Re: [PATCH] mm/damon/reclaim: fix 'enabled' is incorrectly set because 'system_wq' is not initialized

[songyuanzheng]

Hello SJ,

Thank you very much for your reply, through your reply let me understand 
this principle, so please ignore this patch.

Thanks,

Yuanzheng

On 2022/7/16 1:07, SeongJae Park wrote:
> Hello Yuanzheng,
>
>
> On Fri, 15 Jul 2022 06:16:09 +0000 Yuanzheng Song <songyuanzheng@xxxxxxxxxx> wrote:
>
> > The 'enabled' will be incorrectly set because the 'system_wq'
> > might not initialized yet. This results in 'enabled=true',
> > but the 'damon_reclaim_timer' is inactive. So fix it by moving
> > the judgment logic of the 'damon_reclaim_initialized' to the
> > start position of the enable_store().
> Thank you for this patch!
>
> In the case, however, 'damon_reclaim_init()' will activate
> 'damon_reclaim_timer', which will check the 'enabled' and start DAMON later.
> So 'enabled' will inconsistently set while DAMON_RECLAIM is not really enabled
> for a moment, but those will eventually be consistent.  This patch could reduce
> the duration of the inconsistent state.
>
> However, this would break boot-time DAMON_RECLAIM enabling, which adds
> 'damon_reclaim.enabled=true' to the kernel parameter, as this change will set
> 'enabled' as 'false' in the early 'damon_reclaim_enabled_store()', so that the
> later 'damon_reclaim_initialized()' activated 'damon_reclaim_timer' shows
> 'enabled' as 'false' and therefore doesn't start DAMON.
>
> If there is anything I'm missing, please let me know.
>
>
> Thanks,
> SJ
>
> > Fixes: 294928293813 ("mm/damon/reclaim: schedule 'damon_reclaim_timer' only after 'system_wq' is initialized")
> > Signed-off-by: Yuanzheng Song <songyuanzheng@xxxxxxxxxx>
> > ---
> >   mm/damon/reclaim.c | 9 +++++----
> >   1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
> > index e69b807fefe4..b13d5a02bf2e 100644
> > --- a/mm/damon/reclaim.c
> > +++ b/mm/damon/reclaim.c
> > @@ -374,13 +374,14 @@ static bool damon_reclaim_initialized;
> >   static int damon_reclaim_enabled_store(const char *val,
> >   		const struct kernel_param *kp)
> >   {
> > -	int rc = param_set_bool(val, kp);
> > -
> > -	if (rc < 0)
> > -		return rc;
> > +	int rc;
> >   
> >   	/* system_wq might not initialized yet */
> >   	if (!damon_reclaim_initialized)
> > +		return -EINVAL;
> > +
> > +	rc = param_set_bool(val, kp);
> > +	if (rc < 0)
> >   		return rc;
> >   
> >   	schedule_delayed_work(&damon_reclaim_timer, 0);
> > --
> > 2.25.1
> .