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 C55BC44C4E9 for ; Fri, 2 Oct 2026 10:02:23 +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=1790935345; cv=none; b=laZQMxJwG0KZsL6b0pGLq2rmMS3uOA/zVbvPDVIjGi9hIRq+hpDHKjH+IgNYn80+vKiCyCf9tI39hG0ZP2vU+SGcuCggxlEv5O/x2zkz5T8qNEIdDqw+VxeBfsxENk0i3CKIkMBkfuJwo7s6yhFMfConyqEYWb51+Ck2fJ3tMDM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790935345; c=relaxed/simple; bh=ZJYcqZREpwQkGwKb0QLSDEUZFGMpgVMccIxQzeL7GZ8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=c+ae+92rPTJeticdl46idFgIVT51g9/pGMe/HaBMBKTOirt65I9s21kg1aQdYb7e6H3mOnex2MpUBgsvEpGuvXYK/BEq9PEhwApC4CTyYn07G58yUzZQfYwpWHKhAQ0aGrDEDMwOrJFnlhtel0bf8fLgC8ozQJIvtPuv5z9z/SY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Ecu3Fk6+; 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="Ecu3Fk6+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 881761F000FF; Fri, 2 Oct 2026 10:02:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790935343; bh=2CRmv+Y72dCsjIW8cbfqo0pAeQe5Fj28kqP9pHRvXyY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ecu3Fk6+BBTt28XCj/0byPzPkF3oH2shcUtw4oGlJngrRwn1Kj8+1wkGu9qx2r/Dm EqZ7Ms6pF87RRV7v8j7pImt2PTOzQt9j2kIo7RHaXDzq8rDlmqAB9qpi2rb3C/4qCn Pb8wbIDWzOymVkDaUEBUeKNjSJICBfmbIb/dGwBOSluCbLl27Tb8HIyHSCKTSYfmjC CkUyGNTvklmlPMrIsljFGeMQko9k9oWWc1U+ky3Y6tzCPJ9BgT4oB1llYYNFFl/evs 2piDM0acF9bX0KpAjim+Cwelo79TjED+uOqgLLoTD7OvYLMKK8QiZ4vhQ6gwsM5SbX r5GMYWudfthVQ== Message-ID: Date: Fri, 2 Oct 2026 12:02:15 +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: don't ioremap COWed anon pages in generic_access_phys() To: "Lorenzo Stoakes (ARM)" Cc: Nguyen Ngoc Thang , akpm@linux-foundation.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, peterx@redhat.com, dave.hansen@linux.intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+49b1021becba70c1f3f6@syzkaller.appspotmail.com References: <699ae98c.050a0220.340abe.0d31.GAE@google.com> <20261001152524.171115-1-ngocthang2710.1999@gmail.com> <32fe40e2-854a-47ec-9d95-93e23f9a7af4@kernel.org> 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 10/2/26 10:38, Lorenzo Stoakes (ARM) wrote: > On Thu, Oct 01, 2026 at 10:22:29PM +0200, David Hildenbrand (Arm) wrote: >> On 10/1/26 17:25, Nguyen Ngoc Thang wrote: >>> A MAP_PRIVATE mapping of iomem (e.g. a PCI sysfs resourceN file) is a >>> COW pfnmap: a write fault replaces the pfn with an anonymous page, which >>> remap_pfn_range() allows by keeping vm_pgoff equal to the base pfn. >>> >>> generic_access_phys() does not tell those COWed pages apart from the >>> original pfns and ioremaps whatever the PTE points to. Reading such an >>> address via /proc/pid/mem or ptrace then ioremaps RAM: >>> >>> ioremap on RAM at 0x0000000045623000 - 0x0000000045623fff >>> WARNING: arch/x86/mm/ioremap.c:216 at __ioremap_caller.isra.0+0x4c2/0x5f0 >>> Call Trace: >>> generic_access_phys+0x130/0x4d0 mm/memory.c:7178 >>> kernfs_vma_access+0x1ce/0x280 fs/kernfs/file.c:437 >>> __access_remote_vm+0x58f/0x890 mm/memory.c:7256 >>> mem_rw+0x2a1/0x670 fs/proc/base.c:912 >>> >> > > Sorry to say this looks schlopped. > > This guy has sent 10 series across 6 subsystems over ~21 hrs: > > https://lore.kernel.org/all/?q=f%3Angocthang2710.1999%40gmail.com > > Nguyen - please do not flood the kernel with patches, and please use the > Assisted-by tag for generated content. > > The original code you submitted is really not great even if the issue may > be valid. > > So I'd say somebody from the core team should take over this if we want to > come up with a patch. Yes, I'll take care of it. [...] >> >> Signed-off-by: David Hildenbrand (Arm) > > This looks reasonable but I hate that we have 'special' CoW overrides like > this :) After sending this yesterday, I concluded that we can do this cleaner: just have bool normal_page; (naming suggestions?) that express that this is something refcounted with a struct page, like documented for vm_normal_page(). Then we can just refuse all of these. > >> --- >> include/linux/mm.h | 4 +++- >> mm/memory.c | 48 ++++++++++++++++++++++++++++++++++++++++------ >> 2 files changed, 45 insertions(+), 7 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index c49ef99b4413..b90e547797a9 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -3193,6 +3193,8 @@ struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma, >> unsigned long addr, pmd_t pmd); >> struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, >> pmd_t pmd); >> +struct folio *vm_normal_folio_pud(struct vm_area_struct *vma, >> + unsigned long addr, pud_t pud); > > Hmm, if not defined before why would this need a new PUD handler? Do we > even have PUD-leaf PFN mappings? Yes we do. In any case, good for consistency. [...] >> + args->cow = folio && folio_test_anon(folio); > > I actually wonder if this should be changed to folio_has_anon_rmap() at > some point :) Not a fan. > > Since a folio being 'anon' is vague, because we stupidly made 'anon' vague > in general. For folios it's an established term :) > > Anyway I was going to ask does this suffice for CoW but having an anon rmap > implies CoW so it does. The downside of using "bool normal_page;" is that we should check vm_normal_page() for any mapping, not just cow mappings. I suspect performance-wise we don't really care. -- Cheers, David