From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794277504; b=EY61O82SyEuPzcENExEzNeBJyhrWXVAvaGUugkZj77kxgUZwzfc3yfjksFF2TI/5WB4e enIorPlumdNwJxnUNgsI6KbBCowMpbNz6t7SepmcIMJa0B5PvK6jz8kUAEW/ZvVG0JFuk 1LZjvuTSV0KycIhOyb2MHCRCI638bahRuuySk1T9sMbHdXBITVkOSQ4Wckq4cOy8rg0ka sf/vVi6c0v/XGtjwc50Xh9qoAI8rHVExBm5/Fa6En3c76xl+n1X7QNWsy3YdyDuLuYb2N ohR99BIy7Y9pXWdFTL0H3Hl5KPLgvntYfBWyuB18TpJ6Ykzy3YFDhwpVexGxlnK1h9g== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794277504; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=lM4m5T67vL8+cPiiCKZ45w4hjL/d5OYgwK/JlLAImo4=; b=vBJ6iQ5OnuhEbhTr5a6ghqRSfr65xVpWUXshJbVeiTkEOjVbjDTc6AfTZ+1AvvIi0ly3 AIF9eqPCJRug/q9bmiQqsnnufEsvmvGZAAlHrQUP/SluKBda7UNJohWDc7Kmz1rp0RwTi vpAoZBT0E1LpS3dzgVIcKDNxoH96Bre6cQFmDFfGm8dQCmgVzuiwcU0xlNCI4wUCKDbRO YAdw1eNoVjgiNCS/BANetR4lKU4XYljaepOQBShuu/9soa61j8uR7T7JijKbSbrBe7aGb aGNxFsAbULJMvmWQRg9Ap3UCPMXzP8Y/sU9w1oIY8ZRTj4IZdpL4DOaeKkPI38PijTw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Bud2HuED; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F2EA1606A31C for ; Tue, 10 Nov 2026 02:25:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Bud2HuED Received: by smtp.kernel.org (Postfix) with ESMTPSA id 65F851F0089F; Tue, 10 Nov 2026 02:25:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794277503; bh=lM4m5T67vL8+cPiiCKZ45w4hjL/d5OYgwK/JlLAImo4=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Bud2HuEDb4wFfHjEbkpu5s0cw24FBicJGs/BkbhGWcztoMFGzKZEDxwAc6xX+xvsX wvFw1qdULKep0GYY69U8NvRv8beHWZ2bssL9ljNFxw25bcUuPxiGbuSz2XYmPoLhpp pXDo4qwtx+SV4s4eF5YRsIMKwO2lhTgEN/wciTWpqBeHMU0vwQG7MBLuAXiw507x0a G2QAv9yYwhTv1baqm1XUkerDQZx2EZrvIhtppa8AfD2NVUv8avuQRpQlJWwQFaAdq8 zH0PDaeSygdZapPROL/ybL2BfCTrus4+Quy7onvBoProiOuic8g0xYznjynoe/IlAv EqxVzIl5kGaHQ== From: SJ Park To: stable@vger.kernel.org Cc: SJ Park , Andrew Morton , Fernand Sieber , SeongJae Park Subject: [PATCH 6.18.y] mm/damon/core: move static variable last_nr_regions to damon_ctx Date: Mon, 9 Nov 2026 18:24:54 -0800 Message-ID: <20261110022454.112647-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261109200407-failed-6.18-26ae50a0901a@kernel.org> References: <20261109200407-failed-6.18-26ae50a0901a@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit kdamond_split_regions() uses a static variable last_nr_regions, to infer if the region boundaries are too stable. If it looks too stable, it does a more aggressive region split because it might be a fake stability coming from non-aggressive split. Because the variable is static, the inference can be racy and wrong when multiple kdamonds are running in parallel. The user impact is not critical. No crash, no leak. DAMON's best-effort monitoring accuracy can be degraded when multiple kdamonds are used in parallel and the workload has certain access patterns. Still it is a bug that is better to fix. Move the variable into the damon_ctx struct to fix the problem. Link: https://lore.kernel.org/20261025164808.152048-3-sj@kernel.org Fixes: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts") Signed-off-by: SJ Park Signed-off-by: Andrew Morton Closes: https://sashiko.dev/#/bug/linux-eebfcdde-2881-48ef-8df1-e2019d2a47f2 Cc: Fernand Sieber Cc: SeongJae Park Cc: # 5.15.x (cherry picked from commit 26ae50a0901a44f7d50053f87dc38abd9f9a269c) Signed-off-by: SJ Park --- include/linux/damon.h | 5 +++++ mm/damon/core.c | 5 ++--- 2 files changed, 7 insertions(+), 3 deletions(-) diff --git a/include/linux/damon.h b/include/linux/damon.h index 656a314ad2159..46332310fef87 100644 --- a/include/linux/damon.h +++ b/include/linux/damon.h @@ -804,6 +804,11 @@ struct damon_ctx { struct list_head adaptive_targets; struct list_head schemes; + /* + * number of regions after last merge. Read kdamond_split_regions() + * for more details. + */ + unsigned long last_nr_regions; }; static inline struct damon_region *damon_next_region(struct damon_region *r) diff --git a/mm/damon/core.c b/mm/damon/core.c index 56f2343ffefac..701173dadd7bc 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2525,7 +2525,6 @@ static void kdamond_split_regions(struct damon_ctx *ctx) { struct damon_target *t; unsigned int nr_regions = 0; - static unsigned int last_nr_regions; int nr_subregions = 2; damon_for_each_target(t, ctx) @@ -2535,14 +2534,14 @@ static void kdamond_split_regions(struct damon_ctx *ctx) return; /* Maybe the middle of the region has different access frequency */ - if (last_nr_regions == nr_regions && + if (ctx->last_nr_regions == nr_regions && nr_regions < ctx->attrs.max_nr_regions / 3) nr_subregions = 3; damon_for_each_target(t, ctx) damon_split_regions_of(t, nr_subregions, ctx->min_sz_region); - last_nr_regions = nr_regions; + ctx->last_nr_regions = nr_regions; } /* -- 2.47.3