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b=ErTE6kd7JtWaVBDAoXu2s7i33/fLSNJOgv99y2jirEIh5FRREhKyHEBpPSt60wuGB AF/SOzPfIvWwO0hK4X+wKLOyMMpot4HDZOhFrm7Sj/C3AKPURyGBLHLnNVflxDtTgB dBfmMUVNRAHFI1XwUKnpoxRso8AxGDhPJwVh+jfDUkiaMr529dMx/JZ4Ossvp9Y3KS hBplQJ73LWwxWvMO/09GaoCXEwwOdgi15VCVTwkzR5ITTHqJ8rlCkZCdIbRmYtOwM8 t4MJQK6U335Nd8iHwPDAoAaDYkjqoEojWMw8xzoyXjqqKTDiA1RqrlWDGy931gkTgu EvuS8mB+7V7Ow== Date: Sat, 30 May 2026 19:52:25 +0300 From: Mike Rapoport To: Lorenzo Stoakes 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-Rspamd-Server: rspam12 X-Stat-Signature: 9d9m1yzw7qtwfejbcwq7wouczannwpo6 X-Rspam-User: X-Rspamd-Queue-Id: EB05E80010 X-HE-Tag: 1780159956-103190 X-HE-Meta: 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 RxdJ4ZmC 1r6He7SG22zdyPcvlgE5iuXAQ3XhXwfrgu2Fq62XnXfx5oWwkHr6mX9YcpeIvrOdTtMLTZSEtx08e5UYJiY2C8eRpYQ+jo1Fv9rhgqPEqGVXFNvDy/t17gD3M8BHKPuJVeDJCqY0u9E5RSkvEt6pvM0r7FjhYMAiI8vGb1cQ8xcKnr71LUj7L7y1UPLmQABwXHOdj5o02I8jIPJtHAMEgmLcr0g/0Ap0McLEUcd9zDHQ2goVLj3HN+EVMDcHx2DuzDBWCDqvPgu0bdecvk0tXvAc/xkc5sRiN85HPA5aWhvHO0KLj5fo9lJDiaVPlcurSZlxyua4zguaV+xTG7rOnTixyCz4zk8fQ4OZ8Cp/HfFAv1pQz2FZJBhs3k6H+qTcVXySDC1HERA+TbfVFEE+0sJkowEFjU/PVLqbfWysVm87coQghrKQAXXjV6g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. -- Sincerely yours, Mike.