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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OESRhMpe; 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 DA2B7606A319 for ; Tue, 20 Oct 2026 10:33:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OESRhMpe Received: by smtp.kernel.org (Postfix) with ESMTPSA id DA6B91F000C4; Tue, 20 Oct 2026 10:33:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792492389; bh=FihuiNID/Pk5K/0ONXyOSOSNJje8QjS2ThIwMHmFPbk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=OESRhMpeLDUSaKFGrPf+6FmN6yHuBITxtUHklGUlHeJRR6FAaH6Yj3spEbEXpAejG oZO5WDKIm+m7WJLnydupi9P2UsZuT10iJxyqQr+/aG3ByqyQyl7/AtOqrw0QXMLgcu nvfjWrVd1F5fvqAIN0QxmCqMMdo7XwuMbfJDjMbQtuHBlFz4YH+Swf3+vv+YldUTMq qNnhocpZRPCy4vgJmr9QaoPVR4BKXcJSQFg5LDCvz2UuGIZFOREUm6eFO4RnpXopzv zvmTiuyZlMRYaoeRUwM63AFx++PevK7IAQtttP2Q1A1LMFVIH6q0ptEp7QZkOgmH63 w0/bi/wRRsSbQ== Message-ID: Date: Tue, 20 Oct 2026 12:33:05 +0200 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 2/3] mm/hugetlb: disable PMD sharing for UFFD missing mode To: Mike Rapoport , Li Zhe Cc: muchun.song@linux.dev, osalvador@suse.de, akpm@linux-foundation.org, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261019113034.97690-1-lizhe.67@bytedance.com> <20261019113034.97690-3-lizhe.67@bytedance.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 10/20/26 12:11, Mike Rapoport wrote: > On Mon, Oct 19, 2026 at 07:30:33PM +0800, Li Zhe wrote: >> From: "David Hildenbrand (Arm)" >> >> For UFFDIO_POISON to install a poison marker in a hugetlb mapping, it is >> sufficient to have the VMA registered for missing mode. >> >> However, UFFD missing mode does currently not block PMD table sharing, so >> installing the marker will be visible to everyone using the shared PMD >> table, which is wrong as UFFDIO_POISON must stay local to the process. >> >> To fix it, include VM_UFFD_MISSING in uffd_disable_huge_pmd_share(). Add >> a debug check on the marker installation path. >> >> The existing registration path will then unshare PMD tables, and >> subsequent faults will not share them while the mapping remains >> registered. >> >> The alternative would be to unshare PMD tables selectively when >> installing poison markers, which results in way more complexity. Or >> alternatively, simply failing the marker installation, which might break >> some use cases. >> >> This effectively disables PMD sharing for missing-mode registrations >> that never use UFFDIO_POISON. >> >> Suggested-by: Li Zhe >> Fixes: 8a13897fb0da ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs") >> Signed-off-by: David Hildenbrand (Arm) >> Signed-off-by: Li Zhe >> --- >> include/linux/userfaultfd_k.h | 13 +++++++++---- >> mm/hugetlb.c | 1 + >> 2 files changed, 10 insertions(+), 4 deletions(-) >> >> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h >> index a4351cffc60c..fe78ba14dc68 100644 >> --- a/include/linux/userfaultfd_k.h >> +++ b/include/linux/userfaultfd_k.h >> @@ -169,7 +169,10 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma, >> } >> >> /* >> - * Never enable huge pmd sharing on some uffd registered vmas: >> + * Never enable huge pmd sharing on the following VMAs: >> + * >> + * - VM_UFFD_MISSING VMAs, because UFFDIO_POISON may install markers and we >> + * want PMD tables to be unshared when installing these markers already. >> * >> * - VM_UFFD_WP and VM_UFFD_RWP VMAs, because the write protect / access >> * tracking information is per pgtable entry. >> @@ -182,9 +185,11 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma, >> */ >> static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma) >> { >> - return vma_test_any_mask(vma, >> - mk_vma_flags_from_masks(VMA_UFFD_WP, VMA_UFFD_RWP, >> - VMA_UFFD_MINOR)); >> + const vma_flags_t uffd_flags = >> + mk_vma_flags_from_masks(VMA_UFFD_MISSING, VMA_UFFD_WP, >> + VMA_UFFD_RWP, VMA_UFFD_MINOR); > > Since this essentially disable pmd sharing for any uffd mode, maybe replace > the flags check with the check for non-NULL userfaultfd_ctx? That would likely even better and we can just simplify the comments as well. -- Cheers, David