From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 66F70606A323 for ; Tue, 27 Oct 2026 08:06:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=vbj43Q1Z; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=BoIvLyS4 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Unsubscribe-Post:List-Owner; bh=tGP0LjdymEX/O70OpFiXykL3tGWdkhpJkzQJ0xiHk2s=; b=vbj43Q1ZudlFBYcXIIJ0VON+Im /nbGJU13EQDshWJQtiinl8krOirnXI4HZQf/P5EhkEgDfch89kHZv1OoMIyaMBkzFJaZZErGwTRLl y6tqG6FKH96EOetjQ+7acnKa/AdeLenB5Re5JJQXVHCDFLI7VNMlL4S2g2FvC7KN/1qs0Ij/HleL1 VOnbpOZL5IHteluiha3iQwtYtY3XtVRuHZpWST+oyWqQXh+2jahPElAIyYxqMpZp0LXOeB/3UqqYR AW8mQfAHRjJo/ZVVz4mWLOH9gVLWzcNxW7Fn+sYHc0w4T6GUZHUBfteaKJC2BDnFQh2cCMN384TxY A4tTYbGw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLcCo-00000003zh1-3Vdp; Tue, 27 Oct 2026 08:06:50 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLcCm-00000003zgk-3lJJ for kexec@lists.infradead.org; Tue, 27 Oct 2026 08:06:48 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 44F2243BFB; Tue, 27 Oct 2026 08:06:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7669F1F01825; Tue, 27 Oct 2026 08:06:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793088408; bh=tGP0LjdymEX/O70OpFiXykL3tGWdkhpJkzQJ0xiHk2s=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=BoIvLyS4BeMWr65hedSikSZ5vE8O9JRgtibUW/piTLatgtmus034g8LURA3R+gHJc ydIDPqS/Oq7d7JsbwgaioD3d1cV8JjOomy7xzfXW42dsVa5rjkstyyKLDXODrEKei/ vB2Qwp8dvj5O3AKG6ZMVvpIywISsqf/obj5nk6KOVMT8qLApC1PN0PM0ZrjYBAf2Xu 5g6N2XtIXQ3TbybogpP04e5dcBVdBVy+cHCzH1jEUnk9T+8vtYavS1rB+QxhcGQdgH FThLB2pNS03O7e0SOprSN4uGF5bozDM1GQt3OLElmNxQv6g/clWdchbL3nMOAmhEFL j+r5zdUmZqxow== Date: Tue, 27 Oct 2026 08:06:31 +0000 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, Josef Bacik , stable@vger.kernel.org Subject: Re: [PATCH 01/46] mm/mglru: don't use stack pointers when walking PUD and PMD tables Message-ID: References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-1-6f74bec9a5bb@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261027-fixes_core-v1-1-6f74bec9a5bb@kernel.org> X-BeenThere: kexec@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "kexec" X-Spam-Status: No, score=-16.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Oct 27, 2026 at 12:17:21AM +0100, David Hildenbrand (Arm) wrote: > From: Josef Bacik > > MGLRU passes stack copies of page-table entries in walk_pud_range() and > walk_pmd_range() when walking to the next level. These copies can become > stale when khugepaged or MADV_DONTNEED (CONFIG_PT_RECLAIM) removes a PTE > table concurrently. > > This pattern dates back to commit bd74fdaea146 ("mm: multi-gen LRU: > support page table walks"). Likely, earlier locking masked the latent > bug. Later changes exposed it: commit 1d65b771bc08 ("mm/khugepaged: > retract_page_tables() without mmap or vma lock") allowed concurrent > retraction, and commit cf4a65539c13 ("memcg-v1: no need for memcg locking > for MGLRU") dropped the walker's outer RCU protection. > > The fix is simple, so just fix the original commit so we don't have to > speculate. To fix it, pass the actual PMD/PUD pointers. This restores the > RCU-protected lookup and validation against the live PMD. Note that the PUD > handling is required as well to handle folded page tables correctly in all > cases. > > Fix generated by an LLM and sent by Josef Bacik in private. > > Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks") > Cc: stable@vger.kernel.org > Signed-off-by: Josef Bacik > [ david: handle walk_pud_range() as well, clarify+simplify description, > adjust Fixes: ] > Signed-off-by: David Hildenbrand (Arm) > --- God, terrible. And why does MGLRU feel it needs its own independent page walking mechanism?... the whole thing feels like NIH syndrome writ large. Anyway, rant aside: Reviewed-by: Lorenzo Stoakes (ARM) > mm/vmscan.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 9959ca158ce83..6a1af9b92349f 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -3911,7 +3911,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end, > > walk->mm_stats[MM_NONLEAF_FOUND]++; > > - if (!walk_pte_range(&val, addr, next, args)) > + if (!walk_pte_range(pmd + i, addr, next, args)) > continue; > > walk->mm_stats[MM_NONLEAF_ADDED]++; > @@ -3947,7 +3947,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end, > if (!pud_present(val) || WARN_ON_ONCE(pud_leaf(val))) > continue; > > - walk_pmd_range(&val, addr, next, args); > + walk_pmd_range(pud + i, addr, next, args); > > if (need_resched() || walk->batched >= MAX_LRU_BATCH) { > end = (addr | ~PUD_MASK) + 1; > > -- > 2.43.0 > -- Cheers, Lorenzo