[PATCH] md/raid5: allow discard with llbitmap

[Yu Kuai]

From: Yu Kuai <yukuai@xxxxxxx>

Raid5 disables discard unless devices_handle_discard_safely is set
because lower devices that do not return zeroes after discard can
leave a discarded stripe inconsistent. A later partial write can then
reconstruct bad data if a member fails.

The legacy bitmap needs this restriction because it only records
write-intent dirty ranges. It cannot distinguish discarded data from
valid data, so discard can make data inconsistent after later partial
writes and failures.

llbitmap records discarded ranges as unwritten. Raid5 already consults
llbitmap state to force RCW or lazy recovery before using parity for
unwritten data. Therefore non-zeroing discard is safe with llbitmap
while the existing full-stripe granularity and lower-device
discard-size checks still apply.

Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
 drivers/md/raid5.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index debf35342ae0..4e9a758a8cc9 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -7829,11 +7829,12 @@ static int raid5_set_limits(struct mddev *mddev)
 	 * We only allow DISCARD if the sysadmin has confirmed that only safe
 	 * devices are in use by setting a module parameter.  A better idea
 	 * might be to turn DISCARD into WRITE_ZEROES requests, as that is
 	 * required to be safe.
 	 */
-	if (!devices_handle_discard_safely ||
+	if ((!devices_handle_discard_safely &&
+	     mddev->bitmap_id != ID_LLBITMAP) ||
 	    lim.max_discard_sectors < (stripe >> 9) ||
 	    lim.discard_granularity < stripe)
 		lim.max_hw_discard_sectors = 0;
 
 	/*
-- 
2.51.0