) (struct super_block *);
int (*statfs) (struct dentry *, struct kstatfs *);
int (*remount_fs) (struct super_block *, int *, char *);
diff --git a/block/bdev.c b/block/bdev.c
index 21c63bfef323..e8032c5beae0 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -248,9 +248,9 @@ int freeze_bdev(struct block_device *bdev)
if (!sb)
goto sync;
if (sb->s_op->freeze_super)
- error = sb->s_op->freeze_super(sb);
+ error = sb->s_op->freeze_super(sb, FREEZE_HOLDER_USERSPACE);
else
- error = freeze_super(sb);
+ error = freeze_super(sb, FREEZE_HOLDER_USERSPACE);
deactivate_super(sb);

if (error) {
@@ -291,9 +291,9 @@ int thaw_bdev(struct block_device *bdev)
goto out;

if (sb->s_op->thaw_super)
- error = sb->s_op->thaw_super(sb);
+ error = sb->s_op->thaw_super(sb, FREEZE_HOLDER_USERSPACE);
else
- error = thaw_super(sb);
+ error = thaw_super(sb, FREEZE_HOLDER_USERSPACE);
if (error)
bdev->bd_fsfreeze_count++;
else
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 61c5f9d26018..bca4e75c14e0 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -2166,7 +2166,7 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
if (err)
return err;

- freeze_super(sbi->sb);
+ freeze_super(sbi->sb, FREEZE_HOLDER_USERSPACE);
f2fs_down_write(&sbi->gc_lock);
f2fs_down_write(&sbi->cp_global_sem);

@@ -2217,6 +2217,6 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
out_err:
f2fs_up_write(&sbi->cp_global_sem);
f2fs_up_write(&sbi->gc_lock);
- thaw_super(sbi->sb);
+ thaw_super(sbi->sb, FREEZE_HOLDER_USERSPACE);
return err;
}
diff --git a/fs/gfs2/glops.c b/fs/gfs2/glops.c
index 01d433ed6ce7..6bffb7609d01 100644
--- a/fs/gfs2/glops.c
+++ b/fs/gfs2/glops.c
@@ -584,7 +584,7 @@ static int freeze_go_sync(struct gfs2_glock *gl)
if (gl->gl_state == LM_ST_SHARED && !gfs2_withdrawn(sdp) &&
!test_bit(SDF_NORECOVERY, &sdp->sd_flags)) {
atomic_set(&sdp->sd_freeze_state, SFS_STARTING_FREEZE);
- error = freeze_super(sdp->sd_vfs);
+ error = freeze_super(sdp->sd_vfs, FREEZE_HOLDER_USERSPACE);
if (error) {
fs_info(sdp, "GFS2: couldn't freeze filesystem: %d\n",
error);
diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index a84bf6444bba..3965b00a7503 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -682,7 +682,7 @@ void gfs2_freeze_func(struct work_struct *work)
gfs2_assert_withdraw(sdp, 0);
} else {
atomic_set(&sdp->sd_freeze_state, SFS_UNFROZEN);
- error = thaw_super(sb);
+ error = thaw_super(sb, FREEZE_HOLDER_USERSPACE);
if (error) {
fs_info(sdp, "GFS2: couldn't thaw filesystem: %d\n",
error);
@@ -702,7 +702,7 @@ void gfs2_freeze_func(struct work_struct *work)
*
*/

-static int gfs2_freeze(struct super_block *sb)
+static int gfs2_freeze(struct super_block *sb, enum freeze_holder who)
{
struct gfs2_sbd *sdp = sb->s_fs_info;
int error;
@@ -747,7 +747,7 @@ static int gfs2_freeze(struct super_block *sb)
*
*/

-static int gfs2_unfreeze(struct super_block *sb)
+static int gfs2_unfreeze(struct super_block *sb, enum freeze_holder who)
{
struct gfs2_sbd *sdp = sb->s_fs_info;

diff --git a/fs/gfs2/sys.c b/fs/gfs2/sys.c
index 454dc2ff8b5e..9d04a2907869 100644
--- a/fs/gfs2/sys.c
+++ b/fs/gfs2/sys.c
@@ -166,10 +166,10 @@ static ssize_t freeze_store(struct gfs2_sbd *sdp, const char *buf, size_t len)

switch (n) {
case 0:
- error = thaw_super(sdp->sd_vfs);
+ error = thaw_super(sdp->sd_vfs, FREEZE_HOLDER_USERSPACE);
break;
case 1:
- error = freeze_super(sdp->sd_vfs);
+ error = freeze_super(sdp->sd_vfs, FREEZE_HOLDER_USERSPACE);
break;
default:
return -EINVAL;
diff --git a/fs/gfs2/util.c b/fs/gfs2/util.c
index 7a6aeffcdf5c..357457b7c5b3 100644
--- a/fs/gfs2/util.c
+++ b/fs/gfs2/util.c
@@ -191,7 +191,7 @@ static void signal_our_withdraw(struct gfs2_sbd *sdp)
/* Make sure gfs2_unfreeze works if partially-frozen */
flush_work(&sdp->sd_freeze_work);
atomic_set(&sdp->sd_freeze_state, SFS_FROZEN);
- thaw_super(sdp->sd_vfs);
+ thaw_super(sdp->sd_vfs, FREEZE_HOLDER_USERSPACE);
} else {
wait_on_bit(&i_gl->gl_flags, GLF_DEMOTE,
TASK_UNINTERRUPTIBLE);
diff --git a/fs/ioctl.c b/fs/ioctl.c
index 5b2481cd4750..a56cbceedcd1 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -396,8 +396,8 @@ static int ioctl_fsfreeze(struct file *filp)

