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 7DE2D42E429; Fri, 9 Oct 2026 20:24:01 +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=1791577442; cv=none; b=d3Jo+1t8/pwh8zdLe+sLOmPjSgjW0wwMO2kY4sgUXAoY+Cpx9K2zsXCNpTsOEN48RVTNsu5UCm1h8IOdP/D2+/dM5I1MPXRA2k2IqONvhaJZddxiu0ngrnLWNBhsaB37cYleKMd3Pu0wBHpeeQ3E5OY/lhHJeErw+xJ0KhPfFNo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791577442; c=relaxed/simple; bh=iqfZ336YK8sH24FWfln/9shTDe0VBAM0ax4mJsQvODc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JElt/HT5kfALrSWKMNu8jC/XR140fd7DmLtopy0uVR4v8b5yNqtTgCykLSaUIw2PGDeTllGLv0s80M2Fi+58SLrKCn6gYVLNhLeQ3gH5sOTMds8NEJJlcIBbcDm6ITK2IrM9IyOZgGqJ+QAqoYIBckdB5gmtMENb3/XDN65yLfk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Eqg3X0hc; 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="Eqg3X0hc" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6D7791F000FF; Fri, 9 Oct 2026 20:23:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791577441; bh=wCWQOptUF8jyyHqMADNE6+YL1lMNmRc+Jp6ycI/TeyM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Eqg3X0hcDl6uYeqx8paTuM4ul5j58X9NkSbvaiUbh3DZbjI0Nli2t1c3p5CYydvYp GDWlI5RahfJcXnkDAle0s7tUtplBeedzgpEenX+zpQKqsmpDoXBEnL1Q6gCyTcXlXk qmpadTTJJzn274+83PezcoAmemAEG7p/CrhOAX1wdmTkYh1vE28FxAsYTao5iGjey8 PORlHEJYdkAvMbd0JnRh9Ihn1PiKLYt13216sIw08UVYTI1s82l1YEK7Ty1Pa1UOM7 m6LVY7j437q4llZ8qLyElsoxfS9LmDc/1BGrHJkDLO1twn0ICCLRSmrnHzwWcruEDt 2lgN73spZM5aA== Message-ID: <8db35dc0-eb06-42e4-90f9-534522e810e1@kernel.org> Date: Fri, 9 Oct 2026 22:23:54 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: avoid transient none PMDs during lazyfree reclaim To: Zi Yan , Kyle Zeng , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , stable@vger.kernel.org References: <20261009165214.40212-2-kylebot@openai.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/9/26 20:17, Zi Yan wrote: > On Fri Oct 9, 2026 at 12:52 PM EDT, Kyle Zeng wrote: >> __discard_anon_folio_pmd_locked() clears and flushes a huge PMD before >> checking whether the folio can be discarded. If the folio was redirtied >> or has unexpected references, it restores the original PMD. Reclaim holds >> the PMD lock and the anon_vma read lock, but not the owning mmap_lock. >> >> Both zap_pmd_range() and mremap's get_old_pmd() can skip a none PMD >> without taking its lock. A concurrent munmap() or whole-VMA >> MREMAP_DONTUNMAP can therefore miss the PMD and later unlink the source >> VMA from its anon_vma, leaving a restored mapping behind. The anon_vma >> write lock taken by unlink_anon_vmas() waits for reclaim to finish, but >> does not repeat the skipped page-table walk. Once the remaining VMA >> links are removed, the folio's positive mapcount no longer guarantees a >> live anon_vma. Racing lazyfree reclaim against MREMAP_DONTUNMAP as an >> unprivileged user reproduces a KASAN use-after-free in >> folio_lock_anon_vma_read(). >> >> Use pmdp_invalidate() to keep a recognizable, non-none huge PMD while >> the discard can still fail. Concurrent unmap and move operations then >> have to synchronize on the PMD lock instead of skipping the mapping. >> Clear the invalidated PMD only after the dirty and reference checks have >> succeeded, before removing the rmap and withdrawing the deposited page >> table. The full invalidation retains the TLB flush required for the >> dirty and GUP-fast checks. >> >> Fixes: 735ecdfaf4e8 ("mm/vmscan: avoid split lazyfree THP during shrink_folio_list()") >> Cc: stable@vger.kernel.org >> Assisted-by: Codex:gpt-6-astra >> Signed-off-by: Kyle Zeng >> --- >> mm/huge_memory.c | 11 +++++++++-- >> 1 file changed, 9 insertions(+), 2 deletions(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 1e5d68acf62a..20262195385a 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3569,11 +3569,17 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma, >> return false; >> } >> >> - orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp); >> + /* >> + * The discard may fail, so keep the PMD non-none until we're >> + * committed to discarding it. Otherwise, concurrent munmap() or >> + * mremap() can skip the PMD without taking the PTL and later unlink >> + * the VMA from its anon_vma despite a restored mapping. >> + */ >> + orig_pmd = pmdp_invalidate(vma, addr, pmdp); > > There are some gaps we need to close before getting this fix in: > > 1. GUP-fast cannot follow the invalidated PMD: riscv and LoongArch need > pmd_access_permitted() that requires _PAGE_PRESENT; s390's > pmdp_invalidate() needs a change. Ugh. We really need pmdp_invalidate() to have reasonable semantics. This whole PMD locking is a mess :( > > 2. sparc64's thp_pte_count can be imbalanced with this change (based on > Lance's offlist feedback). Ack. > > In addition, powerpc has a page table check issue similr to riscv, where riscv > fixed it with commit 9f4a88b8d01a6. This is not related to this issue > but discovered along with the investigation. Zi, do you have the capacity to take over this patch? -- Cheers, David