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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: weekjtiz6j1setyjjyuz5bk61yardkru X-Rspamd-Queue-Id: EB6FDC0007 X-Rspamd-Server: rspam07 X-Rspam-User: X-HE-Tag: 1780299777-182719 X-HE-Meta: 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 /oU8FFI1 6uugXEYn0nPc45uDNtDF4GTgw59BI9RaKiRYiAONxb6qIVukLwMReY4iFmEv0woNuEps0PzSDklvBIX8DpH2ThoCQHE1CS7b0LChgmKV3y9tQwr2rN2g5Yr15OBE2uTjUKT0LlW2yHboL4CPxDFPGVyJHQNlBmFZdmcgJLzpkqGFtMho6CJcyGfZOZfZKWBbp2noJggtX35vlTyOrtKYlYEM3u56ZjIkCm28s6hBfmfmNMQBiy6zRbDd7syiSlO4ZyFnXbp68NqW0FyMA1nmvUil27sZ52mS38xbwzet78MH4f6vb3pfniXbnGJp9TakPn6Qz Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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