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 668C93B2FDD for ; Thu, 25 Jun 2026 12:07:43 +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=1782389265; cv=none; b=mV8qnAIARie/dzEGUSqkyU0gdnkTMLVeLumAHnn8Rlu8YCSYDF4YDq2FryXn9Te2DCmzGyEE1AYCRTRgUQFR6/bDDAhhWlQYxetPS1WuFVK67FRAyucQU4HlX5S8WmUS2bH851KfeFLkNFhgYOWIT6RBEPZoGwT+ANdd5JtafdY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782389265; c=relaxed/simple; bh=DGEYPSMZatAFzoel0W4xfdrN6GGWroswCUlOm6bptzw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=CPUV2PqFRNdVUnJ0LmPAF8C9xw2y5xd9j+Jkgan2Ybi8IuSgQEvmcIcpxEA0+XG203yEtHkXfF6XHZuy1aoothPDOcsTbSA08azvqopw+JdQMfYl2P3idbfgE/3CfUWLnQlKQLL5bJhT6CHMqYuRYIWZcEUb8U4/c4+ZKUR9H+Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KDZmqBG2; 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="KDZmqBG2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E4B0D1F000E9; Thu, 25 Jun 2026 12:07:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782389263; bh=hzOWIM48785YXF4+EkAFQi3dSVtrixHEzc/TOh6UnkM=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=KDZmqBG2siXwFLEI6Sm2qljEbRdd2HnaDa1c+SIy7VYes8nW+jYo4yiomKt9moDpu HHnqrswZpDGY9vW1YmPtGYEparelZcqK79bL1AJCcjKe340mRooWJZM0hNAv0w1rxZ 5vRimF0BM88IfJ7VPpu/lAXjVLtg3kcqGzP3hYc9NTFFo3ai4ddM+SPQmJdUJgiaK6 KcQ0uwfATmwgG2FEI97wtBkEk3idv5jozP4m8O+ymKllZ+rYTRXU3YCe5EpcSKfRHX xvbAhwRTHnpac978/L40OsZt8ePDZTedhfnXLHVdpackdCvohA2WeX1r3t3lzJ0dIT qOFrpXnyWiZcQ== Date: Thu, 25 Jun 2026 13:07:33 +0100 From: Lorenzo Stoakes To: "David Hildenbrand (Arm)" Cc: Hui Zhu , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kairui Song , Qi Zheng , Shakeel Butt , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hui Zhu Subject: Re: [PATCH v5] mm: assert exclusive nid/zonenum bits at the page/folio access sites Message-ID: References: <20260625071830.996043-1-hui.zhu@linux.dev> <2c4dd46a-4755-4bf5-8f14-2d73eb356e3e@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2c4dd46a-4755-4bf5-8f14-2d73eb356e3e@kernel.org> On Thu, Jun 25, 2026 at 01:53:14PM +0200, David Hildenbrand (Arm) wrote: > On 6/25/26 09:18, Hui Zhu wrote: > > From: Hui Zhu > > > > KCSAN reports a data race between page_to_nid()/folio_pgdat() reading > > page->flags and folio_trylock()/folio_lock() concurrently doing > > test_and_set_bit_lock(PG_locked, ...) on the same word, e.g.: > > > > BUG: KCSAN: data-race in __lruvec_stat_mod_folio / shmem_get_folio_gfp > > > > The node id and zone id occupy fixed bit-ranges of page->flags that > > are set once at page init and never modified afterwards, so they can > > never overlap with the low PG_locked/PG_waiters bits touched by the > > folio lock path. > > > > ASSERT_EXCLUSIVE_BITS(mdf.f, ...) inside memdesc_nid()/memdesc_zonenum() > > checks a by-value copy of the flags word, not the actual shared > > page->flags/folio->flags being modified concurrently, so it doesn't > > reliably assert anything about the real race. Move the assertion to > > page_to_nid(), folio_nid(), page_zonenum() and folio_zonenum(), where > > flags is dereferenced directly from the page/folio. > > > > On CONFIG_NUMA=n, NODES_MASK is 0 and the old memdesc_nid() body > > folded to a constant, so page->flags/folio->flags was never actually > > read. ASSERT_EXCLUSIVE_BITS() is a real runtime check that can't be > > folded away, so doing it unconditionally would add a pointless read > > of page->flags/folio->flags and a check that can never fire. Keep > > page_to_nid()/folio_nid() as plain "return 0" static inline stubs > > under CONFIG_NUMA=n instead. > > > > Signed-off-by: Hui Zhu > > Acked-by: David Hildenbrand (Arm) > > --- > > Changelog: > > v5: > > According to the comments of Sashiko, guard the ASSERT_EXCLUSIVE_BITS() > > calls with #ifndef NODE_NOT_IN_PAGE_FLAGS (for nid) and #if > > ZONES_WIDTH != 0 (for zonenum). > > According to the comments of David, avoid calling > > PF_POISONED_CHECK(page) twice in page_to_nid(). > > According to the warning of lkp, switch the CONFIG_NUMA=n > > page_to_nid()/folio_nid() stubs from macros to static inline functions. > > v4: > > According to the comments of Andrew and Sashiko, set > > page_to_nid()/folio_nid() as static inline stubs returning 0 > > under CONFIG_NUMA=n. > > v3: > > According to the comments of Andrew and Sashiko, move > > ASSERT_EXCLUSIVE_BITS out of memdesc_nid()/memdesc_zonenum() > > into the page/folio call sites. > > v2: > > According to the comments of David, remove useless comments and use > > ASSERT_EXCLUSIVE_BITS() in memdesc_nid() instead of data_race() in > > page_to_nid(). > > > > include/linux/mm.h | 23 ++++++++++++++++++++++- > > include/linux/mmzone.h | 7 ++++++- > > 2 files changed, 28 insertions(+), 2 deletions(-) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 485df9c2dbdd..772bd1fc6fe7 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -2294,15 +2294,36 @@ static inline int memdesc_nid(memdesc_flags_t mdf) > > } > > #endif > > > > +#ifdef CONFIG_NUMA > > static inline int page_to_nid(const struct page *page) > > { > > - return memdesc_nid(PF_POISONED_CHECK(page)->flags); > > + const struct page *p = PF_POISONED_CHECK(page); > > + > > +#ifndef NODE_NOT_IN_PAGE_FLAGS > > + ASSERT_EXCLUSIVE_BITS(p->flags, NODES_MASK << NODES_PGSHIFT); > > +#endif > > + return memdesc_nid(p->flags); > > } > > > > static inline int folio_nid(const struct folio *folio) > > { > > +#ifndef NODE_NOT_IN_PAGE_FLAGS > > + ASSERT_EXCLUSIVE_BITS(folio->flags, > > + NODES_MASK << NODES_PGSHIFT); > > +#endif47 > > This is getting ugly, really. We're leaking implementation details from > memdesc_nid() into folio_nid(). > > Maybe just turn memdesc_nid() into a macro where we can just do that check > internally? Not the best thing in this world, but better than this here. Could also do: if (!IS_ENABLED(NODE_NOT_IN_PAGE_FLAGS)) ASSERT_EXCLUSIVE_BITS(folio->flags, NODES_MASK << NODES_PGSHIFT); But not sure if it's that much better. (There's precedent for that form of it in mm/numa_memblks.c) > > -- > Cheers, > > David Thanks, Lorenzo