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=1794087936; b=jT4mfArrlrxg0A+JacS/dWLCt5UY4wI6IaDWw82OXh0qgz4xJm71wgVwnDOI3uUyy9VI EN6hbxtydHMgE5awJ0u2T7xSxUAaLo4dyiExSUD3T491wZvEZtYv4yZFBT2cUveq/SRhp kVUEBNKswHUq4jl2pCe2/w+/dL+/hS/2BCD/PYt/ZIdIXpISHT/0q2H7wqfBCc9f48NGy zN1Tpizvj708/yXvFyaUQk5Oss88IpabfwPUU6LOYeNpf1lfKsVrkA/iO4qVXmV2j9YU+ Q7aBpgEAR/9lDElMin6KU+g5byqO8ys7eNmsxARbyM3xmtHArr3B8XAPlLz2wp/Agfw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794087936; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=vOx4qL3rBGLSIPXxU4NuO61z7Gu3DhBSLmuQAAlPYBc=; b=QjjfnZQJJhmBUqGM6XaYDakoVqaFoEXAYIjTRxbrcn74tgmPuLFuO99mZ4BdZfR3E0Uv W9n/+g81EPP9tojRAcrkNuxQ6HTh8vge3TA8LfypVUK0pL7X76IzTUrOoI/v9v0qlZIwf VUkBGqThHvNGaf1IaQ5fWthxo41qZAx6+/OxlbGPoSJhQeGUPBEEQlMK3zin0H/KjYNpL PPixSZcnPp3VpT2gjQbdMnDMojAsM0OB3igUzLPbIfF4TF9wUEpMgfn+6SgGakaMyi5V/ p3/PXM7QtukT9TzUjZdgnHUguENdUl7F0vans1Cb2sVsuuAEOzAZNQq3MEL/esdn9IQ== 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=Rgob1eXt; 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 06736606D804; Sat, 7 Nov 2026 21:45:35 +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=Rgob1eXt Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5291D1F008A2; Sat, 7 Nov 2026 21:45:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794087935; bh=vOx4qL3rBGLSIPXxU4NuO61z7Gu3DhBSLmuQAAlPYBc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Rgob1eXtdv2UmztDyt5IdCdFEy+qFB0tzYedbFmfzDILucvsc8rrSXoEwNLtcwRFk kKpjego6wf039GawfVoixG/fSb7lMgSO3E7HV0uu5VROGB7sBhrLAxtrzLmPmxOqxS rLxpyzJu/ffGgADA1ds/IAly+g+vQn7chEO4rcgQmJWtgnsItzmljoJuA8KlRMSlnK fYL+ADxIdyfWilnN/WYL+/no6p6UHlv/dDssTn2V7M2juJF/h06kAnZpC/8mWeHYnx EXF1oNrB4L5dYJcTPfIjhLpcetwUYJ/+dIwGrJGKPJgMvQzKAGsLERk76KBQR2r2nx bFedBA+U6vyZg== From: SJ Park To: Cc: SJ Park , stable@vger.kernel.org, Andrew Morton , Brendan Higgins , David Gow , Quanmin Yan , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Lian Wang , Kunwu Chan Subject: [RFC PATCH 6.12.y 3/5] mm/damon/core: handle uninitialized damos_quota_goal->last_psi_total Date: Sat, 7 Nov 2026 13:45:22 -0800 Message-ID: <20261107214525.304638-4-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261107214525.304638-1-sj@kernel.org> References: <20261107214525.304638-1-sj@kernel.org> X-Mailing-List: linux-kernel@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 When DAMOS_QUOTA_SOME_MEM_PSI_US metric damos quota goal is set, the PSI delta for the feedback loop is calculated using damos_quota_goal->last_psi_total. However, it is initialized only after the first feedback loop. The first iteration of the loop uses the uninitialized value. As a result, the feedback loop can change the effective quota in an unexpected way at the first iteration. The user impact of the issue is not big, because the issue impacts only the first iteration of the feedback loop. The feedback loop also has an internal cap of the quota adjustment. The wrong adjustment will soon be corrected over a few iterations. For this reason, doing no initialization at commit time was intentional. It is also explicitly commented. That said, nobody likes behaviors that are unexpected or difficult to be expected. Check last_psi_total initialization and skip the tuning round when it is not initialized. For this, initialize last_psi_total with U64_MAX in the goal creation and the goal commit time. U64_MAX means the field is not initialized. The tuning round shows the value and adjusts it to guarantee the current quota is maintained for the round, and last_psi_total is correctly initialized on the next round. Before this change, committing a new PSI goal on an existing PSI goal with goal-only DAMON sysfs command (commit_schemes_quota_goals) just worked. After this change, the tuning round right after the commit will be unnecessarily skipped, because last_psi_total is unconditionally marked as not initialized in the damos_commit_quota_goal_union(). This is an intended tradeoff for simplicity. Skipping just one round of tuning is no problem. Meanwhile it makes both the code and the behavior simple to understand. Also update the quota goal commit unit test for changed last_psi_total setup behavior. The issue was discovered [1] by Sashiko. Link: https://lore.kernel.org/20260902002725.108635-3-sj@kernel.org Link: https://lore.kernel.org/20260718005316.89585-1-sj@kernel.org [1] Fixes: 2dbb60f789cb ("mm/damon/core: implement PSI metric DAMOS quota goal") Signed-off-by: SJ Park Signed-off-by: Andrew Morton Reviewed-by: Lian Wang Tested-by: Lian Wang Reviewed-by: Kunwu Chan Cc: Brendan Higgins Cc: David Gow Cc: Quanmin Yan Cc: # 6.9.x (cherry picked from commit 29c6084658af41b5b1fefd873d26d441830ced34) Signed-off-by: SJ Park --- mm/damon/core.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 6ef77741b783b..52f01a6508b66 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -324,6 +324,8 @@ struct damos_quota_goal *damos_new_quota_goal( return NULL; goal->metric = metric; goal->target_value = target_value; + if (metric == DAMOS_QUOTA_SOME_MEM_PSI_US) + goal->last_psi_total = U64_MAX; INIT_LIST_HEAD(&goal->list); return goal; } @@ -705,7 +707,8 @@ static void damos_commit_quota_goal( dst->target_value = src->target_value; if (dst->metric == DAMOS_QUOTA_USER_INPUT) dst->current_value = src->current_value; - /* keep last_psi_total as is, since it will be updated in next cycle */ + else if (dst->metric == DAMOS_QUOTA_SOME_MEM_PSI_US) + dst->last_psi_total = U64_MAX; } /** @@ -1608,7 +1611,12 @@ static void damos_set_quota_goal_current_value(struct damos_quota_goal *goal) break; case DAMOS_QUOTA_SOME_MEM_PSI_US: now_psi_total = damos_get_some_mem_psi_total(); - goal->current_value = now_psi_total - goal->last_psi_total; + /* uninitialized last_psi_total; make no effect this round */ + if (goal->last_psi_total == U64_MAX) + goal->current_value = goal->target_value; + else + goal->current_value = now_psi_total - + goal->last_psi_total; goal->last_psi_total = now_psi_total; break; default: -- 2.47.3