[PATCH v4] mm/damon/core: set quota->charged_from to jiffies at first charge window

[Sang-Heon Jeon <ekffu200098@gmail.com>]

Kernel initialize "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()

Fixes: 2b8a248d5873 ("mm/damon/schemes: implement size quota for schemes application speed control") # 5.16
Cc: stable@vger.kernel.org
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
---
Changes from v3 [3]
- fix checkpatch script errors

Changes from v2 [2]
- remove unnecessary example about time_after_eq()
- remove description of unexpected reset of quota->charged_from
- clarify user impacts and when bug happens

Changes from v1 [1]
- not change current default value of quota->charged_from
- set quota->charged_from when it is consider first charge below
- add more description of jiffies and wraparound example to commit
  messages

[1] https://lore.kernel.org/damon/20250818183803.1450539-1-ekffu200098@gmail.com/
[2] https://lore.kernel.org/damon/20250819150123.1532458-1-ekffu200098@gmail.com/
[3] https://lore.kernel.org/damon/20250821163346.1690784-1-ekffu200098@gmail.com/
---
I checked patch by checkpatch.pl script, just leave result for reference.

./scripts/checkpatch.pl ../patch/250822/v4-0001-mm-damon-core-set-quota-charged_from-to-jiffies-at-f.patch
total: 0 errors, 0 warnings, 10 lines checked

../patch/250822/v4-0001-mm-damon-core-set-quota-charged_from-to-jiffies-at-f.patch has no obvious style 
problems and is ready for submission.
---
 mm/damon/core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index cb41fddca78c..93bad6d0da5b 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2130,6 +2130,10 @@ 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))) {
-- 
2.43.0