Re: [PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Brian Foster <bfoster@redhat.com>]

On Mon, Nov 28, 2022 at 09:22:53AM -0800, Darrick J. Wong wrote:
> On Mon, Nov 28, 2022 at 11:08:13AM -0500, Brian Foster wrote:
> > generic_remap_checks() can reduce the effective request length (i.e.,
> > after the reflink extend to EOF case is handled) down to zero. If this
> > occurs, __generic_remap_file_range_prep() proceeds through dio
> > serialization, file mapping flush calls, and may invoke file_modified()
> > before returning back to the filesystem caller, all of which immediately
> > check for len == 0 and return.
> > 
> > While this is mostly harmless, it is spurious and not completely
> > without side effect. A filemap write call can submit I/O (but not
> > wait on it) when the specified end byte precedes the start but
> > happens to land on the same aligned page boundary, which can occur
> > from __generic_remap_file_range_prep() when len is 0.
> > 
> > The dedupe path already has a len == 0 check to break out before doing
> > range comparisons. Lift this check a bit earlier in the function to
> > cover the general case of len == 0 and avoid the unnecessary work.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> 
> Looks correct,
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> 
> Should there be an(other) "if (!*len) return 0;" after the
> generic_remap_check_len call to skip the mtime update if the remap
> request gets shortened to avoid remapping an unaligned eofblock into the
> middle of the destination file?
> 

Looks sensible to me, though I guess I would do something like the
appended diff. Do you want to just fold that into this patch?

Brian

--- 8< ---

diff --git a/fs/remap_range.c b/fs/remap_range.c
index 32ea992f9acc..2f236c9c5802 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -347,7 +347,7 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 
 	ret = generic_remap_check_len(inode_in, inode_out, pos_out, len,
 			remap_flags);
-	if (ret)
+	if (ret || *len == 0)
 		return ret;
 
 	/* If can't alter the file contents, we're done. */