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@xxxxxxxxxx> > > Looks correct, > Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx> > > 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. */
