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 6D9B71DF248; Thu, 27 Aug 2026 00:44:46 +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=1787791487; cv=none; b=lNjbzrQpk5n7QwdH9wXFC0WPCkVnGEzm9J6tVyKLOhNAF05Jb1zDhGpbMfQQLQDfBLeICUpFVZxsP78bLKD4QO+2daU4LBIwSwU25oC4kGb+ujlfbkIGPBleAf1LA+Z//LlJgB5f5ehIeUHYZgm0L7tb15UBFH+arBCAhobtw0M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787791487; c=relaxed/simple; bh=pGhu6XrTs9bVFSm/qf3E45KwxUkrOzPuYMOvPxI/2ek=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ryUbcihp8fMCvyhjAYfififcDUrdxflOMq29DB6Gvla8wRkjN6X5RJ5YPQVdn0DVHUJdfhAcz22dp5ugVToyQOLUfyYS/A23C5muQUJvCsEryPUGhDxnqNyZk+8aZbB6fBT33rWMPojhadNSCfpJBojtoc8hnsnTNgxMoS7TXgA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ioYAphxG; 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="ioYAphxG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C1E151F000E9; Thu, 27 Aug 2026 00:44:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787791486; bh=cT+bFHA2IGIehxSdYCsujzJSXJKR74Lanqfi3RW9l04=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=ioYAphxGg/RgkfSz2okyrbrrruZuqy2+lG/2q/sJh7qBoSoxCPgmI6/CSl5Lc+nfF M+/5yJ2jXXxg8s8B5oU+JUcUwgF+j2SM0hbiDuD3OIfYoiVtWJZH0skzXXA2f4SXZa bzn+qGvtdXYxgHgSpd9NEuLpUV43VGLyUmeL/RVSDrYo4csgg2uWdP3xx8uHHhlnjy zDhWvmfh9VzG1nhiGDfe66ciTyUQcb+BpnxY8gsLLtYeIqf9TqTV9WO8isFann0RBB 8H7YMIXUY0APO5bNz8EEzJjH+BxPaX764jMCP4JWeZqcN/rbhdqxFpLR5YH/5Lql/l qiUcYXeFbeMyg== From: SJ Park To: SJ Park Cc: Liew Rui Yan , 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: Wed, 26 Aug 2026 17:44:38 -0700 Message-ID: <20260827004438.82746-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260826140508.74447-1-sj@kernel.org> 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 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) { ''' > > > > > > > > > > > 2. The cursor only advances when the quota becomes full. Regions that > > > > are filtered out do not move the cursor, and the scheme can remain > > > > stuck on the same regions. > > > > > > I don't fully understand this, either. Could you pleae clarify more? > > > > > > > Problem 2 is a false positive. As long as the quota is not full, DAMON > > will continue iterating to find applicable regions. I mistakenly > > assumed in the commit message that encountering filtered-out or invalid > > regions would cause the cursor to stall. > > Thank you for clarifying. Please try to reduce this kind of false positives > from the next time. > > > > > This patch ensures that every target is traversed sequentially and > > deterministically, even when the quota is set very low. I omitted this > > benefit in the initial problem description. If you think it is okay, I > > will add it in the next revision. > > What's the problem and benefit? I still don't get it. More clarification > would be nice. > > > Thanks, > SJ > > [...] Thanks, SJ