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 86D09263C8C; Thu, 25 Jun 2026 08:28:20 +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=1782376101; cv=none; b=mpUvtlRrl2yu2lc6KEv07L8MaHXrsZYMuujdx6bJot2rEYLeNlS7ddsiHKN0q5m3pRJXmAQjVJYBlz0fJd0hvz+c0xmOBKHXT2u8yoxnu/eVqCk7NaIYYapUZcZBcSKfTnTXsTluHU9btjQtI4GAcf9agf6Zty4qqVnmPN3CuXA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782376101; c=relaxed/simple; bh=0arGaawYfZJvGNMwZzYpIeBiIHt3Cff+d5T1fy2Fglc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ADPXSBVuRRKEtlp1PnV21B6/yg0EbfK9UVtUHWOICmbd11oF7KmVOdR7LgZWUM2knunFi5wvo82Dw/RFWBjPnTGQjPzGeIfht4RwBBUWE+FjdKC0tNacFJjtudCmbRkbcnIn2nBFqQBIUuZyD3Qn5GVq65+A64evcM+5Nlmy5rI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Vv/BFSEI; 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="Vv/BFSEI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1395E1F000E9; Thu, 25 Jun 2026 08:28:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782376100; bh=egKHugauFVcD2XmzNTTw3d2qyTB8B7rTgdVmFNmqNrg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Vv/BFSEIrXCF8hk5q6CGRIvgWvOV5dQsjW4VlCOMp/qdSdxk12UM4sQxtefRbTzOp QdXuqVGkYsFyLEwaNMZag5GRtXKhEo8ddH4seU+ZQbFr4eI/RZwrVn0nrHd8cMr8Al hQGiJp1NVjM/TA4fwEyDEMewqt8B9VGV1NsEvo9G/Zbb6vVJV5UN2EZlr7XWRJpOco +VQoEcH1OlXNI1jpeU76sFyrYPyzIAS+aGyItEhL1vfE6sdJ36CvFo/azd+A8TfX4x Bru//XwEltp7BC/88i5rhtt3Op8hWVaFunUX/pAWuHTU+hoxF9iYMfsoqOtj42gs8M P+Prj8fBTAaLw== Message-ID: <237cdb97-5abc-4c89-a0cf-1a961425f947@kernel.org> Date: Thu, 25 Jun 2026 10:28:15 +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/rmap: use huge_ptep_get() in try_to_unmap_one() To: Dev Jain , akpm@linux-foundation.org, ljs@kernel.org Cc: riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, kas@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable@vger.kernel.org References: <20260625042853.2752898-1-dev.jain@arm.com> <614e89a8-5108-4ac1-bdf8-fecca48bd91b@arm.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 6/25/26 10:03, Dev Jain wrote: > > > On 25/06/26 1:26 pm, David Hildenbrand (Arm) wrote: >> On 6/25/26 06:28, Dev Jain wrote: >>> try_to_unmap_one() handles hugetlb folios when memory failure needs >>> to replace a poisoned hugetlb mapping with a hwpoison entry. In that >>> case page_vma_mapped_walk() returns the hugetlb entry in pvmw.pte, but >>> the code reads it with ptep_get() before decoding the PFN. >>> >>> That is wrong on architectures where hugetlb entries are not encoded as >>> regular PTEs. On s390, for example, a raw huge RSTE must be converted >>> by huge_ptep_get() before helpers such as pte_pfn() can inspect it. A >>> raw decode can select the wrong subpage, so try_to_unmap_one() can >>> install a hwpoison entry for the wrong PFN. >>> >>> The userspace-visible result is that a later access to the poisoned >>> hugetlb subpage can miss the expected SIGBUS. With DEBUG_VM, the wrong >>> subpage can also trip the PageHWPoison check. >>> >>> Use huge_ptep_get() for hugetlb mappings before decoding the PFN. >>> >>> Before c7ab0d2fdc84, the bug existed in the form of a plain dereference: >>> we would check the head page pfn of the hugetlb with pte_pfn(*pte), and >>> bail out on mismatch. This would mean that the hwpoisoned entry will not >>> get installed. >>> >>> I am not sure what is the procedure on such kinds of very old bugs - how >>> back should I really go? >>> >>> Fixes: c7ab0d2fdc84 ("mm: convert try_to_unmap_one() to use page_vma_mapped_walk()") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Dev Jain >>> --- >>> Applies on mm-unstable (d17fe8a046a2). >>> There are similar old bugs present, in try_to_migrate_one(), check_pte(), >>> remove_migration_pte(), prot_none_hugetlb_entry(). >> >> Yeah, we should handle all these cases properly. Can you send fixes? >> >> Using ptep_get() on something that's not a PTE entry is shaky on some architectures. > > I can send the fixes blaming the commit till which backport is relatively simple. The bug will > still remain before that, where we don't even do ptep_get(), just a plain dereference, if > that is fine. Probably no one is running pre-2017 kernels. The issue is that we would have to analyze in which cases exactly it would cause problems, like when migrating prot-none hugetlb folios on s390x, where pte_present() would not work as expected. I don't think any of us has time (or motivation) for that detailed analysis to make some odd hugetlb cases happy. So I'd say, let's just fix it in a simple way and be done with it. Use best-effort Fixes: but rather state in the patch description that this was found by code inspection and that the actual effects are unclear (e.g., pte_present() misbehaving on s390x), and using huge_ptep_get() is just the right thing to do. -- Cheers, David