From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1081B83A14; Wed, 25 Mar 2026 01:17:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774401422; cv=none; b=DUZ6plWIPHFaEpzRva0NbNPvnxgJDheBqVWA2s45irR6bNw4SmS/YTAtAAJ6JkYleEAFk1axJ1FlWYf+KioAVQ27FY6z9XLvVS0x45tN5a1havdscOexcLPbYGBUIxDu/Z3fPNk8kL6K8drj9dWsqIevyLbdFd6pX/VIWXNsVUM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774401422; c=relaxed/simple; bh=nbpn1HlNS+cTXXOL5D8pJ35n5ChNxTXKV92YCz9TAoU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=aWhS9KMwQq+oeRbhm8G/7aem+soykBSvXnSMq6V3OC3Zt0j9CIZhHtigq1IWL29HHbe2/s/i1emPRsOq7kADyRVvBjjtQ/4g6iFGL/k/PAOiKynrDiVfXK5zqs6GXVhCmACngK9AU9taCd/bfUU7TiOJhHZ8gtmXVpLFdHijpZU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HDebcREA; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HDebcREA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7FD2CC19424; Wed, 25 Mar 2026 01:17:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774401421; bh=nbpn1HlNS+cTXXOL5D8pJ35n5ChNxTXKV92YCz9TAoU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; 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: Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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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