/* Freeze */
if (sb->s_op->freeze_super)
- return sb->s_op->freeze_super(sb);
- return freeze_super(sb);
+ return sb->s_op->freeze_super(sb, FREEZE_HOLDER_USERSPACE);
+ return freeze_super(sb, FREEZE_HOLDER_USERSPACE);
}

static int ioctl_fsthaw(struct file *filp)
@@ -409,8 +409,8 @@ static int ioctl_fsthaw(struct file *filp)

/* Thaw */
if (sb->s_op->thaw_super)
- return sb->s_op->thaw_super(sb);
- return thaw_super(sb);
+ return sb->s_op->thaw_super(sb, FREEZE_HOLDER_USERSPACE);
+ return thaw_super(sb, FREEZE_HOLDER_USERSPACE);
}

static int ioctl_file_dedupe_range(struct file *file,
diff --git a/fs/super.c b/fs/super.c
index 34afe411cf2b..81fb67157cba 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -39,7 +39,7 @@
#include <uapi/linux/mount.h>
#include "internal.h"

-static int thaw_super_locked(struct super_block *sb);
+static int thaw_super_locked(struct super_block *sb, enum freeze_holder who);

static LIST_HEAD(super_blocks);
static DEFINE_SPINLOCK(sb_lock);
@@ -1027,7 +1027,7 @@ static void do_thaw_all_callback(struct super_block *sb)
down_write(&sb->s_umount);
if (sb->s_root && sb->s_flags & SB_BORN) {
emergency_thaw_bdev(sb);
- thaw_super_locked(sb);
+ thaw_super_locked(sb, FREEZE_HOLDER_USERSPACE);
} else {
up_write(&sb->s_umount);
}
@@ -1638,11 +1638,22 @@ static void sb_freeze_unlock(struct super_block *sb, int level)
/**
* freeze_super - lock the filesystem and force it into a consistent state
* @sb: the super to lock
+ * @who: context that wants to freeze
*
* Syncs the super to make sure the filesystem is consistent and calls the fs's
- * freeze_fs. Subsequent calls to this without first thawing the fs will return
+ * freeze_fs. Subsequent calls to this without first thawing the fs may return
* -EBUSY.
*
+ * @who should be:
+ * * %FREEZE_HOLDER_USERSPACE if userspace wants to freeze the fs;
+ * * %FREEZE_HOLDER_KERNEL if the kernel wants to freeze the fs.
+ *
+ * The @who argument distinguishes between the kernel and userspace trying to
+ * freeze the filesystem. Although there cannot be multiple kernel freezes or
+ * multiple userspace freezes in effect at any given time, the kernel and
+ * userspace can both hold a filesystem frozen. The filesystem remains frozen
+ * until there are no kernel or userspace freezes in effect.
+ *
* During this function, sb->s_writers.frozen goes through these values:
*
* SB_UNFROZEN: File system is normal, all writes progress as usual.
@@ -1668,12 +1679,30 @@ static void sb_freeze_unlock(struct super_block *sb, int level)
*
* sb->s_writers.frozen is protected by sb->s_umount.
*/
-int freeze_super(struct super_block *sb)
+int freeze_super(struct super_block *sb, enum freeze_holder who)
{
int ret;

atomic_inc(&sb->s_active);
down_write(&sb->s_umount);
+
+ if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
+ if (sb->s_writers.freeze_holders & who) {
+ deactivate_locked_super(sb);
+ return -EBUSY;
+ }
+
+ WARN_ON(sb->s_writers.freeze_holders == 0);
+
+ /*
+ * Someone else already holds this type of freeze; share the
+ * freeze and assign the active ref to the freeze.
+ */
+ sb->s_writers.freeze_holders |= who;
+ up_write(&sb->s_umount);
+ return 0;
+ }
+
if (sb->s_writers.frozen != SB_UNFROZEN) {
deactivate_locked_super(sb);
return -EBUSY;
@@ -1686,6 +1715,7 @@ int freeze_super(struct super_block *sb)

if (sb_rdonly(sb)) {
/* Nothing to do really... */
+ sb->s_writers.freeze_holders |= who;
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
up_write(&sb->s_umount);
return 0;
@@ -1731,6 +1761,7 @@ int freeze_super(struct super_block *sb)
* For debugging purposes so that fs can warn if it sees write activity
* when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
*/
+ sb->s_writers.freeze_holders |= who;
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
lockdep_sb_freeze_release(sb);
up_write(&sb->s_umount);
@@ -1738,16 +1769,39 @@ int freeze_super(struct super_block *sb)
}
EXPORT_SYMBOL(freeze_super);

