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

[Quanmin Yan <yanquanmin1@huawei.com>]

在 2025/10/29 9:30, SeongJae Park 写道:
> On Tue, 28 Oct 2025 07:32:49 -0700 SeongJae Park <sj@kernel.org> wrote:
>
>> On Tue, 28 Oct 2025 07:19:14 -0700 SeongJae Park <sj@kernel.org> wrote:
>>
>>> 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;
> Please ignore the above suggestion.  It will even not build, like below...
>
> .../mm/damon/stat.c: In function ‘damon_stat_damon_call_fn’:
> .../mm/damon/stat.c:133:53: error: initializer element is not constant
>    133 |         static unsigned long last_refresh_jiffies = jiffies;
>        |                                                     ^~~~~~~
>
>> Actually, a similar issue can happen again if DAMON_STAT is stopped and
>> restarted by user.  That is, if user stops DAMON_STAT just after
>> last_refresh_jiffies is updated, and restart it after 5 seconds or more, the
>> time_before_eq() on damon_call_fn() will return true, so stat updates will
>> happen earlier than expected.  Shouldn't be a real problem, but better to avoid
>> if possible.
>>
>> How about making last_refresh_jiffies a global variable and initialize it on
>> damon_stat_start()?  To avoid unnecessary name conflicts, the variable name
>> would also better to be changed, e.g., damon_stat_last_refresh_jiffies.
> But, please consider the above one.
>
> And I just realized a similar issue exist for next_update_jiffies in
> mm/damon/sysfs.c file.  Please feel free to send a patch for that if you
> willing to.
>
OK, I’ll review all these issues and send a new patch set once everything
is ready.🙂

Thanks,
Quanmin Yan

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