Re: [RFC v3 01/24] fs: unify locking semantics for fs freeze / thaw

From: Jan Kara
Date: Mon Jan 16 2023 - 10:20:59 EST

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On Fri 13-01-23 16:33:46, Luis Chamberlain wrote:
> Right now freeze_super() and thaw_super() are called with
> different locking contexts. To expand on this is messy, so
> just unify the requirement to require grabbing an active
> reference and keep the superblock locked.
>
> Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>

The cleanup is nice but now freeze_super() does not increment s_active
anymore so nothing prevents the superblock from being torn down while it is
frozen. This is a behavioral change that needs documenting in the changelog
if nothing else but I think it may be actually problematic if the
filesystem's ->put_super method gets called on frozen filesystem. So I
think we may need to also block attempts to unmount frozen filesystem -
actually GFS2 needs this as well [1].

Honza

[1] lore.kernel.org/r/20221129230736.3462830-1-agruenba@xxxxxxxxxx

> ---
> block/bdev.c | 5 ++++-
> fs/f2fs/gc.c | 5 +++++
> fs/gfs2/super.c | 9 +++++++--
> fs/gfs2/sys.c | 6 ++++++
> fs/gfs2/util.c | 5 +++++
> fs/ioctl.c | 12 ++++++++++--
> fs/super.c | 51 ++++++++++++++-----------------------------------
> 7 files changed, 51 insertions(+), 42 deletions(-)
>
> diff --git a/block/bdev.c b/block/bdev.c
> index edc110d90df4..8fd3a7991c02 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -251,7 +251,7 @@ int freeze_bdev(struct block_device *bdev)
> error = sb->s_op->freeze_super(sb);
> else
> error = freeze_super(sb);
> - deactivate_super(sb);
> + deactivate_locked_super(sb);
>
> if (error) {
> bdev->bd_fsfreeze_count--;
> @@ -289,6 +289,8 @@ int thaw_bdev(struct block_device *bdev)
> sb = bdev->bd_fsfreeze_sb;
> if (!sb)
> goto out;
> + if (!get_active_super(bdev))
> + goto out;
>
> if (sb->s_op->thaw_super)
> error = sb->s_op->thaw_super(sb);
> @@ -298,6 +300,7 @@ int thaw_bdev(struct block_device *bdev)
> bdev->bd_fsfreeze_count++;
> else
> bdev->bd_fsfreeze_sb = NULL;
> + deactivate_locked_super(sb);
> out:
> mutex_unlock(&bdev->bd_fsfreeze_mutex);
> return error;
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index 7444c392eab1..4c681fe487ee 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -2139,7 +2139,10 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
> if (err)
> return err;
>
> + if (!get_active_super(sbi->sb->s_bdev))
> + return -ENOTTY;
> freeze_super(sbi->sb);
> +
> f2fs_down_write(&sbi->gc_lock);
> f2fs_down_write(&sbi->cp_global_sem);
>
> @@ -2190,6 +2193,8 @@ 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);
> + /* We use the same active reference from freeze */
> thaw_super(sbi->sb);
> + deactivate_locked_super(sbi->sb);
> return err;
> }
> diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
> index 999cc146d708..48df7b276b64 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -661,7 +661,12 @@ void gfs2_freeze_func(struct work_struct *work)
> struct gfs2_sbd *sdp = container_of(work, struct gfs2_sbd, sd_freeze_work);
> struct super_block *sb = sdp->sd_vfs;
>
> - atomic_inc(&sb->s_active);
> + if (!get_active_super(sb->s_bdev)) {
> + fs_info(sdp, "GFS2: couldn't grap super for thaw for filesystem\n");
> + gfs2_assert_withdraw(sdp, 0);
