Re: [PATCH v3] mm/damon/core: set quota->charged_from to jiffies at first charge window

[SeongJae Park <sj@kernel.org>]

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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