Re: [RFC PATCH 4/9] mm/damon/core: use damos_quota_is_set() in damos_adjust_quota()

[SJ Park <sj@kernel.org>]

On Mon, 14 Sep 2026 23:01:48 +0800 Kunwu Chan <kunwu.chan@gmail.com> wrote:

> On Sat, 12 Sep 2026 13:08:05 -0700 SJ Park <sj@kernel.org> wrote:
> 
> > damos_adjust_quota() is manually checking if the user set the DAMOS
> > quota.  There is a dedicated helper, damos_quota_is_set(), for the
> > purpose.  Use the helper.
> > 
> > Signed-off-by: SJ Park <sj@kernel.org>
> > ---
> >  mm/damon/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index cf10f6d725c19..dd27068cb83e3 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -3344,7 +3344,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
> >  	unsigned long cumulated_sz, cached_esz;
> >  	unsigned int score, max_score = 0;
> >  
> > -	if (!quota->ms && !quota->sz && list_empty(&quota->goals))
> > +	if (!damos_quota_is_set(quota))
> 
> Hi SJ,
> This is more than a cleanup, because damos_quota_is_set() also checks
> quota->esz, which the original condition did not.
> 
> The path that makes this matter: damos_commit_quota() copies ms and sz
> from sysfs but not esz.  So if a scheme runs with a non-zero ms, then
> the user sets ms=0 and commits, the running scheme gets ms=0 while esz
> keeps its old value.
> 
> The old check would then return early, leaving esz stale.  That stale
> esz is visible to damos_valid_target() and damos_quota_is_full(),
> which also use damos_quota_is_set().
> 
> With the helper, damos_adjust_quota() proceeds into
> damos_set_effective_quota(), where the first branch resets esz from sz
> when ms and goals are unset.
> 
> The code change is correct, but worth noting the behavioral change in
> the commit message.

So, if user sets only time quota, and then commits the quota to zero while the
esz is non-zero, esz is never reset to zero.  As a result, DAMOS keeps applying
the action with the esz forever.  Correct?

If so, this may deserve Cc-ing stable@.  Kunwu, what do you think?


Thanks,
SJ

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