Re: [PATCH] xfs: flush the range before zero partial block range on truncate down

[Eryu Guan <eguan@redhat.com>]

On Fri, Oct 27, 2017 at 11:05:29PM -0700, Christoph Hellwig wrote:
> On Fri, Oct 27, 2017 at 08:53:28PM +0800, Eryu Guan wrote:
> > But it's possible that a buffer write overwrites the unwritten
> > extent, which won't be converted to a normal extent until I/O
> > completion, and iomap_truncate_page() skips zeroing wrongly because
> > of the not-converted unwritten extent. This would cause a subsequent
> > mmap read sees non-zeros beyond EOF.
> 
> I suspect the right fix is to look at the in-core state im the iomap
> truncate helpers instead of doing a duplicate flush.

I may (and very likely) miss something, but my understanding is that
iomap_truncate_page() already looks at the in-core extent state provided
by xfs_file_iomap_begin()

xfs_setattr_size()
 iomap_truncate_page()
  iomap_zero_range()
   iomap_apply()
    xfs_file_iomap_begin()	# finds extent according to the range
    iomap_zero_range_actor() 	# sees iomap->type == IOMAP_UNWRITTEN

And this in-core extent state won't be converted to XFS_EXT_NORM until
writeback & I/O completion, so I think a flush is required.

Thanks,
Eryu