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Mon, 15 Sep 2025 07:51:59 -0700 (PDT) From: Joshua Hahn To: SeongJae Park Cc: Andrew Morton , damon@lists.linux.dev, kernel-team@meta.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 1/6] mm/damon/core: reset age if nr_accesses changes between non-zero and zero Date: Mon, 15 Sep 2025 07:51:57 -0700 Message-ID: <20250915145158.1425390-1-joshua.hahnjy@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20250915015807.101505-2-sj@kernel.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 2759C4000A X-Stat-Signature: 9mein88a3ifzi6udq5o4pmz7ha6ojrtk X-Rspam-User: X-HE-Tag: 1757947920-642553 X-HE-Meta: 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 s1VbHDCr 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 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, 14 Sep 2025 18:58:02 -0700 SeongJae Park wrote: > DAMON resets the age of a region if its nr_accesses value has > significantly changed. Specifically, the threshold is calculated as 20% > of largest nr_accesses of the current snapshot. This means that regions > changing the nr_accesses from zero to small non-zero value or from a > small non-zero value to zero will keep the age. Since many users treat > zero nr_accesses regions special, this can be confusing. Kernel code > including DAMOS' regions priority calculation and DAMON_STAT's idle time > calculation also treat zero nr_accesses regions special. Make it > unconfusing by resetting the age when the nr_accesses changes between > zero and a non-zero value. Hi SJ, Thank you for the patch, I think the goal of the patch makes sesne to me. I have a small nit / idea which I think makes the code a bit clearer, at least for me. It seems that we basically want to XOR the two values's zero-ness, so maybe something like (!!r->nr_accesses) ^ (!!r->last_nr_access) or (r->nr_accesses == 0) ^ (r->last_nr_access == 0) Can achieve the goal? I know bitwise operations are sometimes harder to understand, so I am just throwing the idea out there : -) Anyways, the rest of it looks good to me, please feel free to add my review! Reviewed-by: Joshua Hahn > Signed-off-by: SeongJae Park > --- > mm/damon/core.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/damon/core.c b/mm/damon/core.c > index be5942435d78..996647caca02 100644 > --- a/mm/damon/core.c > +++ b/mm/damon/core.c > @@ -2261,6 +2261,9 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres, > damon_for_each_region_safe(r, next, t) { > if (abs(r->nr_accesses - r->last_nr_accesses) > thres) > r->age = 0; > + else if ((!r->nr_accesses && r->last_nr_accesses) || > + (r->nr_accesses && !r->last_nr_accesses)) > + r->age = 0; > else > r->age++; > > -- > 2.39.5 Sent using hkml (https://github.com/sjp38/hackermail)