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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) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E917340002 X-Stat-Signature: zguj9kc9mnbbnbq1brem66gdgsanc7nc X-Rspam-User: X-HE-Tag: 1789447751-33443 X-HE-Meta: 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 EHtgAadf 7MvsnfvrHecjS/mx2uf6J2t643nYRBgmE4IucZ3CkmrIE/YguYvUWp8VTduUhwbGaSmLMfzP+lBnb31EqTpv2yzOLder5QIeG0gNj0r/uHn7knv9ni7UPDULQMqsdtuAFZCYMx9/ItWHZbEWLLrzL81/ULlZhcnUr0ZKAUKhJjkCnp5WUXyoU0Mq5ld6gdOC79B2Auo3F8jsUE+Qjw3eWGdmR7i9knJjiqTBbZQfB2UDmmz8e/2+WAf/vTnEtU0yM9cn6abzgr1exyGK2BIhXXUm6o8KO6PW7SjyFzUe5r0htHGu3IN5JgUHG4qZjswps8gUjVp0a09leV/fn3/uI9fDuPFEjzfQiM6bbTPnidYCla9eL8KuqRgXR4gloi5pF927J Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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))