Re: dm: use queue_limits_set

[Mike Snitzer <snitzer@kernel.org>]

Hi Ted,

On Fri, May 17, 2024 at 10:26:46PM -0400, Theodore Ts'o wrote:
> #regzbot introduced: 1c0e720228ad
> 
> While doing final regression testing before sending a pull request for
> the ext4 tree, I found a regression which was triggered by generic/347
> and generic/405 on on multiple fstests configurations, including
> both ext4/4k and xfs/4k.
> 
> It bisects cleanly to commit 1c0e720228ad ("dm: use
> queue_limits_set"), and the resulting WARNING is attached below.  This
> stack trace can be seen for both generic/347 and generic/405.  And if
> I revert this commit on top of linux-next, the failure goes away, so
> it pretty clearly root causes to 1c0e720228ad.
> 
> For now, I'll add generic/347 and generic/405 to my global exclude
> file, but maybe we should consider reverting the commit if it can't be
> fixed quickly?

Commit 1c0e720228ad is a red herring, it switches DM over to using
queue_limits_set() which I now see is clearly disregarding DM's desire
to disable discards (in blk_validate_limits).

It looks like the combo of commit d690cb8ae14bd ("block: add an API to
atomically update queue limits") and 4f563a64732da ("block: add a
max_user_discard_sectors queue limit") needs fixing.

This being one potential fix from code inspection I've done to this
point, please see if it resolves your fstests failures (but I haven't
actually looked at those fstests yet _and_ I still need to review
commits d690cb8ae14bd and 4f563a64732da further -- will do on Monday,
sorry for the trouble):

diff --git a/block/blk-settings.c b/block/blk-settings.c
index cdbaef159c4b..c442f7ec3a6b 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -165,11 +165,13 @@ static int blk_validate_limits(struct queue_limits *lim)
 	lim->max_discard_sectors =
 		min(lim->max_hw_discard_sectors, lim->max_user_discard_sectors);
 
-	if (!lim->max_discard_segments)
-		lim->max_discard_segments = 1;
+	if (lim->max_discard_sectors) {
+		if (!lim->max_discard_segments)
+			lim->max_discard_segments = 1;
 
-	if (lim->discard_granularity < lim->physical_block_size)
-		lim->discard_granularity = lim->physical_block_size;
+		if (lim->discard_granularity < lim->physical_block_size)
+			lim->discard_granularity = lim->physical_block_size;
+	}
 
 	/*
 	 * By default there is no limit on the segment boundary alignment,
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 88114719fe18..e647e1bcd50c 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1969,6 +1969,7 @@ int dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 		blk_queue_flag_clear(QUEUE_FLAG_NOWAIT, q);
 
 	if (!dm_table_supports_discards(t)) {
+		limits->max_user_discard_sectors = 0;
 		limits->max_hw_discard_sectors = 0;
 		limits->discard_granularity = 0;
 		limits->discard_alignment = 0;