Re: [PATCH] writeback: avoid double-writing the inode on a lazytime expiration

Eric Biggers
2020-03-11T03:20:12Z

On Fri, Mar 06, 2020 at 09:00:43PM -0500, Theodore Ts'o wrote:
> In the case that an inode has dirty timestamp for longer than the
> lazytime expiration timeout (or if all such inodes are being flushed
> out due to a sync or syncfs system call), we need to inform the file
> system that the inode is dirty so that the inode's timestamps can be
> copied out to the on-disk data structures. That's because if the file
> system supports lazytime, it will have ignored the dirty_inode(inode,
> I_DIRTY_TIME) notification when the timestamp was modified in memory.q
>
> Previously, this was accomplished by calling mark_inode_dirty_sync(),
> but that has the unfortunate side effect of also putting the inode the
> writeback list, and that's not necessary in this case, since we will
> immediately call write_inode() afterwards.
>
> Eric Biggers noticed that this was causing problems for fscrypt after
> the key was removed[1].
>
> [1] https://lore.kernel.org/r/20200306004555.GB225345@gmail.com
>
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> ---
> fs/fs-writeback.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 76ac9c7d32ec..32101349ba97 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1504,8 +1504,9 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
>
> spin_unlock(&inode->i_lock);
>
> - if (dirty & I_DIRTY_TIME)
> - mark_inode_dirty_sync(inode);
> + /* This was a lazytime expiration; we need to tell the file system */
> + if (dirty & I_DIRTY_TIME_EXPIRED && inode->i_sb->s_op->dirty_inode)
> + inode->i_sb->s_op->dirty_inode(inode, I_DIRTY_TIME_EXPIRED);
> /* Don't write the inode if only I_DIRTY_PAGES was set */
> if (dirty & ~I_DIRTY_PAGES) {
> int err = write_inode(inode, wbc);
> --

Thanks Ted! This fixes the fscrypt test failure.

However, are you sure this works correctly on all filesystems? I'm not sure
about XFS. XFS only implements ->dirty_inode(), not ->write_inode(), and in its
->dirty_inode() it does:

static void
xfs_fs_dirty_inode(
struct inode *inode,
int flag)
{
struct xfs_inode *ip = XFS_I(inode);
struct xfs_mount *mp = ip->i_mount;
struct xfs_trans *tp;

if (!(inode->i_sb->s_flags & SB_LAZYTIME))
return;
if (flag != I_DIRTY_SYNC || !(inode->i_state & I_DIRTY_TIME))
return;

if (xfs_trans_alloc(mp, &M_RES(mp)->tr_fsyncts, 0, 0, 0, &tp))
return;
xfs_ilock(ip, XFS_ILOCK_EXCL);
xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
xfs_trans_log_inode(tp, ip, XFS_ILOG_TIMESTAMP);
xfs_trans_commit(tp);
}

So flag=I_DIRTY_TIME_EXPIRED will be a no-op.

Maybe you should be using I_DIRTY_SYNC instead? Or perhaps XFS should be
checking for either I_DIRTY_TIME_EXPIRED or I_DIRTY_SYNC?

- Eric