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 7A91E341AC7; Mon, 1 Jun 2026 07:42:57 +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=1780299778; cv=none; b=jhjk0OWkuVVf9xZGYKOeFSb9JWrkVaefyIkjTi9PkgdxBURT2YqDM3pVRtuVHAzzy2ZROeCwaIhcQNOs8O+Z3yLdAVpCncZE29+W/kPPaNQF/8Y1+4O9WeEoIFzGbeHIbzFPAs3oXLoNsKIEyRliMgyvc5p7S7mDy9ghw5yoHCg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780299778; c=relaxed/simple; bh=LBRASCQNybdy4by4isiOs6IW4BLbct2fZE2vZspbdHw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=IzgNpXg4SzHqkEV3jy6DD1xs65JuT0TnZa9qYD3NlCqFhUOktQYQKvtFGQMkj2KGi4kzMXQPusDRA6QA+aPnSSbTtjee6efqBxpDnT9d14dyKHlr7cpDJc3XKIbfLlVZjvmMFGTqRVtYsxhg6xJH9yS5umG8Lg6NEAWbgeysW0g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RJ1pWFC1; 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="RJ1pWFC1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 342CD1F00893; Mon, 1 Jun 2026 07:42:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780299777; bh=G6E/427vKjNMrNFnfzJ5w/oioAQsHShjLOWHUrX6Zb8=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=RJ1pWFC19TPXp/omPArHxv5x2qOq16KOXdKlqOHHweNUSE0B0kXCuHNmYIa29VeAv x1xukkrYT0vEFqTjKybI9CMv+G5k3YQfnYB7JlHdtGpKnzVF72J3cklM5PUgBuWYPT Zb85JfREbCwhnT+mX5TVtU5r6x789OLRNYsvMsTkvBSMq9BEmXMyFMwfSMfWkTM+YD zJfWmnK/xP4OSww3ENvvdeuDgg8mYVJjxGxRuRKS8rHEcyubNuUc6uwlcM8SfIhTRP fN0lzfo7D0m4HnqdibY2CcGk5s9ocXceGp/B64TNx44+GE5hErGZbYDwEUb79kCi5M xO2iV1myeeJyw== Date: Mon, 1 Jun 2026 08:42:48 +0100 From: Lorenzo Stoakes To: Mike Rapoport Cc: Kiryl Shutsemau , akpm@linux-foundation.org, peterx@redhat.com, david@kernel.org, surenb@google.com, vbabka@kernel.org, Liam.Howlett@oracle.com, ziy@nvidia.com, corbet@lwn.net, skhan@linuxfoundation.org, seanjc@google.com, pbonzini@redhat.com, jthoughton@google.com, aarcange@redhat.com, sj@kernel.org, usama.arif@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org, kernel-team@meta.com, "Kiryl Shutsemau (Meta)" , stable@vger.kernel.org Subject: Re: [PATCH v5 04/18] mm: skip out-of-range bits in mk_vma_flags() Message-ID: References: <20260526130509.2748441-1-kirill@shutemov.name> <20260526130509.2748441-5-kirill@shutemov.name> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sat, May 30, 2026 at 07:52:25PM +0300, Mike Rapoport wrote: > On Fri, May 29, 2026 at 03:00:14PM +0100, Lorenzo Stoakes wrote: > > On Tue, May 26, 2026 at 02:04:52PM +0100, Kiryl Shutsemau wrote: > > > From: "Kiryl Shutsemau (Meta)" > > > > > > vma_flags_t is one unsigned long on 32-bit -- NUM_VMA_FLAG_BITS == > > > BITS_PER_LONG by design, so VM_xxx-declared bits sit in the first > > > word and hit the single-long fast path. But the bit enum declares > > > some bits unconditionally above BITS_PER_LONG (VMA_UFFD_MINOR_BIT > > > == 41 today, with VM_UFFD_MINOR == VM_NONE on 32-bit so no VMA > > > actually carries the bit). > > > > Yeah ugh. > > > > > Passing such a bit to mk_vma_flags() goes through __set_bit(41, > > > &one_long) and writes one word past the end. The compiler folds > > > the OOB store with wraparound (1UL << (41 % 32) == bit 9) into > > > the first word. Bit 9 is already in __VMA_UFFD_FLAGS so the mask > > > happens to come out right today, but any high-numbered bit whose > > > > That is... helpful :) but not great that this is the situation, an > > oversight, clearly! How I hate 32-bit kernels :) > > > > > mod-BITS_PER_LONG position is otherwise unused would silently OR > > > an extra bit into the mask. > > > > > > Add VMA_NO_BIT and have DECLARE_VMA_BIT() resolve any bitnum out > > > of range to it. vma_flags_set_flag() drops negative bit values. > > > The ternary collapses at compile time, the runtime check folds > > > away when the bit is in range, and the common path is unchanged. > > > > Hmm are you sure it does? > > > > A key design goal was that mk_vma_flags() generates compile-time constants > > the same as if the bitmap were constructed independently. > > > > This surely must generate code? Or at least runs a significant risk of it? > > ... > > > A simple solution that doesn't require change to the core is to just uglify > > userfaultfd_k.h a bit with: > > > > #ifdef HAVE_ARCH_USERFAULTFD_MINOR > > #define __VMA_UFFD_FLAGS mk_vma_flags(VMA_UFFD_MISSING_BIT, VMA_UFFD_WP_BIT, \ > > VMA_UFFD_MINOR_BIT) > > #else > > #define __VMA_UFFD_FLAGS mk_vma_flags(VMA_UFFD_MISSING_BIT, VMA_UFFD_WP_BIT) > > #endif > > > > But of course that becomes much more horrible with your changes... > > > > Another alternative, which I used for VMA_DROPPABLE is to add something > > like this in mm.h: > > > > #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR > > #define VM_UFFD_MINOR INIT_VM_FLAG(UFFD_MINOR) > > +define VMA_UFFD_MINOR mk_vma_flags(VMA_UFFD_MINOR_BIT) > > #else > > #define VM_UFFD_MINOR VM_NONE > > +define VMA_UFFD_MINOR EMPTY_VMA_FLAGS > > #endif > > I have a PoC of yet another alternative: > > https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=uffd/vm-flags > > The idea there is to keep a single VMA flag, VMA_UFFD_BIT/VM_UFFD and move > all the rest into what's now struct vm_userfaultfd_ctx. *Gives Mike a big kiss* YES PLEASE! > > -- > Sincerely yours, > Mike. Thanks, Lorenzo