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AFNElJ+iLKK3zMTuKb2ChIPlckVJVnuYSW28iBNALw5nG/WJi8NNyM7xU4Oyi08I4oHh//Mo8NltZryPqA==@kvack.org X-Gm-Message-State: AOJu0Yzf0yrIPdClSmhgUfyICbWqN0YhF0+gNeibs15b84t8hEDeLffy v/qDmwgxkPFtZK5ByeaJX/jRVhQj4ZOyJQ60OuOQUTNfITysk3qXktutCd6lj26n3cyTxem5K7g m6xMWcg== X-Received: from plsr2.prod.google.com ([2002:a17:902:be02:b0:2b2:48d8:c695]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:3c43:b0:2bd:c310:f7c9 with SMTP id d9443c01a7336-2bdc310fd7fmr94710675ad.2.1779136962102; Mon, 18 May 2026 13:42:42 -0700 (PDT) Date: Mon, 18 May 2026 13:42:41 -0700 In-Reply-To: Mime-Version: 1.0 References: <20260402041156.1377214-1-rppt@kernel.org> <20260402041156.1377214-14-rppt@kernel.org> Message-ID: Subject: Re: [PATCH v4 13/15] KVM: guest_memfd: implement userfaultfd operations From: Sean Christopherson To: Ackerley Tng Cc: Mike Rapoport , Andrew Morton , Andrea Arcangeli , Andrei Vagin , Axel Rasmussen , Baolin Wang , David Hildenbrand , Harry Yoo , Hugh Dickins , James Houghton , "Liam R. Howlett" , "Lorenzo Stoakes (Oracle)" , "Matthew Wilcox (Oracle)" , Michal Hocko , Muchun Song , Nikita Kalyazin , Oscar Salvador , Paolo Bonzini , Peter Xu , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , kvm@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, tarunsahu@google.com Content-Type: text/plain; charset="us-ascii" X-Rspam-User: X-Rspamd-Queue-Id: 7730240010 X-Rspamd-Server: rspam04 X-Stat-Signature: 34frpzccg7ddm76m4kfbddqkrokh31zw X-HE-Tag: 1779136963-964944 X-HE-Meta: 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 eR66AeXM 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, May 15, 2026, Ackerley Tng wrote: > Sean Christopherson writes: > > > > > [...snip...] > > > >> +#ifdef CONFIG_USERFAULTFD > >> +static bool kvm_gmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags) > >> +{ > >> + struct inode *inode = file_inode(vma->vm_file); > >> + > >> + /* > >> + * Only support userfaultfd for guest_memfd with INIT_SHARED flag. > >> + * This ensures the memory can be mapped to userspace. > >> + */ > > Is the principle here that any memory that is allowed to be mapped to > userspace can be userfault-ed? Yes? It's more the opposite though: memory that can't be SHARED and thus can't be mapped into userspace can't possibly support userfaultfd. Six of one, half dozen of the other, but I think it's important to highlight that there's no judgment call being made, i.e. we aren't deciding to support only SHARED just because we're lazy, it's a hard requirement.