[PATCH v2 2/3] md/raid5: validate discard support at request time

[Yu Kuai]

From: Yu Kuai <yukuai@xxxxxxx>

Raid5 used to disable discard limits when devices_handle_discard_safely
was not set or when stacked member limits could not support a full-stripe
discard. That hides discard from userspace before raid5 can decide whether
a request can be handled safely.

Follow other virtual drivers and advertise a UINT_MAX discard limit for the
md device. Cache lower discard support in r5conf when setting queue limits,
and reject unsupported discard bios before queuing stripe work.

Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
 drivers/md/raid5.c | 34 +++++++++++++++++++---------------
 drivers/md/raid5.h |  1 +
 2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index debf35342ae0..76e736ee48d3 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -1132,10 +1132,22 @@ static void defer_issue_bios(struct r5conf *conf, sector_t sector,
 	spin_unlock(&conf->pending_bios_lock);
 
 	dispatch_bio_list(&tmp);
 }
 
+static bool raid5_discard_limits(struct mddev *mddev, struct bio *bi)
+{
+	struct r5conf *conf = mddev->private;
+
+	if (!conf->raid5_discard_unsupported)
+		return true;
+
+	bi->bi_status = BLK_STS_NOTSUPP;
+	bio_endio(bi);
+	return false;
+}
+
 static void
 raid5_end_read_request(struct bio *bi);
 static void
 raid5_end_write_request(struct bio *bi);
 
@@ -5702,10 +5714,13 @@ static void make_discard_request(struct mddev *mddev, struct bio *bi)
 
 	if (mddev->reshape_position != MaxSector)
 		/* Skip discard while reshape is happening */
 		return;
 
+	if (!raid5_discard_limits(mddev, bi))
+		return;
+
 	stripe_sectors = conf->chunk_sectors *
 		(conf->raid_disks - conf->max_degraded);
 	first_stripe = DIV_ROUND_UP_SECTOR_T(bi->bi_iter.bi_sector,
 					     stripe_sectors);
 	last_stripe = bio_end_sector(bi);
@@ -7815,36 +7830,25 @@ static int raid5_set_limits(struct mddev *mddev)
 	mddev_stack_rdev_limits(mddev, &lim, 0);
 	rdev_for_each(rdev, mddev)
 		queue_limits_stack_bdev(&lim, rdev->bdev, rdev->new_data_offset,
 				mddev->gendisk->disk_name);
 
-	/*
-	 * Zeroing is required for discard, otherwise data could be lost.
-	 *
-	 * Consider a scenario: discard a stripe (the stripe could be
-	 * inconsistent if discard_zeroes_data is 0); write one disk of the
-	 * stripe (the stripe could be inconsistent again depending on which
-	 * disks are used to calculate parity); the disk is broken; The stripe
-	 * data of this disk is lost.
-	 *
-	 * 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 ||
 	    lim.max_discard_sectors < (stripe >> 9) ||
 	    lim.discard_granularity < stripe)
-		lim.max_hw_discard_sectors = 0;
+		conf->raid5_discard_unsupported = true;
+	else
+		conf->raid5_discard_unsupported = false;
 
 	/*
 	 * Requests require having a bitmap for each stripe.
 	 * Limit the max sectors based on this.
 	 */
 	lim.max_hw_sectors = RAID5_MAX_REQ_STRIPES << RAID5_STRIPE_SHIFT(conf);
 	if ((lim.max_hw_sectors << 9) < lim.io_opt)
 		lim.max_hw_sectors = lim.io_opt >> 9;
+	lim.max_hw_discard_sectors = UINT_MAX;
 
 	/* No restrictions on the number of segments in the request */
 	lim.max_segments = USHRT_MAX;
 
 	return queue_limits_set(mddev->gendisk->queue, &lim);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 1c7b710fc9c1..ba06cf88aa24 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -687,10 +687,11 @@ struct r5conf {
 	struct r5pending_data	*pending_data;
 	struct list_head	free_list;
 	struct list_head	pending_list;
 	int			pending_data_cnt;
 	struct r5pending_data	*next_pending_data;
+	bool			raid5_discard_unsupported;
 
 	mempool_t		*ctx_pool;
 	int			ctx_size;
 };
 
-- 
2.51.0