[RFC v3 03/24] fs: distinguish between user initiated freeze and kernel initiated freeze

[Luis Chamberlain <mcgrof@kernel.org>]

Userspace can initiate a freeze call using ioctls. If the kernel decides
to freeze a filesystem later it must be able to distinguish if userspace
had initiated the freeze, so that it does not unfreeze it later
automatically on resume.

Likewise if the kernel is initiating a freeze on its own it should *not*
fail to freeze a filesystem if a user had already frozen it on our behalf.
This same concept applies to thawing, even if its not possible for
userspace to beat the kernel in thawing a filesystem. This logic however
has never applied to userspace freezing and thawing, two consecutive
userspace freeze calls will results in only the first one succeeding, so
we must retain the same behaviour in userspace.

This doesn't implement yet kernel initiated filesystem freeze calls,
this will be done in subsequent calls. This change should introduce
no functional changes, it just extends the definitions of a frozen
filesystem to account for future kernel initiated filesystem freeze
and let's us keep record of when userpace initiated it so the kernel
can respect a userspace initiated freeze upon kernel initiated freeze
and its respective thaw cycle.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 block/bdev.c       |  4 ++--
 fs/f2fs/gc.c       |  4 ++--
 fs/gfs2/glops.c    |  2 +-
 fs/gfs2/super.c    |  2 +-
 fs/gfs2/sys.c      |  4 ++--
 fs/gfs2/util.c     |  2 +-
 fs/ioctl.c         |  4 ++--
 fs/super.c         | 31 ++++++++++++++++++++++++++-----
 include/linux/fs.h | 16 ++++++++++++++--
 9 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/block/bdev.c b/block/bdev.c
index 8fd3a7991c02..668ebf2015bf 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -250,7 +250,7 @@ int freeze_bdev(struct block_device *bdev)
 	if (sb->s_op->freeze_super)
 		error = sb->s_op->freeze_super(sb);
 	else
-		error = freeze_super(sb);
+		error = freeze_super(sb, true);
 	deactivate_locked_super(sb);
 
 	if (error) {
@@ -295,7 +295,7 @@ int thaw_bdev(struct block_device *bdev)
 	if (sb->s_op->thaw_super)
 		error = sb->s_op->thaw_super(sb);
 	else
-		error = thaw_super(sb);
+		error = thaw_super(sb, true);
 	if (error)
 		bdev->bd_fsfreeze_count++;
 	else
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 4c681fe487ee..8eac3042786b 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -2141,7 +2141,7 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
 
 	if (!get_active_super(sbi->sb->s_bdev))
 		return -ENOTTY;
-	freeze_super(sbi->sb);
+	freeze_super(sbi->sb, true);
 
 	f2fs_down_write(&sbi->gc_lock);
 	f2fs_down_write(&sbi->cp_global_sem);
@@ -2194,7 +2194,7 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
 	f2fs_up_write(&sbi->cp_global_sem);
 	f2fs_up_write(&sbi->gc_lock);
 	/* We use the same active reference from freeze */
-	thaw_super(sbi->sb);
+	thaw_super(sbi->sb, true);
 	deactivate_locked_super(sbi->sb);
 	return err;
 }
diff --git a/fs/gfs2/glops.c b/fs/gfs2/glops.c
index 081422644ec5..62a7e0693efa 100644
--- a/fs/gfs2/glops.c
+++ b/fs/gfs2/glops.c
@@ -574,7 +574,7 @@ 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, true);
 		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 48df7b276b64..9c55b8042aa4 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -672,7 +672,7 @@ 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, true);
 		if (error) {
 			fs_info(sdp, "GFS2: couldn't thaw filesystem: %d\n",
 				error);
diff --git a/fs/gfs2/sys.c b/fs/gfs2/sys.c
index b98be03d0d1e..69514294215b 100644
--- a/fs/gfs2/sys.c
+++ b/fs/gfs2/sys.c
@@ -167,10 +167,10 @@ 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, true);
 		break;
 	case 1:
-		error = freeze_super(sdp->sd_vfs);
+		error = freeze_super(sdp->sd_vfs, true);
 		break;
 	default:
 		deactivate_locked_super(sb);
diff --git a/fs/gfs2/util.c b/fs/gfs2/util.c
index 3a0cd5e9ad84..be9705d618ec 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, true);
 	} else {
 		wait_on_bit(&i_gl->gl_flags, GLF_DEMOTE,
 			    TASK_UNINTERRUPTIBLE);
diff --git a/fs/ioctl.c b/fs/ioctl.c
index 3d2536e1ea58..0ac1622785ad 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -401,7 +401,7 @@ static int ioctl_fsfreeze(struct file *filp)
 	/* Freeze */
 	if (sb->s_op->freeze_super)
 		ret = sb->s_op->freeze_super(sb);
-	ret = freeze_super(sb);
+	ret = freeze_super(sb, true);
 
 	deactivate_locked_super(sb);
 
