On Fri, 22 Aug 2025 01:33:46 +0900 Sang-Heon Jeon ekffu200098@gmail.com wrote:
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()
Thank you for this patch, Sang-Heon! But, checkpatch.pl raises below three warnings. Could you please fix those and send yet another version?
WARNING: Commit log lines starting with '#' are dropped by git as comments #16: #define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
WARNING: Commit log lines starting with '#' are dropped by git as comments #21: #define time_after_eq(a,b) \
WARNING: Prefer a maximum 75 chars per line (possible unwrapped commit description?) #26: When quota->charged_from is initialized to 0, time_after_eq() can incorrectly
Thanks, SJ
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