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 7214837E5D7; Tue, 15 Sep 2026 04:49:11 +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=1789447752; cv=none; b=uTjFmo6zp7uv4uqyedqqdI7bDCERvQ3+r/7QK0t207aJLVkqAFeRdxZHxgResEscSOHZ5XYMzZwIUZe9Yo6OOD+lAo0ZitW5uVJogL/k9dxVbmud/rlrXX8fPrwnET0VZv5XMJeK3Vf7+YgZ67q9AZGAOVgyE4198BPTBGv+ACw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789447752; c=relaxed/simple; bh=vdkCvwfXIzsD0cJPJGZ7snyCG8l/wtQ6Xc3k96wpsF4=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=J1RRKxvAxSZI37M/iSB0uhdUmxd0zvKwGpiSlufnNUkFQjeoi80zdnSQcMkQ3vtA/w1+CaBO39/c8E/3G4M+f1lblS4XhraX0QBeGhcdJr91dWqxq+yGQ/Dbd2LrvJMjA/UyyYPFnb6so8e5LKWSi28YmXYUwdS3+tG+L3P4/qw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=Y1XOSA9m; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="Y1XOSA9m" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 72F641F000FF; Tue, 15 Sep 2026 04:49:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1789447751; bh=SBi1oH0/CTNATCkjFwhK+8zaxvaVE7CGcZmLnTYGtxo=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=Y1XOSA9m0nN9zLzax/SfzWcFsuLnMJKkYnksyk08t120nLnejZYf5YU5lNLBJWkHB eTboNh/c/oQJHhNFJBFoXaPY5X3tRDxtwiwMY94sW+i8jtMoCiWgsfQV82syPf1Fsd o32hT4L6yqA8zPT1Dl1pVl80dSp+UXQmqVu8VcJ4= Date: Mon, 14 Sep 2026 21:49:10 -0700 From: Andrew Morton To: Gregory Price Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, peterx@redhat.com, jgg@ziepe.ca, david@kernel.org, sashiko-bot , stable@vger.kernel.org Subject: Re: [PATCH] proc/task_mmu: handle special PMDs in clear_refs and pagemap Message-Id: <20260914214910.a22cfcdc250bfbb6b14edaf8@linux-foundation.org> In-Reply-To: <20260912122822.3348978-1-gourry@gourry.net> References: <20260912122822.3348978-1-gourry@gourry.net> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Sat, 12 Sep 2026 08:28:22 -0400 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(). > > Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject") > Cc: # v6.19+ Why cc:stable? Is a crafted test driver the only way of hitting this? > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1671,7 +1671,9 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr, > if (!pmd_present(*pmd)) > goto out; > > - folio = pmd_folio(*pmd); > + folio = vm_normal_folio_pmd(vma, addr, *pmd); > + if (!folio) > + goto out; > > /* Clear accessed and referenced bits. */ > pmdp_test_and_clear_young(vma, addr, pmd); > @@ -1991,7 +1993,7 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp, unsigned long addr, > goto populate_pagemap; > > if (pmd_present(pmd)) { > - page = pmd_page(pmd); > + page = vm_normal_page_pmd(vma, addr, pmd); > > flags |= PM_PRESENT; > if (pmd_soft_dirty(pmd))