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bh=QeOHqKFwwRXxKDoOadJ6pEtYZjxux1n/oCSHCxWNGR0=; b=hYVlAnv8P0JolCLi8l6L2XNqLZB/9LMH03IXTZ0JvZ3NefGdO0uY9GSwmUBVp7htZX2h+O 3fBHJO1gp+0YAFbLI6UsWzYUerQnjvn3PpWnb49lxzHS7nf7R8sDBv9TaM/dhBZ048fo1T 5mL7rU45Lgl6jMahQaL+6w20s/F1qos= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=V5vOSBrJ; spf=pass (imf05.hostedemail.com: domain of sj@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=sj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 72E0C41AAA; Fri, 28 Aug 2026 18:29:17 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 183801F000E9; Fri, 28 Aug 2026 18:29:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787941757; bh=QeOHqKFwwRXxKDoOadJ6pEtYZjxux1n/oCSHCxWNGR0=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=V5vOSBrJlYoEX0iEEqFMvHyD/qvqqQD6D8XY4ZUpfg4jNKBDyLgJq5Zb+GQQ3GSI/ ZqWPV5YPCatrwZazeLHntHVDTKLi6fSFRzXWNQs45KQ4n+kQG4BYp5jfIT16Q/H/mh jZezdZv/flg6Ym5n/H7JQPAzZ4VVOf40vu3kb0evCD8LtY2M0tY/YkZ9xDDJy2XYlw gsz9GiW4nm1b9HNHCGwf0QXiqrYgq4k9zjigo3iWlrcJGULrFs5g8emNh+35bHcc3b 5q0NugBsnP6oRcJ2jZGthMU9G5y+5NPSUpvTyzXgTXsBE7EQZDwiBwATFOvhMZtzhx gRtHWBUzW9fbw== From: SJ Park To: Liew Rui Yan Cc: SJ Park , Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/damon: fix unconditionally skip last region Date: Fri, 28 Aug 2026 11:29:09 -0700 Message-ID: <20260828182910.70304-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260828084737.290024-1-aethernet65535@gmail.com> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 12EAE10000B X-Stat-Signature: ewo7n8npp6jezsg348p6zc7p3org3x98 X-HE-Tag: 1787941758-132293 X-HE-Meta: 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 J0O3Tyrh NbgSTbupHgCELKciS8+RNIt+lofCuSrvDmJlt0YHIE1fJrV3NnURyMyWvINHPN5Vwq0jivhAtflRCuNKbA2PiVw3IcTPvqhVTsEwxauf0YK14tKluGiVgo6cPg/z9sUwG+QrvHlE0v1Yir0oQCcy6uUX7QMw2ZnNaz3vMXWtpJQb+vakliNNwP5aT9bmg8Y+C837u6roq6wtqWLuNlHm44H48YpHis4leUtn9hyZzgUi4iI7NLhvr98oTbtKRbRRAn/i6 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 28 Aug 2026 16:47:37 +0800 Liew Rui Yan wrote: > Once quota set, the charge_{target,addr}_from 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. > > Example: > > 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). Cursor means charge_{target,addr}_from, right? Let's explain that, or just keep using the terms (charge_{target,addr}_from). > 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. Let's make example simpler by setting R1 (0-50 bytes) and R2 (50-100 bytes) or quota size 100 bytes per window. Also, it continues being skipped only in a corner case that the region addresses and the access patterns are kept. So the user impact is mild. Let's clarify that to not make users unnecessarily afraid. > > Fix this by only skipping the last region after it has been applied. > > Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions") > Cc: # v5.16.x > Signed-off-by: Liew Rui Yan > --- > > Changes from RFC v1: > - Minimal fix, only fixes the issue where the last-region is skipped. > - Add an example to the commit message to demonstrate that this error > occurs very rarely. > - RFC v1: https://lore.kernel.org/damon/20260825124616.5129-1-aethernet65535@gmail.com > > --- > mm/damon/core.c | 13 +++++++------ > 1 file changed, 7 insertions(+), 6 deletions(-) > > diff --git a/mm/damon/core.c b/mm/damon/core.c > index 644daf5a1656..21dc6b086c42 100644 > --- a/mm/damon/core.c > +++ b/mm/damon/core.c > @@ -2347,14 +2347,15 @@ 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 || > + damon_is_last_region(r, 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) { As Sashiko pointed out, this doesn't work if the the last region's start address is smaller than charge_addr_from and the end address is larger than charge_addr_from, but the size to skip (charge_addr_from - r->ar.start) is smaller than min_region_sz. As you replied to Sashiko, let's do the last region handling in every case. While doing that, let's do the charge_{target,addr}_from reset in only one place, like below. ''' --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2688,36 +2688,40 @@ static bool damos_skip_charged_region(struct damon_target *t, { struct damos_quota *quota = &s->quota; unsigned long sz_to_skip; + bool skip = false; /* Skip previously charged regions */ 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) - return true; + r->ar.end <= quota->charge_addr_from) { + skip = true; + goto out; + } if (quota->charge_addr_from && r->ar.start < quota->charge_addr_from) { sz_to_skip = ALIGN_DOWN(quota->charge_addr_from - r->ar.start, min_region_sz); if (!sz_to_skip) { - if (damon_sz_region(r) <= min_region_sz) - return true; + if (damon_sz_region(r) <= min_region_sz) { + skip = true; + goto out; + } sz_to_skip = min_region_sz; } damon_split_region_at(t, r, sz_to_skip); - return true; + skip = true; } + } +out: + if (r == damon_last_region(t)) { quota->charge_target_from = NULL; quota->charge_addr_from = 0; + return true; } - return false; + return skip; } static void damos_update_stat(struct damos *s, ''' Btw, I think damos_skip_charged_region() may deserve a kunit test. [1] https://lore.kernel.org/20260828090410.40AEA1F000E9@smtp.kernel.org [2] https://lore.kernel.org/20260828115047.332978-1-aethernet65535@gmail.com Thanks, SJ [...]