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 733583CF203; Sat, 11 Jul 2026 18:04:21 +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=1783793062; cv=none; b=cw9II16Aj4XWMVA8LwoAp+WjpqX/fJrzvrEwxZoxi6E7ZU9RdCgLhroiJQb1Pg+aTdM8O7IcYVj56Uyj6ZDj9YOxooxjR8Yx+2cferpznoUBPompF65sT74QIlF4MJA3ASPPyGluH6UuVpFYZ9NhfNfqvCzOfWUuvb3k8L6HmNU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783793062; c=relaxed/simple; bh=eShbpgLz2NbIysbCSJJgbBQ51wiA+Nyw8yXGHdcRa9M=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=gqm+lOg8iXbjid43ww8iolL7+5i0Qg8wR0kPxmKMAZdxDErFrydb7zKenXsn2WqnggXmXzLL6UeQFYDxSMjMuFY4DdgL4MnxJ7huacWBiwAEmJHsm2rPD7TvAOk5myvBVeE6H501bGa7hF5OVfNrrZ3XGBIPJBa6+GfvB99TwDk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=op3h4Blz; 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="op3h4Blz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BC5FC1F00A3E; Sat, 11 Jul 2026 18:04:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783793061; bh=Y5xW2GMWQ2fJulemhF10bJTV6JcOyhTF3brjsm/0yqc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=op3h4BlzzWxIRL9mMkFVjIL13nQaru6+YNQBVr6INCZjs+9GzxN/GXZHy5xhq8qb4 KmOGyQomZpWjvKKZICQR4wvSxurZaj9z4vJIF3lM7OSfHTEK+r1ReYAeCS9nkAvapC +aDBbobBMNq3Ctp/B98N41tFowCT/zJng8zVEH3CALzvf0DY3BDxL+SXsw2TlmVaA9 jfalGPF8MN0H5XO2B96UqZlTg+yvraVNioWHxDFHXApcS3c7f4ciur+GF2U+dGFPr+ ogxUThR+9T77WwaMkv3t7gkUSyOTDqQYGfUqhQ1f//1YcnzMREHk3PWV80X0RuWOQT ohQ7Aarudbv3g== From: SJ Park To: Cc: SJ Park , "# 5 . 15 . x" , Andrew Morton , Fernand Sieber , Leonard Foerster , SeongJae Park , Shakeel Butt , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [RFC PATCH 3/5] mm/damon/vaddr: drop last same folio access check optimization Date: Sat, 11 Jul 2026 11:04:06 -0700 Message-ID: <20260711180409.82093-4-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260711180409.82093-1-sj@kernel.org> References: <20260711180409.82093-1-sj@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The optimization can race when multiple kdamonds are running. Meanwhile, the impact of the optimization is quite doubtful. Just remove it. The user impact of the issue should be quite trivial. After all, the race can happen only when the user intentionally setup DAMON in the way. Even if it happens, it would be rare and only degrade the best-effort monitoring results. No critical consequences like kernel panic or memory corruption happen. The race possibility was discovered [1] by Sashiko. [1] https://lore.kernel.org/20260621204050.10993-1-sj@kernel.org Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces") Cc: # 5.15.x Signed-off-by: SJ Park --- mm/damon/vaddr.c | 26 ++++---------------------- 1 file changed, 4 insertions(+), 22 deletions(-) diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c index d10b8042adb5b..16fe210d2042a 100644 --- a/mm/damon/vaddr.c +++ b/mm/damon/vaddr.c @@ -383,8 +383,6 @@ static void damon_va_prepare_access_checks(struct damon_ctx *ctx) } struct damon_young_walk_private { - /* size of the folio for the access checked virtual memory address */ - unsigned long *folio_sz; bool young; }; @@ -411,7 +409,6 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr, mmu_notifier_test_young(walk->mm, addr)) priv->young = true; - *priv->folio_sz = HPAGE_PMD_SIZE; huge_out: spin_unlock(ptl); return 0; @@ -430,7 +427,6 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr, if (pte_young(ptent) || !folio_test_idle(folio) || mmu_notifier_test_young(walk->mm, addr)) priv->young = true; - *priv->folio_sz = folio_size(folio); out: pte_unmap_unlock(pte, ptl); return 0; @@ -458,7 +454,6 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask, if (pte_young(entry) || !folio_test_idle(folio) || mmu_notifier_test_young(walk->mm, addr)) priv->young = true; - *priv->folio_sz = huge_page_size(h); folio_put(folio); @@ -470,11 +465,9 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask, #define damon_young_hugetlb_entry NULL #endif /* CONFIG_HUGETLB_PAGE */ -static bool damon_va_young(struct mm_struct *mm, unsigned long addr, - unsigned long *folio_sz) +static bool damon_va_young(struct mm_struct *mm, unsigned long addr) { struct damon_young_walk_private arg = { - .folio_sz = folio_sz, .young = false, }; @@ -496,26 +489,15 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr, static void __damon_va_check_access(struct mm_struct *mm, struct damon_region *r, bool same_target) { - static unsigned long last_addr; - static unsigned long last_folio_sz = PAGE_SIZE; - static bool last_accessed; + bool accessed; if (!mm) { damon_update_region_access_rate(r, false); return; } - /* If the region is in the last checked page, reuse the result */ - if (same_target && (ALIGN_DOWN(last_addr, last_folio_sz) == - ALIGN_DOWN(r->sampling_addr, last_folio_sz))) { - damon_update_region_access_rate(r, last_accessed); - return; - } - - last_accessed = damon_va_young(mm, r->sampling_addr, &last_folio_sz); - damon_update_region_access_rate(r, last_accessed); - - last_addr = r->sampling_addr; + accessed = damon_va_young(mm, r->sampling_addr); + damon_update_region_access_rate(r, accessed); } static unsigned int damon_va_check_accesses(struct damon_ctx *ctx) -- 2.47.3