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 DF1EA3A4505; Fri, 28 Aug 2026 18:29:17 +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=1787941759; cv=none; b=HmJFBWifJsEWnslpD5gVNsUqTBrbHNBR7LzxT0AlfQwnupHBuDXAMPQKFcxziMpV712Ymy2iXL5mXEezj4ZtCOJvPXYNDomgeG/9X42X1rU+oPTjVmRpSRerJQR0bylOETdgq4YmS9vS3vZGpViQ0Udc5g8UW6YplpDymiiegqY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787941759; c=relaxed/simple; bh=tYCi2x5F+r0yfVDr8C945+5FuD0jcgPWMELcRlga2pw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=dlWscIAKNfRfnTm2jAcZ7Yr+KQpCXrwWBgzgfxjCorNkDTx+dyQidmiPFdm0480WU2UBoHXTwIHgwYV6pWV3YWLCoU6xiS7d75OpiFUipiStqKFubYSSt0BgE6C5+AjY0hVXS/D27FczRXcO/xk2x3PXKrYn3EmYYqSnKPN7LXs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=V5vOSBrJ; 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="V5vOSBrJ" 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: 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 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 [...]