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 4A49E2F8E88; Fri, 25 Sep 2026 14:02:51 +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=1790344972; cv=none; b=uM3J3t1hMf8GrEFIm+pjKVBe+mZ0RLBWDqFPXY3ti8EAz3DTkIq5G46CCsEubnfM9OVCfMaEZsZeVfCN5SFATmgzIqd87H6mO/p5k6bQcCGyL2ufx6J9G+Fo9B+4IvmkfuZT0I7tds7y0f+unbIavMO/GrCKOYjXnm6GtN4CmKM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790344972; c=relaxed/simple; bh=vJmM53T1gvLVt2GOugt91tr2ZoRcYIz6a2/D9x2PQuA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OX1aA/QxMGkdtNOh8ddLRafAPlPz8vXd0ROZoppHRCUDPwHGNxJT7M98C3NdMHPV0FVwKm9j5b6hMSpw3PHPWM51R+4PJ4Kw6rWK1i1H77CZoWKBgB5lA3iApEPPRVWlmRPVbq66bkrr9gy8k1/mGQY9dku7TMKvQDCz5jmIAeQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jVd9KAcu; 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="jVd9KAcu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A503F1F000FF; Fri, 25 Sep 2026 14:02:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790344970; bh=NNCsrq1Pv4PJ3tCFfQhR0ITmItYI/b9/JCcKk5G3gR4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=jVd9KAcuMu0TNXZeoByHF3KAzOSxm8XMBVZ6mQ6QBVKKxJTc7/cm529qR6DbrR8Io rAoWeoJCZM+eN7ARUYqNxWblUf31m2r8SH6cjJX46DD/Ph0kY85iYkqO+a6eDEYMZC faitM5TpGFrED073qp0WLAr9ejfunVcNc6wbGLBDwxjfqJ6mj9NfeZqPmKsVicfzjB 2cSe6opxwp84fTugfEwBHskJZj9y016emGRNn7jtQLkyaSHF38KgsAmK7vfWQHA37k y0fbpezVJaTCm7/xo1L1ImRh6Rh1nLMVf8GNdqXYNgD2hNuaoKD6HTGx2FeZlD8QO4 5QAZSjCeKoROQ== Message-ID: Date: Fri, 25 Sep 2026 13:28:30 +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] proc/task_mmu: handle special PMDs in clear_refs and pagemap To: Gregory Price , linux-mm@kvack.org Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, peterx@redhat.com, jgg@ziepe.ca, sashiko-bot , stable@vger.kernel.org References: <20260912122822.3348978-1-gourry@gourry.net> 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 9/12/26 14:28, Gregory Price wrote: > mshv_vtl_low can install a special PMD from a PFN supplied in the mmap file > offset. That PFN need not have a memmap entry. Reading /proc/PID/pagemap > or writing /proc/PID/clear_refs for such a mapping oopses the kernel. > > With a test driver mapping the PFN at the 1 TiB mark: > > echo 1 > /proc/self/clear_refs > > BUG: unable to handle page fault for address: fffffb5b80000008 > #PF: supervisor read access in kernel mode > #PF: error_code(0x0000) - not-present page > RIP: 0010:clear_refs_pte_range+0xb1/0x1e0 > Call Trace: > walk_pgd_range+0x50a/0xad0 > __walk_page_range+0x6a/0x1d0 > walk_page_range_mm_unsafe+0x193/0x230 > clear_refs_write+0x18e/0x3f0 > > pread(pagemap_fd, buf, 512 * 8, addr / PAGE_SIZE * 8) > > BUG: unable to handle page fault for address: fffff57840000008 > RIP: 0010:pagemap_pmd_range+0x3cf/0x6b0 > Call Trace: > walk_pgd_range+0x50a/0xad0 > __walk_page_range+0x6a/0x1d0 > walk_page_range_mm_unsafe+0x193/0x230 > pagemap_read+0x1dc/0x350 > > clear_refs_pte_range() passes the PMD to pmd_folio(), while > pagemap_pmd_range_thp() uses pmd_page() followed by page_folio(). > > Both paths assume the PFN has a struct page and dereference bad address. > > Use vm_normal_folio_pmd() and vm_normal_page_pmd() so clear_refs skips > folio operations and pagemap skips folio-derived flags for special PMDs. > This matches the PTE paths, which already use vm_normal_folio() and > vm_normal_page(). > > User-facing notes: > - Pagemap still reports PM_PRESENT and the PFN to a CAP_SYS_ADMIN reader. > > - For a special PMD backed by a valid memmap entry and for the huge > zero PMD, PM_FILE changes from set to clear. > > - PM_MMAP_EXCLUSIVE is already clear in both cases. > > - The output for an anonymous THP is unchanged. > > Tested in QEMU with the test driver on broken and fixed kernels. Both proc > operations oops before the fix and return 0 after it. > > Reported-by: sashiko-bot > Closes: https://sashiko.dev/#/patchset/20260912034833.2952750-1-gourry%40gourry.net > Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject") > Cc: # v6.19+ > Assisted-by: LLM > Signed-off-by: Gregory Price > --- Simple fix, let's CC-stable indeed (unless I am too late and this is already upstream :) ) Acked-by: David Hildenbrand (Arm) -- Cheers, David