Re: [PATCH] xfs: short circuit xfs_get_acl() if no acl is possible

[Brian Foster <bfoster@redhat.com>]

On Wed, Jun 26, 2019 at 01:16:27PM -0500, Eric Sandeen wrote:
> 
> 
> On 6/26/19 1:12 PM, Darrick J. Wong wrote:
> > On Thu, May 09, 2019 at 09:05:39AM -0400, Brian Foster wrote:
> >> On Wed, May 08, 2019 at 01:10:33PM -0700, Darrick J. Wong wrote:
> >>> On Wed, May 08, 2019 at 02:28:09PM -0500, Eric Sandeen wrote:
> >>>> If there are no attributes on the inode, don't go through the
> >>>> cost of memory allocation and callling xfs_attr_get when we
> >>>> already know we'll just get -ENOATTR.
> >>>>
> >>>> Reported-by: David Valin <dvalin@redhat.com>
> >>>> Suggested-by: Dave Chinner <david@fromorbit.com>
> >>>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> >>>> ---
> >>>>
> >>>> diff --git a/fs/xfs/xfs_acl.c b/fs/xfs/xfs_acl.c
> >>>> index 8039e35147dd..b469b44e9e71 100644
> >>>> --- a/fs/xfs/xfs_acl.c
> >>>> +++ b/fs/xfs/xfs_acl.c
> >>>> @@ -132,6 +132,9 @@ xfs_get_acl(struct inode *inode, int type)
> >>>>  		BUG();
> >>>>  	}
> >>>>  
> >>>> +	if (!xfs_inode_hasattr(ip))
> >>>> +		return NULL;
> >>>
> >>> This isn't going to cause problems if someone's adding an ACL to the
> >>> inode at the same time, right?
> >>>
> >>> I'm assuming that's the case since we only would load inodes when
> >>> setting up a vfs inode but before any userspace can get its sticky
> >>> fingers all over the inode, but it sure would be nice to know that
> >>> for sure. :)
> >>>
> >>
> >> Hmm, that's a good question. At first I was thinking it wouldn't matter,
> >> but then I remembered the fairly recent issue around writing back an
> >> empty leaf buffer on format conversion a bit too early. That has me
> >> wondering if that would be an issue here as well. For example, suppose a
> >> non-empty local format attr fork is being converted to extent format due
> >> to a concurrent (and unrelated) xattr set. That involves
> >> xfs_attr_shortform_to_leaf() -> xfs_bmap_local_to_extents_empty(), which
> >> looks like it creates a transient empty fork state. Might
> >> xfs_inode_hasattr() catch that as a false negative here? If so, that
> >> would certainly be a problem if the existing xattr was the ACL the
> >> caller happens to be interested in. It might be prudent to surround this
> >> check with ILOCK_SHARED...
> > 
> > <shrug> But xfs_inode_hasattr checks forkoff > 0, so as long as the
> 
> It does do that ...
> 
> int
> xfs_inode_hasattr(
>         struct xfs_inode        *ip)
> {
>         if (!XFS_IFORK_Q(ip) ||
> 
> 
> > shortform to leaf conversion doesn't zero forkoff we'd be fine, I think.
> > AFAICT it doesn't...?
> 
> but there's that pesky || part :
> 
>             (ip->i_d.di_aformat == XFS_DINODE_FMT_EXTENTS &&
>              ip->i_d.di_anextents == 0))
>                 return 0;
>         return 1;
> }
> 
> and I think it's the latter state Brian was concerned about?
> 

Yep, pretty much.

Brian

> I can play with sandwiching it in a shared lock...
> 
> -Eric
>