-static int thaw_super_locked(struct super_block *sb)
+/*
+ * Undoes the effect of a freeze_super_locked call. If the filesystem is
+ * frozen both by userspace and the kernel, a thaw call from either source
+ * removes that state without releasing the other state or unlocking the
+ * filesystem.
+ */
+static int thaw_super_locked(struct super_block *sb, enum freeze_holder who)
{
int error;

- if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
+ if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
+ if (!(sb->s_writers.freeze_holders & who)) {
+ up_write(&sb->s_umount);
+ return -EINVAL;
+ }
+
+ /*
+ * Freeze is shared with someone else. Release our hold and
+ * drop the active ref that freeze_super assigned to the
+ * freezer.
+ */
+ if (sb->s_writers.freeze_holders & ~who) {
+ sb->s_writers.freeze_holders &= ~who;
+ deactivate_locked_super(sb);
+ return 0;
+ }
+ } else {
up_write(&sb->s_umount);
return -EINVAL;
}

if (sb_rdonly(sb)) {
+ sb->s_writers.freeze_holders &= ~who;
sb->s_writers.frozen = SB_UNFROZEN;
goto out;
}
@@ -1765,6 +1819,7 @@ static int thaw_super_locked(struct super_block *sb)
}
}

+ sb->s_writers.freeze_holders &= ~who;
sb->s_writers.frozen = SB_UNFROZEN;
sb_freeze_unlock(sb, SB_FREEZE_FS);
out:
@@ -1776,13 +1831,19 @@ static int thaw_super_locked(struct super_block *sb)
/**
* thaw_super -- unlock filesystem
* @sb: the super to thaw
+ * @who: context that wants to freeze
*
- * Unlocks the filesystem and marks it writeable again after freeze_super().
+ * Unlocks the filesystem and marks it writeable again after freeze_super()
+ * if there are no remaining freezes on the filesystem.
+ *
+ * @who should be:
+ * * %FREEZE_HOLDER_USERSPACE if userspace wants to thaw the fs;
+ * * %FREEZE_HOLDER_KERNEL if the kernel wants to thaw the fs.
*/
-int thaw_super(struct super_block *sb)
+int thaw_super(struct super_block *sb, enum freeze_holder who)
{
down_write(&sb->s_umount);
- return thaw_super_locked(sb);
+ return thaw_super_locked(sb, who);
}
EXPORT_SYMBOL(thaw_super);

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 133f0640fb24..88bd81a1b980 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1145,7 +1145,8 @@ enum {
#define SB_FREEZE_LEVELS (SB_FREEZE_COMPLETE - 1)

struct sb_writers {
- int frozen; /* Is sb frozen? */
+ unsigned short frozen; /* Is sb frozen? */
+ unsigned short freeze_holders; /* Who froze fs? */
wait_queue_head_t wait_unfrozen; /* wait for thaw */
struct percpu_rw_semaphore rw_sem[SB_FREEZE_LEVELS];
};
@@ -1899,6 +1900,10 @@ extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
struct file *dst_file, loff_t dst_pos,
loff_t len, unsigned int remap_flags);

+enum freeze_holder {
+ FREEZE_HOLDER_KERNEL = (1U << 0),
+ FREEZE_HOLDER_USERSPACE = (1U << 1),
+};

struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
@@ -1911,9 +1916,9 @@ struct super_operations {
void (*evict_inode) (struct inode *);
void (*put_super) (struct super_block *);
int (*sync_fs)(struct super_block *sb, int wait);
- int (*freeze_super) (struct super_block *);
+ int (*freeze_super) (struct super_block *, enum freeze_holder who);
int (*freeze_fs) (struct super_block *);
- int (*thaw_super) (struct super_block *);
+ int (*thaw_super) (struct super_block *, enum freeze_holder who);
int (*unfreeze_fs) (struct super_block *);
int (*statfs) (struct dentry *, struct kstatfs *);
int (*remount_fs) (struct super_block *, int *, char *);
@@ -2286,8 +2291,8 @@ extern int unregister_filesystem(struct file_system_type *);
extern int vfs_statfs(const struct path *, struct kstatfs *);
extern int user_statfs(const char __user *, struct kstatfs *);
extern int fd_statfs(int, struct kstatfs *);
-extern int freeze_super(struct super_block *super);
-extern int thaw_super(struct super_block *super);
+int freeze_super(struct super_block *super, enum freeze_holder who);
+int thaw_super(struct super_block *super, enum freeze_holder who);
extern __printf(2, 3)
int super_setup_bdi_name(struct super_block *sb, char *fmt, ...);
extern int super_setup_bdi(struct super_block *sb);