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

[SeongJae Park <sj@kernel.org>]

On Fri, 22 Aug 2025 11:50:57 +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()
> 
> 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>

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
> Changes from v3 [3]
> - fix checkpatch script errors

Thank you for doing this, Sang-Heon!


Thanks,
SJ

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