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 D403B331A61; Fri, 9 Oct 2026 18:00:58 +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=1791568859; cv=none; b=CmwHv/Bh//nzWwTFjsW38SW9UuvQ2jTQA4YpQqmzUUbCEl40o1Jx/q24exCWQY8jGvgClmPvMpuFC1zvjtitZrCYNtz0hWYhSwNfyUNVqecc12de2hkK5SmZjbo6jndT4hPUSVvla9/bjS6J1HeAXmJpRjj3vfV+u7tKMvgTA7E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791568859; c=relaxed/simple; bh=a19n9BGRcuFVkg5Gk4SBfmtopyOgzvmAYzjjWMeCkEQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=reynOA1F0dkrZILmcQP/JfmJL5vEh5AyTMxEo9Sbo25vMhq0dltVrhJW7zLlz49Fl0alCo6eaB1en09wtykIdjRU3mgDtVV6ee0lSfRl3K0CZ8REf6eNZQqzEJKxn/H/JTmx8awp0rQtGPzXp3ysNtkJqm4WPMh0VDlTAbcyzkw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TpnCNeDk; 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="TpnCNeDk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B79C41F00893; Fri, 9 Oct 2026 18:00:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791568858; bh=UAqgduf0BQOvueONoyBqpc3wCA4v82uCh+xQrLLFBpI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TpnCNeDkFmKf8caNwk0vroIg025H+aS+gYvM6CtupjAtLu+gXm/BExjdya8nvjpWB t+ckV8jBREMsltwvhamM2VNB7rNhmEodT7Be0ZnXinw6l8X+JH67BSNgGcub1ombmp hbW8neetl2m2yRHXCi0UYPIgnzZrPGpoe6/dKcQ5mbRxaJM3PhMEi9/PxFHwJMdabN yVSH39t/34uF1d1e2fhRIiZqz3hfHe8nPVbCCa6eo/Ua1aHszkZGqI10ZzHw9KKAD/ h9FyFTGX4r4NpQ07MmS2w1RJVj4hw8e+PSnpFz8D5L1qB/XaOvfoxipGgMNwxaayDq 37G8bUkgklBRA== Message-ID: <54b5a63d-34fc-4bd8-beaf-19673c8b6ad7@kernel.org> Date: Fri, 9 Oct 2026 20:00:51 +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: Kyle Zeng , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Zi Yan , 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 18:52, 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. > + */ You can drop the entire comment. This is what we should do in most cases where the mapping might be restored/changed, for example, in set_pmd_migration_entry(). > + orig_pmd = pmdp_invalidate(vma, addr, pmdp); > > /* > * Syncing against concurrent GUP-fast: > - * - clear PMD; barrier; read refcount > + * - invalidate PMD; barrier; read refcount > * - inc refcount; barrier; read PMD > */ > smp_mb(); > @@ -3607,6 +3613,7 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma, > return false; > } > > + pmdp_huge_get_and_clear(mm, addr, pmdp); > folio_remove_rmap_pmd(folio, pmd_page(orig_pmd), vma); > zap_deposited_table(mm, pmdp); > add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR); Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David