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=1794424433; b=5BLfP2NbH6yamxmET76YQ7MWxUCLm88jDJzS+cPql1Vo20wpXLGIte3z49kWl5o8dIjc 1Gibapyoqx271Smo9pjwiWmTlIuK/lqjNUpbU74VNWIqykP/lmD6+Y8qhgaJd6j6ptg6+ PtLv9gn8KPHMydQ931xo+0Kn6lEfetfDqJTfvzvhloKpQa1U9SwKCJmIXNo2D/L/bANmx WhI0r6xfJbb420o9xLJhULku9/TpgKDMLMn6XLdkQ5akowIy/RLRFoUkhDQearB+23X6u +mNuJsCW1MEb06nEkhem5HZWcMEMxlhwLGYIIoOS6cHgMzgxU91L/tqvR7yLAvtozTA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794424433; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=2B3besPZA71mAbj8ksEVmeM3RSoTUvUOJQH78TzUjD4=; b=5kb75mdEXsTfcFlGjXrO1c4W/6QoejtlQ5/7UbqAukIger57mepj1GehhkbFApxidwSl mrNKOsuVnefVlJAW+YJ3pQxtnT4wWwv+c5UoQjryFCB/CSxWB0rz6VHyD5CN67ZLMaY8a veEMTXEwTZ8/+iy2G33ianUJP1EcdWFh1hGa32PGpZQjWZERnC0lFsws5szB0vXO+JvFF HTxUio1HQIalMy9bKGSNYounWklrgHTU1l+JNPxzlPpXWbfePjKlGzqEBKl8d0lPY0bIQ 4CTlbzerygvGa+3iyeYkEN9xmIOru3l7YFeZPnBmRWiTuNgGnNzmVW8075dMccsXgyw== 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=Rom0bMUY; 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 1C8C1606A330 for ; Wed, 11 Nov 2026 19:13:52 +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=Rom0bMUY Received: by smtp.kernel.org (Postfix) with ESMTPSA id E6D081F00A06; Wed, 11 Nov 2026 19:13:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794424432; bh=2B3besPZA71mAbj8ksEVmeM3RSoTUvUOJQH78TzUjD4=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Rom0bMUYU74zrXCslkfIfzq2kvv1rC6obKeJA8arKZzN9eczSfJyHBGWjQrl0dWzp N6xZKXt895sciCOPNUJDVFhR3foB5ElO4BALM449BH3KkgBCCOytrWfgYXCaFW0OUH 7aDQXaXL4wwOHQ6fQTRg/+NlHIL5+GApHxsfM06f1PquST2chzg/0VsZgnu4aACx14 F88VmBBJlLo2pPu7wUj1i8GdK2Rmv8+ItPhkqFXSpXm6bZrPhCukHwflvD16X2EOCA gofHyFYhEGpYUid44Wd5Zu7hPaRlwCsOu+8m0Zg/KcqSfA8Yzx6A7nUQkXJ75XjsuM bdjUMaQ36ALlg== From: Sasha Levin To: stable@vger.kernel.org Cc: Chengfeng Ye , Andrew Morton , Xu Xin , "David Hildenbrand (Arm)" , Hugh Dickins , Sasha Levin Subject: [PATCH 6.6.y] mm/ksm: mark migration stores with WRITE_ONCE() Date: Wed, 11 Nov 2026 14:13:50 -0500 Message-ID: <20261111191332.3939168-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261107053152-failed-6.6-d3863210464f@kernel.org> References: <20261107053152-failed-6.6-d3863210464f@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 From: Chengfeng Ye [ Upstream commit d3863210464f69c18615cbe400d48a6e53e770d4 ] ksm_get_folio() deliberately samples stable_node->kpfn and folio->mapping without taking the folio lock because the KSM folio may be migrated concurrently. folio_migrate_ksm() updates the same state using plain assignments. The reader can load the old kpfn, then the migrator can store the new kpfn, execute smp_wmb(), and clear the old folio's mapping before the reader checks that mapping. Thus the initial kpfn load can overlap its update and the subsequent mapping load can overlap the clear, with no common lock. This leaves marked READ_ONCE() accesses racing with plain stores. The kernel reported: BUG: KCSAN: data-race in folio_migrate_ksm / ksm_get_folio read (marked) to 0xffff8ce401421330 of 8 bytes by task 48 on cpu 3: ksm_get_folio+0x7f/0x2a0 ksm_scan_thread+0x1635/0x3330 kthread+0x1af/0x1f0 write to 0xffff8ce401421330 of 8 bytes by task 102 on cpu 1: folio_migrate_ksm+0x6a/0xd0 folio_migrate_flags+0x193/0x420 __migrate_folio.isra.0+0x162/0x1a0 migrate_folio+0x4c/0x70 move_to_new_folio+0xd6/0x170 Use WRITE_ONCE() for both stores to pair them with the existing lockless reads. This preserves the existing smp_wmb()/smp_rmb() migration protocol and control flow while preventing compiler transformations of the shared accesses. Link: https://lore.kernel.org/20260824112433.191301-1-nicoyip.dev@gmail.com Fixes: c8d6553b9580 ("ksm: make KSM page migration possible") Signed-off-by: Chengfeng Ye Signed-off-by: Andrew Morton Acked-by: Xu Xin Acked-by: David Hildenbrand (Arm) Cc: Hugh Dickins Cc: [ sashal: Adapted folio_set_stable_node() to set_page_stable_node() with page->mapping and PAGE_MAPPING_KSM. ] Signed-off-by: Sasha Levin --- mm/ksm.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index fa4229a224060..a79a2d8b24abd 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -916,7 +916,8 @@ static inline void set_page_stable_node(struct page *page, struct ksm_stable_node *stable_node) { VM_BUG_ON_PAGE(PageAnon(page) && PageAnonExclusive(page), page); - page->mapping = (void *)((unsigned long)stable_node | PAGE_MAPPING_KSM); + WRITE_ONCE(page->mapping, + (void *)((unsigned long)stable_node | PAGE_MAPPING_KSM)); } #ifdef CONFIG_SYSFS @@ -3078,7 +3079,7 @@ void folio_migrate_ksm(struct folio *newfolio, struct folio *folio) stable_node = folio_stable_node(folio); if (stable_node) { VM_BUG_ON_FOLIO(stable_node->kpfn != folio_pfn(folio), folio); - stable_node->kpfn = folio_pfn(newfolio); + WRITE_ONCE(stable_node->kpfn, folio_pfn(newfolio)); /* * newfolio->mapping was set in advance; now we need smp_wmb() * to make sure that the new stable_node->kpfn is visible -- 2.53.0