From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 1258D3093C1; Fri, 28 Aug 2026 00:40:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787877658; cv=none; b=itLbKRfcNCyXwjDZ9MXJQPRlFUvrCVMkUFcM0au3X5WT9AfoSZwCHqkAzhm+Gols6ALfvPuRBGqAX1FEu3b1ymHt6KEd7ZCNJAQ+JVHT+jcnsh+nl9rdsQep1DA7wHOhg1IwT9tvQrZGiyCw3WglTl9RwAqTi7CXi2htetS09QU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787877658; c=relaxed/simple; bh=/sqbOqBC2VV7YhvVIfvJyn4Cst9mP3+3SLC9Z70RTnE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ol+6Au6dEoaqUF9pF6BwNP/vCbMd3b715VCvc1COZ4jk9StwJ/QHdQjW5If8Kngk9W3yPiuaDnc35z+OkJ3h62zir+FOTxhXvHfLhsyiJR8s1kDp45k1D3uRuxFSLkKEQ4fR0HFoQuPfP/EIWG1rPK4AiFS5j+jKxoFXkkQKqPo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=O0v7eLd9; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="O0v7eLd9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6454F1F000E9; Fri, 28 Aug 2026 00:40:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787877656; bh=C3dYuQBwRk/xR09vNJREZ7Cj/0z5bTktPZ0KJHmO4nE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=O0v7eLd9EuTh23YcZbaWO4XrOIMVKWuFnMqXs9EyOKVDR16db5yb5hnCDj0jjGgaz 91cSy0G2tEhgOhtlwU/D9jwUIK4oEomYN3oLHX2anKUuG6GoP439kfjrCDCsOvb20z 7N4r4MHPD1jwFoESI6l9WYcpYp3Q2Nj9V2sVm1R/EhN1sc5pvRjyJwiA5ImoGtI15y JytMYBFzprHqP/B/rvaoDLfywpsXO5C76QdkcEgdK+jABlbSkuWD+kthvWJgzyjSfR apK1rR+JTfs/0SZuN8fO/Q+Esx9xxa0Ewi1vDHXe4C3ki4C3UnPkAyEETcBZbHxdyu HsBH0IK5oKrrQ== From: SJ Park To: Liew Rui Yan Cc: SJ Park , akpm@linux-foundation.org, damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH] mm/damon: fix damos quota walk-position tracking Date: Thu, 27 Aug 2026 17:40:48 -0700 Message-ID: <20260828004049.62386-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260827180822.4037-1-aethernet65535@gmail.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Fri, 28 Aug 2026 02:08:22 +0800 Liew Rui Yan wrote: > On Wed, 26 Aug 2026 17:44:38 -0700 SJ Park wrote: > > > On Wed, 26 Aug 2026 07:05:08 -0700 SJ Park wrote: > > > > > On Wed, 26 Aug 2026 18:24:13 +0800 Liew Rui Yan wrote: > > > > > > > On Tue, 25 Aug 2026 06:54:57 -0700 SJ Park wrote: > > > > > > > > > On Tue, 25 Aug 2026 20:46:16 +0800 Liew Rui Yan wrote: > > > > > > > > > > > DAMOS uses charge_target_from/charge_addr_from to remember how far a > > > > > > quota-limited walk has progressed. The current implementation has two > > > > > > problems: > > > > > > > > > > > > 1. Once set, the cursor unconditionally skips and resets at the last > > > > > > region of the tracked target, so the last region can be skipped even > > > > > > when it has not been processed. > > > > > > > > > > I don't fully understand this. Could you please clarify more? Maybe adding a > > > > > realistic example scenario would be helpful. > > > > > > > > > > > > > Problem: Unconditional skip of the last region > > > > > > > > In the current damos_skip_charged_region(), there is this logic: > > > > > > > > if (r == damon_last_region(t)) { > > > > quota->charge_target_from = NULL; > > > > quota->charge_addr_from = 0; > > > > return true; /* Skip */ > > > > } > > > > > > > > Scenario: > > > > 1. Target has 2 regions: R1 (0-100 bytes) and R2 (100-200 bytes). > > > > > > > > 2. Quota is configured to process only 50 bytes per window. > > > > > > > > 3. Window 1: Processes R1 (0-50). Quota is full. Cursor is saved at > > > > (Target, 50). > > > > > > > > 4. Window 2: Skips R1 (0-50). Processes R1 (50-100). Quota is full. > > > > Cursor is saved at (Target, 100), which is exactly the start of R2. > > > > > > > > 5. Window 3: The loop reaches R2. Because R2 is damon_last_region(t), > > > > the old code unconditionally returns true, skipping R2 entirely and > > > > resetting the cursor. > > > > > > > > Result: R2 is permanently skipped even though it has never been > > > > processed. > > > > > > Ok, makes sense. The user impact should be not that big, though. > > > > > > > > > > > To fix this, the patch advances the cursor every time a region is > > > > walked, regardless of whether it is applied or filtered out. This > > > > allows DAMON to accurately track whether the last region has already > > > > been visited, eliminating the need for the unconditional reset. > > > > > > Sounds like a big change compared to the problem. Why we cannot modify the > > > last region case? Have you also considered other possible simpler approaches? > > > > For example, > > > > ''' > > --- a/mm/damon/core.c > > +++ b/mm/damon/core.c > > @@ -2686,14 +2686,15 @@ static bool damos_skip_charged_region(struct damon_target *t, > > if (quota->charge_target_from) { > > if (t != quota->charge_target_from) > > return true; > > - if (r == damon_last_region(t)) { > > - quota->charge_target_from = NULL; > > - quota->charge_addr_from = 0; > > - return true; > > - } > > if (quota->charge_addr_from && > > - r->ar.end <= quota->charge_addr_from) > > + r->ar.end <= quota->charge_addr_from) { > > + if (r->ar.end == quota->charge_addr_from || > > + r == damon_last_region(t)) { > > + quota->charge_target_from = NULL; > > + quota->charge_addr_from = 0; > > + } > > return true; > > + } > > > > if (quota->charge_addr_from && r->ar.start < > > quota->charge_addr_from) { > > ''' > > > > Thank you for the example! > > While your approach works, I am curious, why should the cursor be reset > every time the function returns false (does not skip)? It doesn't. It resets charge_{target,addr}_from only once after the regions to skip are all skipped. Am I missing something? > > In my opinion, a cleaner and more deterministic approach is to reset the > cursor only after the target has been fully iterated through. So I will be able to give you some comments only after the above question is answered. Thanks, SJ [...]