> + return;
> + }
> +
> error = gfs2_freeze_lock(sdp, &freeze_gh, 0);
> if (error) {
> gfs2_assert_withdraw(sdp, 0);
> @@ -675,7 +680,7 @@ void gfs2_freeze_func(struct work_struct *work)
> }
> gfs2_freeze_unlock(&freeze_gh);
> }
> - deactivate_super(sb);
> + deactivate_locked_super(sb);
> clear_bit_unlock(SDF_FS_FROZEN, &sdp->sd_flags);
> wake_up_bit(&sdp->sd_flags, SDF_FS_FROZEN);
> return;
> diff --git a/fs/gfs2/sys.c b/fs/gfs2/sys.c
> index d87ea98cf535..d0b80552a678 100644
> --- a/fs/gfs2/sys.c
> +++ b/fs/gfs2/sys.c
> @@ -162,6 +162,9 @@ static ssize_t freeze_store(struct gfs2_sbd *sdp, const char *buf, size_t len)
> if (!capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> + if (!get_active_super(sb->s_bdev))
> + return -ENOTTY;
> +
> switch (n) {
> case 0:
> error = thaw_super(sdp->sd_vfs);
> @@ -170,9 +173,12 @@ static ssize_t freeze_store(struct gfs2_sbd *sdp, const char *buf, size_t len)
> error = freeze_super(sdp->sd_vfs);
> break;
> default:
> + deactivate_locked_super(sb);
> return -EINVAL;
> }
>
> + deactivate_locked_super(sb);
> +
> if (error) {
> fs_warn(sdp, "freeze %d error %d\n", n, error);
> return error;
> diff --git a/fs/gfs2/util.c b/fs/gfs2/util.c
> index 7a6aeffcdf5c..3a0cd5e9ad84 100644
> --- a/fs/gfs2/util.c
> +++ b/fs/gfs2/util.c
> @@ -345,10 +345,15 @@ int gfs2_withdraw(struct gfs2_sbd *sdp)
> set_bit(SDF_WITHDRAW_IN_PROG, &sdp->sd_flags);
>
> if (sdp->sd_args.ar_errors == GFS2_ERRORS_WITHDRAW) {
> + if (!get_active_super(sb->s_bdev)) {
> + fs_err(sdp, "could not grab super on withdraw for file system\n");
> + return -1;
> + }
> fs_err(sdp, "about to withdraw this file system\n");
> BUG_ON(sdp->sd_args.ar_debug);
>
> signal_our_withdraw(sdp);
> + deactivate_locked_super(sb);
>
> kobject_uevent(&sdp->sd_kobj, KOBJ_OFFLINE);
>
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 80ac36aea913..3d2536e1ea58 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -386,6 +386,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
> static int ioctl_fsfreeze(struct file *filp)
> {
> struct super_block *sb = file_inode(filp)->i_sb;
> + int ret;
>
> if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
> return -EPERM;
> @@ -394,10 +395,17 @@ static int ioctl_fsfreeze(struct file *filp)
> if (sb->s_op->freeze_fs == NULL && sb->s_op->freeze_super == NULL)
> return -EOPNOTSUPP;
>
> + if (!get_active_super(sb->s_bdev))
> + return -ENOTTY;
> +
> /* Freeze */
> if (sb->s_op->freeze_super)
> - return sb->s_op->freeze_super(sb);
> - return freeze_super(sb);
> + ret = sb->s_op->freeze_super(sb);
> + ret = freeze_super(sb);
> +
> + deactivate_locked_super(sb);
> +
> + return ret;
> }
>
> static int ioctl_fsthaw(struct file *filp)
> diff --git a/fs/super.c b/fs/super.c
> index 12c08cb20405..a31a41b313f3 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -39,8 +39,6 @@
> #include <uapi/linux/mount.h>
> #include "internal.h"
>
> -static int thaw_super_locked(struct super_block *sb);
> -
> static LIST_HEAD(super_blocks);
> static DEFINE_SPINLOCK(sb_lock);
>
> @@ -830,7 +828,6 @@ struct super_block *get_active_super(struct block_device *bdev)
> if (sb->s_bdev == bdev) {
> if (!grab_super(sb))
> goto restart;
> - up_write(&sb->s_umount);
> return sb;
> }
> }
> @@ -1003,13 +1000,13 @@ void emergency_remount(void)
>
