md: skip bitmap accounting for empty write ranges

mkfs.ext4 can submit zero-sector flush/FUA bios. These bios are WRITE
bios for md_write_start() purposes, but they do not cover any data sector
and must not dirty bitmap bits.

md bitmap accounting currently passes such bios to bitmap start_write().
For llbitmap this reaches llbitmap_start_write() with sectors == 0,
which underflows the end chunk calculation.

Personality bitmap mapping can also turn a non-empty bio into an empty
bitmap range when the requested sectors are outside the active bitmap
geometry. Treat both cases as not started, so the completion path will not
call end_write() for an empty range.

Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-11-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

0 files changed, 0 insertions, 0 deletions