Re: [PATCH v2] mm/damon/core: set quota->charged_from to jiffies at first charge window

[SeongJae Park <sj@kernel.org>]

On Thu, 21 Aug 2025 20:06:58 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:

> On Thu, Aug 21, 2025 at 2:41 PM SeongJae Park <sj@kernel.org> wrote:
[...]
> > Let's restart.  Could you please rewrite the commit log for this patch and send
> > the draft as a reply to this?
> >
> > We can further discuss on the new draft if it has more things to improve.  And
> > once the discussion is finalized, you can post v4 of this patch with the
> > updated commit message.
> 
> Good Idea. This is the draft for commit message. Also, Thank you for
> your patience and understanding.

Thank you for accepting my humble suggestion.

> 
> 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()

This new draft looks good to me.  I find nothing to further modify.  Could you
please send v3 of this patch with the above commit log?


Thanks,
SJ

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