Re: [PATCH] mm/damon/stat: set last_refresh_jiffies to jiffies at startup

[SeongJae Park <sj@kernel.org>]

On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <yanquanmin1@huawei.com> wrote:

> In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to
> avoid unnecessarily frequent stat update.
> 
> On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make
> jiffies wrap bugs appear earlier. However, this causes time_before_eq()
> in DAMON_STAT to unexpectedly return true during the first 5 minutes
> after boot on 32-bit systems (see [1] for more explanation, which fixes
> another jiffies-related issue in DAMON). As a result, DAMON_STAT does not
> update any monitoring results during that period, which can be more
> confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.
> 
> Fix it by setting last_refresh_jiffies to jiffies at startup.

Nice catch, thank you for this patch!

> 
> [1] https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com
> 
> Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")
> Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
> ---
>  mm/damon/stat.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/damon/stat.c b/mm/damon/stat.c
> index 6c4503d2aee3..6dc3e18de910 100644
> --- a/mm/damon/stat.c
> +++ b/mm/damon/stat.c
> @@ -132,6 +132,9 @@ static int damon_stat_damon_call_fn(void *data)
>  	struct damon_ctx *c = data;
>  	static unsigned long last_refresh_jiffies;
>  
> +	if (unlikely(!last_refresh_jiffies))
> +		last_refresh_jiffies = jiffies;
> +

How about doing the initialization together with the declaration?  E.g.,

 static int damon_stat_damon_call_fn(void *data)
 {
        struct damon_ctx *c = data;
-       static unsigned long last_refresh_jiffies;
+       static unsigned long last_refresh_jiffies = jiffies;



Thanks,
SJ

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