@@ -418,7 +418,7 @@ 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 thaw_super(sb, true);
 }
 
 static int ioctl_file_dedupe_range(struct file *file,
diff --git a/fs/super.c b/fs/super.c
index fdcf5a87af0a..0d6b4de8da88 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1004,7 +1004,7 @@ static void do_thaw_all_callback(struct super_block *sb)
 		return;
 	if (sb->s_root && sb->s_flags & SB_BORN) {
 		emergency_thaw_bdev(sb);
-		thaw_super(sb);
+		thaw_super(sb, true);
 	}
 	deactivate_locked_super(sb);
 }
@@ -1614,6 +1614,8 @@ 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
+ * @usercall: whether or not userspace initiated this via an ioctl or if it
+ * 	was a kernel 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
@@ -1644,11 +1646,14 @@ 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, bool usercall)
 {
 	int ret;
 
-	if (sb->s_writers.frozen != SB_UNFROZEN)
+	if (!usercall && sb_is_frozen(sb))
+		return 0;
+
+	if (!sb_is_unfrozen(sb))
 		return -EBUSY;
 
 	if (!(sb->s_flags & SB_BORN))
@@ -1657,6 +1662,7 @@ int freeze_super(struct super_block *sb)
 	if (sb_rdonly(sb)) {
 		/* Nothing to do really... */
 		sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+		sb->s_writers.frozen_by_user = usercall;
 		return 0;
 	}
 
@@ -1674,6 +1680,7 @@ int freeze_super(struct super_block *sb)
 	ret = sync_filesystem(sb);
 	if (ret) {
 		sb->s_writers.frozen = SB_UNFROZEN;
+		sb->s_writers.frozen_by_user = false;
 		sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
 		wake_up(&sb->s_writers.wait_unfrozen);
 		return ret;
@@ -1699,6 +1706,7 @@ int freeze_super(struct super_block *sb)
 	 * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
 	 */
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+	sb->s_writers.frozen_by_user = usercall;
 	lockdep_sb_freeze_release(sb);
 	return 0;
 }
@@ -1707,18 +1715,30 @@ EXPORT_SYMBOL(freeze_super);
 /**
  * thaw_super -- unlock filesystem
  * @sb: the super to thaw
+ * @usercall: whether or not userspace initiated this thaw or if it was the
+ * 	kernel which initiated it
  *
  * Unlocks the filesystem and marks it writeable again after freeze_super().
  */
-int thaw_super(struct super_block *sb)
+int thaw_super(struct super_block *sb, bool usercall)
 {
 	int error;
 
-	if (sb->s_writers.frozen != SB_FREEZE_COMPLETE)
+	if (!usercall) {
+		/*
+		 * If userspace initiated the freeze don't let the kernel
+		 * thaw it on return from a kernel initiated freeze.
+		 */
+		if (sb_is_unfrozen(sb) || sb_is_frozen_by_user(sb))
+			return 0;
+	}
+
+	if (!sb_is_frozen(sb))
 		return -EINVAL;
 
 	if (sb_rdonly(sb)) {
 		sb->s_writers.frozen = SB_UNFROZEN;
+		sb->s_writers.frozen_by_user = false;
 		goto out;
 	}
 
@@ -1735,6 +1755,7 @@ int thaw_super(struct super_block *sb)
 	}
 
 	sb->s_writers.frozen = SB_UNFROZEN;
+	sb->s_writers.frozen_by_user = false;
 	sb_freeze_unlock(sb, SB_FREEZE_FS);
 out:
 	wake_up(&sb->s_writers.wait_unfrozen);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c0cab61f9f9a..3b2586de4364 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1129,6 +1129,7 @@ enum {
 
 struct sb_writers {
 	int				frozen;		/* Is sb frozen? */
+	bool				frozen_by_user;	/* User freeze? */
 	wait_queue_head_t		wait_unfrozen;	/* wait for thaw */
 	struct percpu_rw_semaphore	rw_sem[SB_FREEZE_LEVELS];
 };
@@ -1615,6 +1616,17 @@ static inline bool sb_is_frozen(struct super_block *sb)
 	return sb->s_writers.frozen == SB_FREEZE_COMPLETE;
 }
 
+/**
+ * sb_is_frozen_by_user - was the superblock frozen by userspace?
+ * @sb: the super to check
+ *
+ * Returns true if the super is frozen by userspace, such as an ioctl.
+ */
+static inline bool sb_is_frozen_by_user(struct super_block *sb)
+{
+	return sb_is_frozen(sb) && sb->s_writers.frozen_by_user;
+}
+
 /**
  * sb_is_unfrozen - is superblock unfrozen
  * @sb: the super to check
@@ -2292,8 +2304,8 @@ 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);
+extern int freeze_super(struct super_block *super, bool usercall);
+extern int thaw_super(struct super_block *super, bool usercall);
 extern __printf(2, 3)
 int super_setup_bdi_name(struct super_block *sb, char *fmt, ...);
 extern int super_setup_bdi(struct super_block *sb);
-- 
2.35.1