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 they 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))
In 64bit system, these might not be a problem because wrapround occurs 300 million years after the boot, assuming HZ value is 1000.
With same assuming, In 32bit system, wraparound occurs 5 minutues after the initial boot and every 49 days after the first wraparound. And about 25 days after first wraparound, it continues quota charging window up to next 25 days.
Example 1: initial boot jiffies=0xFFFB6C20, charged_from+interval=0x000003E8 time_after_eq(jiffies, charged_from+interval)=(long)0xFFFB6838; In signed values, it is considered negative so it is false.
Example 2: after about 25 days first wraparound jiffies=0x800004E8, charged_from+interval=0x000003E8 time_after_eq(jiffies, charged_from+interval)=(long)0x80000100; In signed values, it is considered negative so it is false
So, change quota->charged_from to jiffies at damos_adjust_quota() when it is consider first charge window.
In theory; but almost impossible; quota->total_charged_sz and qutoa->charged_from should be both zero even if it is not in first charge window. But It will only delay one reset_interval, So it is not big problem.
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 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
SeongJae, please re-check Fixes commit is valid. Thank you.
[1] https://lore.kernel.org/damon/20250818183803.1450539-1-ekffu200098@gmail.com... --- 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("a->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))) {