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Howlett" , Michal Hocko , Muchun Song , Oscar Salvador , Hugh Dickins , Suren Baghdasaryan Subject: Re: [PATCH v3 1/4] mm: Introduce vm_uffd_ops API Message-ID: References: <20250926211650.525109-1-peterx@redhat.com> <20250926211650.525109-2-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Ra8xXZuBW3h5jcpHMWSOgHEYHt5m0THqKsi1HfBTrq4_1759258134 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: CD1DD1C000C X-Stat-Signature: qk15kxd935zygis1i3deqmw16q1ii6zn X-Rspam-User: X-HE-Tag: 1759258136-219334 X-HE-Meta: U2FsdGVkX18vxpObSwoplFQU3mIwVia8LaGTdXb1X1x8b0hrQrzXHcrdx5rWp9OzYtllzbKWF+78G6UQFKq2zC8BL7QN7jluNuI0TwBLBWAV4dG7zZ9Wzbfap0ZUAmc6wyXuz6cGi7+njb3l0GPAq9sEpsc1FiE0PX20MA5j/odM2FeelYgshoZAVhKJG4H5BHo3alfG/KPPceo9/UO6xp01LL6961zKZzzpH3TxFkTFApk5XE4SPTx0grOqrJDY4mcCw+LipcN9uj3q5x2nuVz3q4HVUt2i2YOjpQTJBgi/ZEEsdI2H2cnZ+irBFoOCWnpnO92unQLDj2LqU7oi+djd8+g7H04oleytGMRSpPROrqAAoGrjur0CHzQsBIiuHAth5XkPyzHKCgH3Z+Ge5oWnJ9d/X3diNWhLlPg17Or1yr6jgifLUl99/6EXLcM7RxLQKLuxkZV5EzsFTcQ0Zz89JWeOeI0cpAQpGFnft1jE03KGXqcsxOhjNGFxf8FM91iR+a5QH+e270QHc4UZ2sOJWSD0zqg3n72f5M4/phgtSsX1TpkHDdTsgaAsxPnUY8H01LYTK2w7eL4yVVsBYfXGKaPLLx9Xzbaf3rk5R1PZT4z0CaXyrYOuupslbg2gtIccC13DsM/4+LA/qDvveO5b8GyI4K4EvKWnV2VggdITVCSjCLHEmTzP3Eeyddy1STODagPUWDeLcxZFzLs5rfvWhaGeBFMUuEbJQPbM3+8+V5PQair990xKN1V0gepgVdAQ7sqHJFepljaxtuxZQAhGEmNHL0wBbnnaZop+Nf6xX/519IycR5XHbTJFB4xcozvBEwj5V3gbiLxESMD/8Il84gUWe5dNrk+fwJEPrshNKTjZEhZK2M/mejxw+0EqHUhjqcfD2/muMGhPRRMV/qTG+jwvAZ1wqcS+QfHLGjQu27gmi2lz+NDoLAnV9+JWcrbcd14fOiwY8ebsnJw 5hT+9EcE 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 30, 2025 at 11:36:53AM +0200, David Hildenbrand wrote: > > +/* VMA userfaultfd operations */ > > +struct vm_uffd_ops { > > + /** > > + * @uffd_features: features supported in bitmask. > > + * > > + * When the ops is defined, the driver must set non-zero features > > + * to be a subset (or all) of: VM_UFFD_MISSING|WP|MINOR. > > + * > > + * NOTE: VM_UFFD_MISSING is still only supported under mm/ so far. > > + */ > > + unsigned long uffd_features; > > This variable name is a bit confusing , because it's all about vma flags, > not uffd features. Just reading the variable, I would rather connect it to > things like UFFD_FEATURE_WP_UNPOPULATED. > > As currently used for VM flags, maybe you should call this > > unsigned long uffd_vm_flags; > > or sth like that. Indeed it's slightly confusing. However uffd_vm_flags is confusing in another way, where it seems to imply some flags similar to vm_flags that is prone to change. How about uffd_vm_flags_supported / uffd_modes_supported? > > I briefly wondered whether we could use actual UFFD_FEATURE_* here, but they > are rather unsuited for this case here (e.g., different feature flags for > hugetlb support/shmem support etc). > > But reading "uffd_ioctls" below, can't we derive the suitable vma flags from > the supported ioctls? > > _UFFDIO_COPY | _UFDIO_ZEROPAGE -> VM_UFFD_MISSING > _UFFDIO_WRITEPROTECT -> VM_UFFD_WP > _UFFDIO_CONTINUE -> VM_UFFD_MINOR Yes we can deduce that, but it'll be unclear then when one stares at a bunch of ioctls and cannot easily digest the modes the memory type supports. Here, the modes should be the most straightforward way to describe the capability of a memory type. If hugetlbfs supported ZEROPAGE, then we can deduce the ioctls the other way round, and we can drop the uffd_ioctls. However we need the ioctls now for hugetlbfs to make everything generic. Do you mind I still keep it as-is? So far that's still the clearest I can think of. It's only set when some support is added to a memory type, so it's a one-time shot. Thanks, -- Peter Xu