> static void do_thaw_all_callback(struct super_block *sb)
> {
> - down_write(&sb->s_umount);
> + if (!get_active_super(sb->s_bdev))
> + return;
> if (sb->s_root && sb->s_flags & SB_BORN) {
> emergency_thaw_bdev(sb);
> - thaw_super_locked(sb);
> - } else {
> - up_write(&sb->s_umount);
> + thaw_super(sb);
> }
> + deactivate_locked_super(sb);
> }
>
> static void do_thaw_all(struct work_struct *work)
> @@ -1651,22 +1648,15 @@ int freeze_super(struct super_block *sb)
> {
> int ret;
>
> - atomic_inc(&sb->s_active);
> - down_write(&sb->s_umount);
> - if (sb->s_writers.frozen != SB_UNFROZEN) {
> - deactivate_locked_super(sb);
> + if (sb->s_writers.frozen != SB_UNFROZEN)
> return -EBUSY;
> - }
>
> - if (!(sb->s_flags & SB_BORN)) {
> - up_write(&sb->s_umount);
> + if (!(sb->s_flags & SB_BORN))
> return 0; /* sic - it's "nothing to do" */
> - }
>
> if (sb_rdonly(sb)) {
> /* Nothing to do really... */
> sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> - up_write(&sb->s_umount);
> return 0;
> }
>
> @@ -1686,7 +1676,6 @@ int freeze_super(struct super_block *sb)
> sb->s_writers.frozen = SB_UNFROZEN;
> sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
> wake_up(&sb->s_writers.wait_unfrozen);
> - deactivate_locked_super(sb);
> return ret;
> }
>
> @@ -1702,7 +1691,6 @@ int freeze_super(struct super_block *sb)
> sb->s_writers.frozen = SB_UNFROZEN;
> sb_freeze_unlock(sb, SB_FREEZE_FS);
> wake_up(&sb->s_writers.wait_unfrozen);
> - deactivate_locked_super(sb);
> return ret;
> }
> }
> @@ -1712,19 +1700,22 @@ int freeze_super(struct super_block *sb)
> */
> sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> lockdep_sb_freeze_release(sb);
> - up_write(&sb->s_umount);
> return 0;
> }
> EXPORT_SYMBOL(freeze_super);
>
> -static int thaw_super_locked(struct super_block *sb)
> +/**
> + * thaw_super -- unlock filesystem
> + * @sb: the super to thaw
> + *
> + * Unlocks the filesystem and marks it writeable again after freeze_super().
> + */
> +int thaw_super(struct super_block *sb)
> {
> int error;
>
> - if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
> - up_write(&sb->s_umount);
> + if (sb->s_writers.frozen != SB_FREEZE_COMPLETE)
> return -EINVAL;
> - }
>
> if (sb_rdonly(sb)) {
> sb->s_writers.frozen = SB_UNFROZEN;
> @@ -1739,7 +1730,6 @@ static int thaw_super_locked(struct super_block *sb)
> printk(KERN_ERR
> "VFS:Filesystem thaw failed\n");
> lockdep_sb_freeze_release(sb);
> - up_write(&sb->s_umount);
> return error;
> }
> }
> @@ -1748,19 +1738,6 @@ static int thaw_super_locked(struct super_block *sb)
> sb_freeze_unlock(sb, SB_FREEZE_FS);
> out:
> wake_up(&sb->s_writers.wait_unfrozen);
> - deactivate_locked_super(sb);
> return 0;
> }
> -
> -/**
> - * thaw_super -- unlock filesystem
> - * @sb: the super to thaw
> - *
> - * Unlocks the filesystem and marks it writeable again after freeze_super().
> - */
> -int thaw_super(struct super_block *sb)
> -{
> - down_write(&sb->s_umount);
> - return thaw_super_locked(sb);
> -}
> EXPORT_SYMBOL(thaw_super);
> --
> 2.35.1
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR

Next message: David Hildenbrand: "Re: [PATCH mm] mm: hugetlb: Add checks for NULL for vma returned from find_vma. find_vma may return NULL, that's why its return value is usually checked for NULL."
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