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[79.179.7.133]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-482fbb32d30sm12171637f8f.34.2026.08.29.11.42.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 29 Aug 2026 11:42:57 -0700 (PDT) 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="utf-8" Content-Transfer-Encoding: 7bit Subject: Re: [PATCH 6/6] userfaultfd: collapse VM_UFFD_{MISSING,WP,MINOR,RWP} into single VM_UFFD From: Tal Zussman To: Mike Rapoport Cc: "David Hildenbrand (Arm)" , "Lorenzo Stoakes (ARM)" , Andrew Morton , Baolin Wang , Barry Song , Dev Jain , Hugh Dickins , Jann Horn , Jason Gunthorpe , John Hubbard , Jonathan Corbet , Lance Yang , "Liam R. Howlett" , Masami Hiramatsu , Mathieu Desnoyers , Michal Hocko , Muchun Song , Nico Pache , Oscar Salvador , Pedro Falcato , Peter Xu , Ryan Roberts , Shakeel Butt , Shuah Khan , Steven Rostedt , Suren Baghdasaryan , Usama Arif , Vlastimil Babka , Zi Yan , linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org In-Reply-To: References: <20260823-uffd-vm-flags-v1-v1-0-3086981b33cf@kernel.org> <20260823-uffd-vm-flags-v1-v1-6-3086981b33cf@kernel.org> <37f6af95-950e-4457-886e-6548d4719f0d@kernel.org> Date: Sat, 29 Aug 2026 14:42:52 -0400 Message-Id: <178802897277.671075.10362065249796431311.b4-reply@b4> X-Mailer: b4 0.17-dev-db0b7 X-Developer-Signature: v=1; a=ed25519-sha256; t=1788028972; l=1768; i=tz2294@columbia.edu; s=20250528; h=from:subject:message-id; bh=8Gj2G76gEDUSTrJh9wyKhjlV0WDG+1riVaR0UmTV1c4=; b=cMv3WyS297uSy6OhyEPKL7TFiV6NgPyRlli6/52vFPU3U+wxk43IDrwyt65mNyPQi/updU28Q fXkQOPOdareAvP+hQQ2rgHqvMZY30DpDTGX/m0/lnTSj2MdkKXGNeAi X-Developer-Key: i=tz2294@columbia.edu; a=ed25519; pk=BIj5KdACscEOyAC0oIkeZqLB3L94fzBnDccEooxeM5Y= X-Proofpoint-Spam-Info: AW1haW4tMjYwODI5MDE2MCBTYWx0ZWRfX4bYrAt8Y0oDn KfkLqPqcaGu+xjJ+H/SifdUMRcTPsxvnr/fSooKJKjjty5PIeuZjLm4p17MgDjjaSKiIWnptINU S2aO9aWH8SyoPXSEnbzVjuzpY1oc2pbhA0mC1sEXsu7jytDG6/0o X-Authority-Analysis: v=2.4 cv=Gr5yPE1C c=1 sm=1 tr=0 ts=6a932833 cx=c_pps a=WeENfcodrlLV9YRTxbY/uA==:117 a=LRATYcvZeiQrLRB5MfbBvg==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=x7bEGLp0ZPQA:10 a=A0y_DWxS2BwA:10 a=VkNPw1HP01LnGYTKEx00:22 a=Da8U98TiO7q1upZEImrf:22 a=HpS3TJQ9O3Ob1ozEcmik:22 a=kX8B4L1VaituP6mSkuYA:9 a=QEXdDO2ut3YA:10 a=kacYvNCVWA4VmyqE58fU:22 X-Proofpoint-GUID: 7tX6LzBjHC9cVMlqnrZclYGiV4B8RWUh X-Proofpoint-ORIG-GUID: 7tX6LzBjHC9cVMlqnrZclYGiV4B8RWUh X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODI5MDE2MCBTYWx0ZWRfX8saCT/XFOoAG Cz1X1ussMgs2Tb46rogYXQtXWiYEysByaTwQeFdy4WZN2qiro/ywGow/dzRkdKva1RPERrwmUnD pVfsyeZQxezENtp4Xd0FXPb2Hp14N9VW6NAM1APdKqOV3cAE/INmtfIoNPKXx0xZpXTIokPgK9K 5Cj4INqp49KGM1ZAjCio4SOy62QgbAG7kMpEiiNHBHKOy9J/fNv+k73r/g/GCztBY7NMofRKYF5 SQMBJdZMoXvwKfltE08qCX/W2D84LedSlAkEG9IWnNz9Oax8K6gz8dGEMe5ylss2mOztho1ofHh QuvJGZXyIgd2b5ziAB9e2FaMQ2HB5C1bFZADZaGaaIgTeZbpmUtiL8tgvJl00JjP93Vf3t6LT3L PgHQHeSq15JucfrL07VGec3LEuLky1y4BJlBjtuAiYdSNdkamuaefU1ezTCvykBhYWTxPRs9aTh GnVlbRhZm23Ke8he7NQ== X-Proofpoint-Virus-Version: vendor=nai engine=6900 definitions=11890 signatures=596817 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 spamscore=0 lowpriorityscore=10 bulkscore=10 adultscore=0 impostorscore=10 suspectscore=0 clxscore=1011 malwarescore=0 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608290160 On 2026-08-29 14:00 +0300, Mike Rapoport wrote: > On Thu, Aug 27, 2026 at 05:19:30PM +0200, David Hildenbrand (Arm) wrote: > > On 8/27/26 13:18, Lorenzo Stoakes (ARM) wrote: > > > On Thu, Aug 27, 2026 at 12:16:32PM +0100, Lorenzo Stoakes (ARM) wrote: > > >> So actually you're increasing by a cacheline and increasing the VMA size by > > >> 64 bytes, i.e. 1/3, which is unacceptable obviously. > > >> > > >> Maybe there's something that can be done with: > > >> > > >> /* forced alignments: 1 */ > > >> > > >> Perhaps? But that looks potentially ugly. > > >> > > > > > > I say elsewhere (or think I do) but to highlight - I think probably we could fix > > > this by putting the flags in the low bits of vm_uffd_state.ctx? > > > > if that's possible that would be clearly preferable memory-wise. > > This gives only 4 bits and makes this completely not extendable. Wouldn't it be 6 bits? struct userfaultfd_ctx is allocated with kmem_cache_create() and SLAB_HWCACHE_ALIGN, and most of the flags are only available on 64 bit, so it should be 64-byte aligned in the relevant cases. (Not that 6 is that much better than 4... but it's a little more wiggle room.) It could in theory also be bumped up to 7 by setting align in kmem_cache_create(). userfaultfd_ctx already takes 192 bytes due to existing alignment. Aligning it to 128 bytes would make it 256 bytes, adding 64 bytes to each uffd rather than each VMA. But this sounds like more pain for little gain :) > So I think I'll drop this for now and wait until VMA grows another cache > line or until having per-VMA uffd state rather than a pointer to per-fd > context is a must. > > > -- > > Cheers, > > > > David > > -- > Sincerely yours, > Mike. > >