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b=uFsz/XzAtqVvI344+B8Hl5ee7p4MwnG2OvMjZusp1xiq2U4nKH7HUerXjJIJ49nTH UBD44jOyppgmpYaAZJSDXUs36NVRlS1rFddNBJ3r6cJE8gQ50D4xG6gz0ShaTJOfs2 +TFuFuZuLK/6/C7J3nvmaX/Ki5/gqzrCBwOcb20IUooMhYMHjlKwQia+f3CvdcScaV 6x7aIvGeZn0Es2k91Kv+YmiiXZN8BKC3RLs0/ad3PlEH9F/nwsNu6bcgw18WSHkk9b y7u1Y3XUcM/wop9TDgqrwr0RBJYbtN8iInVdwRoFLBjQQRc1TdsFiOkhKp0wRPmlXS ZiDqAzTs1BJmA== From: SeongJae Park To: Honggyu Kim Cc: SeongJae Park , damon@lists.linux.dev, Andrew Morton , linux-mm@kvack.org, kernel_team@skhynix.com, Yunjeong Mun Subject: Re: [PATCH 1/3] mm/damon: do not allow creating zero size region Date: Sun, 22 Jun 2025 09:04:53 -0700 Message-Id: <20250622160453.47161-1-sj@kernel.org> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250622120926.1712-2-honggyu.kim@sk.com> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Stat-Signature: oh5xdsf5emkhttwwa8oxrepmtwn6ruao X-Rspam-User: X-Rspamd-Queue-Id: B7D2F40014 X-HE-Tag: 1750608297-652402 X-HE-Meta: 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 KcKyIblA w7jhJymsfjKZ/ppYd+Za8mlFC4mEGa1jJz5wtgTly4G7GvZZABJhdRLOs2thGCDx1PcnTn27N93u8lMoYrysz8CLv0Cv1f9s+gTaYVnyvZxP14cFOL7oPb2RCpQiKoAy+qdC6xIWeMOBE0ZUUjPNQbqrQIFKgnuYCwQFswJFuLKo/b/ihTNfXVh8HXr/Fh0M2VFZ6vTQCB1rO7wxGianatjzqffmnkG2ZCRjeWo9GvhBZmQ99X7AgyuK99lgD/W8VsUwhFiL1JiRaewM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, 22 Jun 2025 21:09:23 +0900 Honggyu Kim wrote: > Creating zero size region leads a divide by zero error inside > damon_get_intervals_score() as follows. > > static unsigned long damon_get_intervals_score(struct damon_ctx *c) > { > struct damon_target *t; > struct damon_region *r; > unsigned long sz_region, max_access_events = 0, access_events = 0; > unsigned long target_access_events; > unsigned long goal_bp = c->attrs.intervals_goal.access_bp; > > damon_for_each_target(t, c) { > damon_for_each_region(r, t) { > sz_region = damon_sz_region(r); > max_access_events += sz_region * c->attrs.aggr_samples; > access_events += sz_region * r->nr_accesses; > } > } > target_access_events = max_access_events * goal_bp / 10000; > return access_events * 10000 / target_access_events; /* divide by zero! */ > } Thank you for finding this issue! Coudl you please further share how zero size region can be made, and if user-space can make the situation? > > This patch makes a NULL return for such cases when creating a region > inside damon_new_region(). I agree zero size region could look silly. But I don't really think it should be prohibited. What about modifying damon_get_intervals_score() instead? Maybe we can set target_access_events as 1 in this case. > > Signed-off-by: Honggyu Kim > Cc: Yunjeong Mun > --- > mm/damon/core.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/damon/core.c b/mm/damon/core.c > index b217e0120e09..44740da337fd 100644 > --- a/mm/damon/core.c > +++ b/mm/damon/core.c > @@ -126,8 +126,11 @@ struct damon_region *damon_new_region(unsigned long start, unsigned long end) > if (!region) > return NULL; > > + if (start == end) > + return NULL; > region->ar.start = start; > region->ar.end = end; > + Above new line is unnecessary in my opinion. > region->nr_accesses = 0; > region->nr_accesses_bp = 0; > INIT_LIST_HEAD(®ion->list); > -- > 2.34.1 Thanks, SJ