[PATCH 2/2] btrfs: reinterpret async discard iops_limit=0 as no delay

[Boris Burkov <boris@bur.io>]

Currently, a limit of 0 results in a hardcoded metering over 6 hours.
Since the default is a set limit, I suspect no one truly depends on this
rather arbitrary setting. Repurpose it for an arguably more useful
"unlimited" mode, where the delay is 0.

Note that if block groups are too new, or go fully empty, there is still
a delay associated with those conditions. Those delays implement
heuristics for not trimming a region we are relatively likely to fully
overwrite soon.

Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/discard.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index aef789bcff8f..ac419ce9ab0a 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -56,8 +56,6 @@
 #define BTRFS_DISCARD_DELAY		(120ULL * NSEC_PER_SEC)
 #define BTRFS_DISCARD_UNUSED_DELAY	(10ULL * NSEC_PER_SEC)
 
-/* Target completion latency of discarding all discardable extents */
-#define BTRFS_DISCARD_TARGET_MSEC	(6 * 60 * 60UL * MSEC_PER_SEC)
 #define BTRFS_DISCARD_MIN_DELAY_MSEC	(1UL)
 #define BTRFS_DISCARD_MAX_DELAY_MSEC	(1000UL)
 #define BTRFS_DISCARD_MAX_IOPS		(10000U)
@@ -577,6 +575,7 @@ void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl)
 	s32 discardable_extents;
 	s64 discardable_bytes;
 	u32 iops_limit;
+	unsigned long min_delay = BTRFS_DISCARD_MIN_DELAY_MSEC;
 	unsigned long delay;
 
 	discardable_extents = atomic_read(&discard_ctl->discardable_extents);
@@ -607,13 +606,16 @@ void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl)
 	}
 
 	iops_limit = READ_ONCE(discard_ctl->iops_limit);
-	if (iops_limit)
+
+	if (iops_limit) {
 		delay = MSEC_PER_SEC / iops_limit;
-	else
-		delay = BTRFS_DISCARD_TARGET_MSEC / discardable_extents;
+	} else {
+		/* unset iops_limit means go as fast as possible, so allow a delay of 0 */
+		delay = 0;
+		min_delay = 0;
+	}
 
-	delay = clamp(delay, BTRFS_DISCARD_MIN_DELAY_MSEC,
-		      BTRFS_DISCARD_MAX_DELAY_MSEC);
+	delay = clamp(delay, min_delay, BTRFS_DISCARD_MAX_DELAY_MSEC);
 	discard_ctl->delay_ms = delay;
 
 	spin_unlock(&discard_ctl->lock);
-- 
2.40.0