Re: [PATCH 1/6] xfs: use XFS_IFORK_BOFF xchk_bmap_check_rmaps

[Brian Foster <bfoster@redhat.com>]

On Tue, May 12, 2020 at 08:38:54AM -0700, Darrick J. Wong wrote:
> On Tue, May 12, 2020 at 11:31:32AM -0400, Brian Foster wrote:
> > On Mon, May 11, 2020 at 05:10:04PM +0530, Chandan Babu R wrote:
> > > On Sunday 10 May 2020 12:53:59 PM IST Christoph Hellwig wrote:
> > > > XFS_IFORK_Q is to be used in boolean context, not for a size.  This
> > > > doesn't make a difference in practice as size is only checked for
> > > > 0, but this keeps the logic sane.
> > > >
> > > 
> > > Wouldn't XFS_IFORK_ASIZE() be a better fit since it gives the space used by the
> > > attr fork inside an inode's literal area?
> > > 
> > 
> > I had the same thought. It's not clear to me what size is really
> > supposed to be between the file size for a data fork and fork offset for
> > the attr fork. I was also wondering if this should use
> > XFS_IFORK_DSIZE(), but that won't be conditional based on population of
> > the fork. At the same time, I don't think i_size != 0 necessarily
> > correlates with the existence of blocks. The file could be completely
> > sparse or could have any number of post-eof preallocated extents.
> 
> TBH I should have made that variable "bool empty" or something.
> 
> case XFS_DATA_FORK:
> 	empty = i_size_read() == 0;
> 

Even that is somewhat unclear because it's tied to i_size. What about
size == 0 && <post-eof extents>?

Brian

> case XFS_ATTR_FORK:
> 	empty = !XFS_IFORK_Q();
> 
> default:
> 	empty = true;
> 
> if ((is not btree) && (empty || nextents > 0))
> 	return 0;
> 
> --D
> 
> > Brian
> > 
> > > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > > > ---
> > > >  fs/xfs/scrub/bmap.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/fs/xfs/scrub/bmap.c b/fs/xfs/scrub/bmap.c
> > > > index add8598eacd5d..283424d6d2bb6 100644
> > > > --- a/fs/xfs/scrub/bmap.c
> > > > +++ b/fs/xfs/scrub/bmap.c
> > > > @@ -591,7 +591,7 @@ xchk_bmap_check_rmaps(
> > > >  		size = i_size_read(VFS_I(sc->ip));
> > > >  		break;
> > > >  	case XFS_ATTR_FORK:
> > > > -		size = XFS_IFORK_Q(sc->ip);
> > > > +		size = XFS_IFORK_BOFF(sc->ip);
> > > >  		break;
> > > >  	default:
> > > >  		size = 0;
> > > > 
> > > 
> > > 
> > > -- 
> > > chandan
> > > 
> > > 
> > > 
> > 
>