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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: BaN5imEnCpW4WETLTQjVfPNRlM-b86tM8jPNGrjfATo_1759264540 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: 3D5E0C0013 X-Rspamd-Server: rspam05 X-Stat-Signature: umsrdfhauof8o1f34b9zwy91rpo8dros X-Rspam-User: X-HE-Tag: 1759264543-890736 X-HE-Meta: U2FsdGVkX1/8vTQeuyqi97GkTuROAckYyy6Yc+668ImffrGifNsAUKTlxCI0bmXgCzgvi8fYmkiNzA9A5JlzG4k/k29BLJt15ZZvzerBj+/wP4lsa6pKQXBxpXiwQQgKVOZ+k2psWduGNUTHvK41Hm9HTvJ7qtP0idYcH6NLRu9rIrZGoF1dRsxMecbcLuv2fBq4M5D/sAwiDYsDChYrhI9s08J/wyhl822sIm3DmF5HyOgeQidkik4dF5oh05nUaPyHqMEmdNy9UYv41mj5aX1XXt+Ho5izy6QDEE0+yxVnb5zknO4UROmHuOELTspT3th1xEdq4dTkVEuN/fwYn8YPOqD9gTQctXssueCqYSzvV3G2Kozk8VT0JHZzpE5junQMkEA/L9618M1A4GCY0BeWXOYdx7tpnlynZQTL0Ez9veEPmcPK9F7MxO3tG2XF6gac28oLL/2h27h8S3tGug3W2rT64z8EtIbiJTXUp27fwim337Hb/zzHhMDw1hB3ARZ5/DLqElGj57YI51njLjvF9EBiO88DrYdYzkXW9apSE/Iiq5lawa9CDTNHHx3m6W0XjH0NqiLjMMEt4LE1juCdnXKRNj/lrqct3HjOHdatefVLJ7sLxAVTvOm2nPdL5ieP0+nqcn/o/vxdB9rpwcfYy6TBlmDlRFDqgrkAYzxuweblrNBiEQufUJbPWruzqjMZgdh8q2PYKl6SUCgad2kk2V5r2ZAimhXHuOPXkncd4N0k8DBXMtWqFcpbthkfWWKb3O5sSznyIgUePRVCMoX+OixVApx8J349RhkN5oyyvjXBErK27ExdnWfUaRxZJTcktRFpYW9NmjafXEaVa2QtQrGQYsUmatTidPNqoFyb3Ck5SeypthOaXFvqcQpW7KBq6gI9zIGobCUwpGrdUeQE7tg16A0Ii4QZB5DU6aLRNkH2Q/immAClPjXDOzhKrr8gZkxLBWuY6qwRgLX 8RAJ5Z+u 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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 09:19:05PM +0200, David Hildenbrand wrote: > On 30.09.25 20:48, Peter Xu wrote: > > 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? > > The former would make things clearer when we are at least not talking about > uffd features. I'll go with it. > > > > > > > > > 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. > > I rather dislike the current split approach between vm-flags and ioctls. > > I briefly thought about abstracting it for internal purposes further and > just have some internal backend ("memory type") flags. > > UFFD_BACKEND_FEAT_MISSING -> _UFFDIO_COPY and VM_UFFD_MISSING > UFFD_BACKEND_FEAT_ZEROPAGE -> _UFDIO_ZEROPAGE > UFFD_BACKEND_FEAT_WP -> _UFFDIO_WRITEPROTECT and VM_UFFD_WP > UFFD_BACKEND_FEAT_MINOR -> _UFFDIO_CONTINUE and VM_UFFD_MINOR > UFFD_BACKEND_FEAT_POISON -> _UFFDIO_POISON This layer of mapping can be helpful to some, but maybe confusing to others.. who is familiar with existing userfaultfd definitions. > > > > 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. > > POISON is not a VM_ flag, so that wouldn't work completely, right? Logically speaking, POISON should be meaningful if MISSING|MINOR is supported. However, in reality, POISON should always be supported across all types.. > > As a side note, hugetlbfs support for ZEROPAGE should be fairly easy: > similar to shmem support, simply allocate a zeroed hugetlb folio. IMHO it'll be good if we do not introduce ZEROPAGE only because we want to remove some flags.. We could be introducing dead codes that nobody uses. I think it'll be good if we put that as a separate discussion, and define the vm_uffd_ops based on the current situation. > > > > > Do you mind I still keep it as-is? > > I would prefer if we find a way to not have this dependency between both > feature/ioctl thingies. It just looks rather odd. > > But let's hear if there are other opinions. Sure. -- Peter Xu