FAILED: patch "[PATCH] mm/damon/core: set quota->charged_from to jiffies at first" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x ce652aac9c90a96c6536681d17518efb1f660fb8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025091357-stapling-walrus-d0f7@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ce652aac9c90a96c6536681d17518efb1f660fb8 Mon Sep 17 00:00:00 2001
From: Sang-Heon Jeon <[email protected]>
Date: Fri, 22 Aug 2025 11:50:57 +0900
Subject: [PATCH] mm/damon/core: set quota->charged_from to jiffies at first
charge window

Kernel initializes the "jiffies" timer as 5 minutes below zero, as shown in include/linux/jiffies.h

/*
* Have the 32 bit jiffies value wrap 5 minutes after boot
* so jiffies wrap bugs show up earlier.
*/
#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))

And jiffies comparison help functions cast unsigned value to signed to cover wraparound

#define time_after_eq(a,b) \
(typecheck(unsigned long, a) && \
typecheck(unsigned long, b) && \
((long)((a) - (b)) >= 0))

When quota->charged_from is initialized to 0, time_after_eq() can incorrectly return FALSE even after reset_interval has elapsed. This occurs when (jiffies - reset_interval) produces a value with MSB=1, which is interpreted as negative in signed arithmetic.

This issue primarily affects 32-bit systems because: On 64-bit systems:
MSB=1 values occur after ~292 million years from boot (assuming HZ=1000), almost impossible.

On 32-bit systems: MSB=1 values occur during the first 5 minutes after boot, and the second half of every jiffies wraparound cycle, starting from day 25 (assuming HZ=1000)

When above unexpected FALSE return from time_after_eq() occurs, the charging window will not reset. The user impact depends on esz value at that time.

If esz is 0, scheme ignores configured quotas and runs without any limits.

If esz is not 0, scheme stops working once the quota is exhausted. It remains until the charging window finally resets.

So, change quota->charged_from to jiffies at damos_adjust_quota() when it is considered as the first charge window. By this change, we can avoid unexpected FALSE return from time_after_eq()

index 106ee8b0f2d5..c2e0b469fd43 100644
@@ -2111,6 +2111,10 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	if (!quota->ms && !quota->sz && list_empty(&quota->goals))
 		return;
 
+	/* First charge window */
+	if (!quota->total_charged_sz && !quota->charged_from)
+		quota->charged_from = jiffies;
+
 	/* New charge window starts */
 	if (time_after_eq(jiffies, quota->charged_from +
 				msecs_to_jiffies(quota->reset_interval))) {

Hi Greg,

[...]

The broken commit was introduced from 5.16, so I don't think this commit need to be cherry-picked on 5.15.y. Please let me knwo if I'm missing something or there is anything I can help.

Thanks,
SJ

[...]

You are right, I was too loose in the version ranges here, thanks.

greg k-h