Re: [PATCH RFC 1/4] xfs: clean up cow fork reservation and tag inodes correctly

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 15, 2016 at 01:11:01PM -0500, Brian Foster wrote:
> On Tue, Nov 15, 2016 at 06:16:21AM -0800, Christoph Hellwig wrote:
> > > +	if (imap->br_startoff != got.br_startoff ||
> > > +	    imap->br_blockcount != got.br_blockcount)
> > >  		xfs_inode_set_cowblocks_tag(ip);
> > 
> > Can't got.br_blockcount be smaller than imap->br_blockcount if we have
> > an existing COW fork reservation lying around behind the whole we're
> > filling?  Also they way xfs_bmapi_reserve_delalloc works the startoff
> > will be the same.  E.g. this check should probably be:
> > 
> 
> Good point, though I think it can be smaller or larger without
> necessarily having preallocation due to being merged with surrounding
> extents. I'm not quite sure what the right answer for that is with
> regard to tagging, but I do think it's better to have false positive
> tagging than false negatives.

Good point, merging can change both the start and length.  Based on
that I think tagging in the caller of xfs_bmapi_reserve_delalloc is
wrong, and we need to do it inside xfs_bmapi_reserve_delalloc where
we know if we did speculative preallocation.