Re: [PATCH 2/4] fs: avoid double-writing the inode on a lazytime expiration

[Dave Chinner <david@xxxxxxxxxxxxx>]

On Wed, Mar 25, 2020 at 01:28:23PM +0100, Christoph Hellwig 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.  Replace the call to
> mark_inode_dirty_sync() with a new lazytime_expired method to clearly
> separate out this case.


hmmm. Doesn't this cause issues with both iput() and
vfs_fsync_range() because they call mark_inode_dirty_sync() on
I_DIRTY_TIME inodes to move them onto the writeback list so they are
appropriately expired when the inode is written back.

i.e.:


> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 2094386af8ac..e5aafd40dd0f 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -612,19 +612,13 @@ xfs_fs_destroy_inode(
>  }
>  
>  static void
> -xfs_fs_dirty_inode(
> -	struct inode			*inode,
> -	int				flag)
> +xfs_fs_lazytime_expired(
> +	struct inode			*inode)
>  {
>  	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);
> @@ -1053,7 +1047,7 @@ xfs_fs_free_cached_objects(
>  static const struct super_operations xfs_super_operations = {
>  	.alloc_inode		= xfs_fs_alloc_inode,
>  	.destroy_inode		= xfs_fs_destroy_inode,
> -	.dirty_inode		= xfs_fs_dirty_inode,
> +	.lazytime_expired	= xfs_fs_lazytime_expired,
>  	.drop_inode		= xfs_fs_drop_inode,
>  	.put_super		= xfs_fs_put_super,
>  	.sync_fs		= xfs_fs_sync_fs,

This means XFS no longer updates/logs the current timestamp because
->dirty_inode(I_DIRTY_SYNC) is no longer called for XFS) before
->fsync flushes the inode data and metadata changes to the journal.
Hence the current in-memory timestamps are not present in the log
before the fsync is run as so we violate the fsync guarantees
lazytime gives for timestamp updates....

I haven't quite got it straight in my head if the iput() case has
similar problems, but the fsync case definitely looks broken.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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