mm/damon/core: use time_in_range() for damos quota window start

RFC PATCH In review SeongJae Park 1 participant 3 messages Mar 28

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SeongJae Park

author maintainer

2026-03-28 16:39 UTC #0 lore raw

RFC PATCH 1/1 mm/damon/core: use time_in_range() for damos quota window start

damos_adjust_quota() uses time_after_eq() to show if it is time to start a new quota charge window, comparing the current jiffies and the scheduled next charge window start time. If it is, the next charge window start time is updated and the new charge window starts.

The time check and next window start time update is skipped while the scheme is deactivated by the watermarks. Let's suppose the deactivation is kept more than LONG_MAX jiffies (assuming CONFIG_HZ of 250, more than 99 days in 32 bit systems and more than one billion years in 64 bit systems), resulting in having the jiffies larger than the next charge window start time + LONG_MAX. Then, the time_after_eq() call can return false until another LONG_MAX jiffies are passed.

This means the scheme can continue working after being reactivated by the watermarks. But, soon, the quota will be exceeded and the scheme will again effectively stop working until the next charge window starts. Because the current charge window is extended to up to LONG_MAX jiffies, however, it will look like it stopped unexpectedly and indefinitely, from the user's perspective.

Fix this by using !time_in_range() instead.

The issue was discovered [1] by sashiko.

[1] https://lore.kernel.org/[email protected]

Fixes ee801b7dd782 ("mm/damon/schemes: activate schemes based on a watermarks mechanism")

Cc <[email protected]> # 5.16.x

Signed-off-by SeongJae Park <[email protected]>

---
 mm/damon/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

mm/damon/core.c +1 -1

index 3298ee8d8f64..19ae388ff1e0 100644
@@ -2449,7 +2449,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	}
 
 	/* New charge window starts */
-	if (time_after_eq(jiffies, quota->charged_from +
+	if (!time_in_range(jiffies, quota->charged_from, quota->charged_from +
 				msecs_to_jiffies(quota->reset_interval))) {
 		if (damos_quota_is_set(quota) &&
 				quota->charged_sz >= quota->esz)

base-commit: f16846f15feb80ea9553b860bd0f70d9072101aa

Signature · 1 line

2.47.3

SeongJae Park

author maintainer

2026-03-28 17:00 UTC #1 lore raw

Re: (sashiko review) [RFC PATCH] mm/damon/core: use time_in_range() for damos quota window start

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/[email protected]
# start of sashiko.dev inline review
commit 2ee07f5a4fcf25b39aeac61c6b88fe3fdfd0c9b6
Author: SeongJae Park <[email protected]>

mm/damon/core: use time_in_range() for damos quota window start

This commit attempts to fix a stall in DAMOS quota charge windows when a scheme is deactivated for more than LONG_MAX jiffies. It replaces time_after_eq() with !time_in_range() to properly handle the extended time wrap-around.

Link https://lore.kernel.org/[email protected] open here

mm/damon/core.c static void damos_adjust_quota(struct damon_ctx *c, struct damos *s) · line 2456

Quoted 11 lines

mm/damon/core.c +1 -1

index db6c67e52d2b..8e47183473d3 100644
@@ -2452,7 +2452,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	}
 
 	/* New charge window starts */
-	if (time_after_eq(jiffies, quota->charged_from +
+	if (!time_in_range(jiffies, quota->charged_from, quota->charged_from +
 				msecs_to_jiffies(quota->reset_interval))) {

Does this introduce an off-by-one regression that extends every quota charge window by 1 jiffy?

time_in_range(a, b, c) evaluates the inclusive interval [b, c], which means it returns true when jiffies exactly equals the end of the interval. The original code used time_after_eq(), which correctly triggered the new charge window exactly on that jiffy.

By replacing it with !time_in_range(), the reset evaluates to false on that exact jiffy and is delayed until the next jiffy. For small reset_interval configurations or low CONFIG_HZ values, this 1-jiffy delay might alter the effective quota replenishment rate.

Would using !time_in_range_open() preserve the exact original timing semantics while still fixing the wrap-around stall issue?

mm/damon/core.c static void damos_adjust_quota(struct damon_ctx *c, struct damos *s) · line 2455

Quoted 2 lines

mm/damon/core.c +0 -0

 		if (damos_quota_is_set(quota) &&
 				quota->charged_sz >= quota->esz)

# end of sashiko.dev inline review
# review url: https://sashiko.dev/#/patchset/[email protected]
#
# hkml [1] generated a draft of this mail.  It can be regenerated
# using below command:
#
#     hkml patch sashiko_dev --for_forwarding \
#             [email protected]
#
# [1] https://github.com/sjp38/hackermail

Sent using hkml (https://github.com/sjp38/hackermail)

SeongJae Park

author maintainer

2026-03-28 17:03 UTC #2 lore raw

Re: (sashiko review) [RFC PATCH] mm/damon/core: use time_in_range() for damos quota window start

Quoted 36 lines · On Sat, 28 Mar 2026 10:00:08 -0700 SeongJae Park <[email protected]> wrote:

Forwarding Sashiko review for doing discussions via mails.
# review url: https://sashiko.dev/#/patchset/[email protected]
# start of sashiko.dev inline review
commit 2ee07f5a4fcf25b39aeac61c6b88fe3fdfd0c9b6
Author: SeongJae Park <[email protected]>
mm/damon/core: use time_in_range() for damos quota window start
This commit attempts to fix a stall in DAMOS quota charge windows when a scheme is deactivated for more than LONG_MAX jiffies. It replaces time_after_eq() with !time_in_range() to properly handle the extended time wrap-around.
Link https://lore.kernel.org/[email protected] open here
mm/damon/core.c static void damos_adjust_quota(struct damon_ctx *c, struct damos *s) · line 2456
Quoted 11 lines
mm/damon/core.c +1 -1
index db6c67e52d2b..8e47183473d3 100644
@@ -2452,7 +2452,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	}
 
 	/* New charge window starts */
-	if (time_after_eq(jiffies, quota->charged_from +
+	if (!time_in_range(jiffies, quota->charged_from, quota->charged_from +
 				msecs_to_jiffies(quota->reset_interval))) {
Does this introduce an off-by-one regression that extends every quota charge window by 1 jiffy?
time_in_range(a, b, c) evaluates the inclusive interval [b, c], which means it returns true when jiffies exactly equals the end of the interval. The original code used time_after_eq(), which correctly triggered the new charge window exactly on that jiffy.

Quoted 4 lines

By replacing it with !time_in_range(), the reset evaluates to false on that exact jiffy and is delayed until the next jiffy. For small reset_interval configurations or low CONFIG_HZ values, this 1-jiffy delay might alter the effective quota replenishment rate.

The effect should be negligible, but good finding.

Quoted 3 lines

Would using !time_in_range_open() preserve the exact original timing semantics while still fixing the wrap-around stall issue?

Good suggestion, I will do so in the next revision.

Thanks,
SJ

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