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Sat, 29 Aug 2026 23:31:55 -0700 (PDT) From: Liew Rui Yan To: SJ Park Cc: Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Liew Rui Yan , stable@vger.kernel.org Subject: [PATCH v2] mm/damon/core: fix unconditionally skip last region Date: Sun, 30 Aug 2026 14:31:50 +0800 Message-ID: <20260830063159.6347-1-aethernet65535@gmail.com> X-Mailer: git-send-email 2.55.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 9095E120004 X-Stat-Signature: gkfh7bomiimyr8nf897hynezfe9c7dix X-HE-Tag: 1788071517-691005 X-HE-Meta: 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 YkAOy6rX kkvGfokAvR5CUn+CYcik5TtlSPxqRK7fL4rCAccQXHEbuMVjFKqVoR0N+e3sn1zA3OeIKmR6gAk5W86r0sU9HUarhYx2CVLRJMJV2leLeqTP59VlFmtM9AZdkv7A4FAmBwMYeG8zLiWqzQSYCGlRhWmUzzJN5Nbw0dATibO48kSvfjsz6koUmJdg4Lo4RFkWH8g+8YQWePge8vHwVpu/UOQt9YR9RBNx+LvRiKFB7Q4F2AjlzU8EzqkxkJZOsn0c5JMsK+W5Hjlwm32SFM4/ZV59ghCFapt+wFQRvx/dbirbLkD3+fm2ar/W8gFjA9ZHZZKZ6oqztWpUhm151jX0WIOxJ6KgMQvPvmJNqrOeC7zMy6UX0lihm43LTQvEaMBi7DH0Gxta4aahqquVdLvrIdgbsK2sFyNBcG26oWyA8MvTDzzfYrCL6/4AottZWuKLs8/1/xbnbeoI55Rx/CAQmD8YCxqL4DPxAVBeiXFvg+mvdObU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 100 bytes per window. 3. Window 1: Processes R1 (0-100). Quota is full. charge_{target, addr}_from is saved at (Target, 100). 4. Window 2: The loop reaches R2. Because R2 is damon_last_region(t), the old code unconditionally returns true, skipping R2 entirely and resetting the charge_{target,addr}_from. Result: R2 is permanently skipped even though it has never been processed. However, it is important to note that this is a very minor issue. This is because it is triggered only when the previous window saved/kept charge_{target,addr}_from, and in the next window, all regions except the last region were skipped by damos_skip_charged_region(). Fix this by only resets the charge_{target,addr}_from when last region is reached, only skips when it applied or cannot split. Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions") Cc: # v5.16.x Signed-off-by: Liew Rui Yan --- Changes from v1: - Simplify example in commit message. - Add clarification regarding the severity of the bug to avoid causing users unnecessary afraid. - Modify the patch code, each time the last region is reached, only charge_{target,addr}_from is reset, but the return value depends on the situation. - Modify patch's title from 'mm/damon' to 'mm/damon/core', since it only changes core.c. - v1: https://lore.kernel.org/damon/20260828084737.290024-1-aethernet65535@gmail.com 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 | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 644daf5a1656..fb76b530bbc6 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2342,36 +2342,39 @@ 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 false; + return skip; } static void damos_update_stat(struct damos *s, -- 2.55.0