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

[SeongJae Park <sj@kernel.org>]

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.


Thanks,
SJ

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