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

[Dave Chinner <david@fromorbit.com>]

On Tue, Mar 10, 2020 at 08:20:09PM -0700, Eric Biggers wrote:
> 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?

Right, XFS does not use the VFS inode writeback code at all - we
track all metadata changes internally via journalling. The VFS uses
I_DIRTY_SYNC to indicate and inode is metadata dirty and a writeback
candidate. Hence if we need to mark an inode dirty for integrity
purposes for _any reason_, then I_DIRTY_SYNC is the correct flag to
be passing to ->dirty_inode.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com