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

[Brian Foster <bfoster@redhat.com>]

On Tue, Nov 29, 2022 at 10:18:53AM -0800, Darrick J. Wong wrote:
> On Tue, Nov 29, 2022 at 06:29:51AM -0500, Brian Foster wrote:
> > 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?
> 
> Yes, could you fold it in and send a v2 with my rvb on it, please?
> 

Sure. I'll change both lines to do 'if (ret || *len == 0),' not sure why
I didn't do that the first time..

Brian

> --D
> 
> > 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. */
> > 
>