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b=HDebcREAm/zGVG4vNA/rTt/xIPC1Jn6idgfjKVKuop5NLhgbCt/C+Vu+dz2smYnGM nCJnd4KBuiHOsNY2ugsIlopjvgIEdwcYdPzi4RYgF5tx82MO+TF6VzKORgv8INEYn3 eUkWkIAm1X9845uVt/Yay+0AKqdT9ClNfMXuQxIKpumvRa+QU/Hr1wc/O4J+HD34De sWarPv8f6bXsBY/BWzAi1mTWH0uY5XcZe8nw/KK3uAQp3rcCAsQO6Set1cI7z05M73 Y+Tdb8PqNDLYsjllOr5GnL1DZTQk7eO6n8xifCDk8xMqjx+7qXQ+lfvqA+ibyAmwpd 7Lgg9nETqZvrw== From: SeongJae Park To: Josh Law Cc: SeongJae Park , akpm@linux-foundation.org, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: (sashiko review) [PATCH] mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses() Date: Tue, 24 Mar 2026 18:16:52 -0700 Message-ID: <20260325011653.85502-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260324154005.83651-1-objecting@objecting.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: bp7ps3x8u757zu1ytb919od38yod9q3p X-Rspamd-Queue-Id: 92CE340005 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1774401422-908215 X-HE-Meta: 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 zdfPpWUm 037A9mm+6JwOcJtp41wv1SDHAhLAGEudlpbKTbt+2rOlHA3ANk54foZ6++vl09I13oKnhtYM3f4rgUQmb3C35LrAKDoOUuWol4mlMfWTCaQfiqDxBxCSAcbzks1BBjtPADx1EU2QxGQxj/1hX+KX4D6X/4KmOyXXAfy1himG2/xq8pE6ciEOP4jy44g+FHM8yb69BWeI6JBPaYVTm2apikMnojkSCredoIAt2bsJ6g5LmzCo+d0WCc9QEqrs2S8gZDrio5NjvE5BADVP4b8hy/wT1aIi4aTSUT4L80nV+XVcs9iFHp05F+3zxyXsuvXx3OlyVqnJwF1mcb2NiRvK664wNKc6FG6JIyLrSUbcotuD0ZgpGCc8Hs82+6ZwnjB0Bp9aEkMqHiTKOKes= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Forwarding Sashiko review for doing discussions via mails. # review url: https://sashiko.dev/#/patchset/20260324154005.83651-1-objecting@objecting.org # start of sashiko.dev inline review commit ff5e227a18cd69435749b0aae0dc2c762c4f0a47 Author: Josh Law mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses() Hardware integer division is slow. The function damon_max_nr_accesses(), which is called very frequently, performs an integer division. However, the struct damon_attrs already caches this exact ratio in the internal field aggr_samples. We can eliminate the hardware division in the hot path by simply returning aggr_samples. > diff --git a/include/linux/damon.h b/include/linux/damon.h > index d9a3babbafc1..04c8a052fcfb 100644 > --- a/include/linux/damon.h > +++ b/include/linux/damon.h > @@ -978,8 +978,7 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx) > static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs) > { > /* {aggr,sample}_interval are unsigned long, hence could overflow */ > - return min(attrs->aggr_interval / attrs->sample_interval, > - (unsigned long)UINT_MAX); > + return min_t(unsigned long, attrs->aggr_samples, UINT_MAX); > } If an administrator dynamically updates aggr_interval via sysfs and commits the inputs, will attrs->aggr_samples be correctly recalculated? If aggr_samples becomes stale, kdamond_fn() will compute the real aggregation loop count and allow r->nr_accesses to increment up to the new bounds (for example, 80 loops). Meanwhile, damon_max_nr_accesses() will return the stale aggr_samples cache (for example, 20). When evaluating schemes, damon_hot_score() normalizes r->nr_accesses against this stale maximum, producing a freq_subscore that severely exceeds DAMON_MAX_SUBSCORE. This causes the calculated hotness to exceed DAMOS_MAX_SCORE. For DAMOS_PAGEOUT actions used by damon_reclaim, damon_cold_score() returns DAMOS_MAX_SCORE - hotness, yielding a highly negative score. Could this negative score cause an out-of-bounds write on the kernel heap when used as an array index? mm/damon/core.c:damos_walk() { ... c->regions_score_histogram[score] += damon_sz_region(r); ... } > diff --git a/mm/damon/core.c b/mm/damon/core.c > index db6c67e52d2b..afb058f7da5c 100644 > --- a/mm/damon/core.c > +++ b/mm/damon/core.c > @@ -587,6 +587,7 @@ struct damon_ctx *damon_new_ctx(void) > ctx->attrs.sample_interval = 5 * 1000; > ctx->attrs.aggr_interval = 100 * 1000; > ctx->attrs.ops_update_interval = 60 * 1000 * 1000; > + ctx->attrs.aggr_samples = 20; Since this value is cached here, but not updated when intervals change, does it remain permanently stale during dynamic updates? # end of sashiko.dev inline review # review url: https://sashiko.dev/#/patchset/20260324154005.83651-1-objecting@objecting.org # # hkml [1] generated a draft of this mail. 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