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b=ouTe4zgol/qqvYdtUesO6rnH51nqxNfQqJrFucoXdQT8nSDiy+4iUGLreqe9CTCts k34aRchUC67G4GJScTMgV7qLie0WngGjy0TGU84hGnBUS4j1bWOapDB6yiCQ2vjx3p IBIygbkEI4qXzBFZarYkRXARmIWZaoxRV/Hzb2+brMGPMV4WUu721CKEsFszcUlPzA AXoy597hfwzuk2KgOe2vTy9TXE2vAzRkMFHW0BjCj+Ke2BLD0t9Y2focow7JFgFOYr 90bvIuzU5MPLvWTusXIo5ChW1D09UBJYJnMloc+dtMa44O+5bIyDXrs8C6asHc5JwO /UHnkSkoCPfRw== Date: Thu, 18 Jun 2026 12:21:52 +0300 From: Mike Rapoport To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Linus Torvalds , Alexander Viro , Christian Brauner , Jan Kara , Oleg Nesterov , Peter Xu , vova tokarev , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org Subject: Re: [PATCH] userfaultfd: prevent registration of special VMAs Message-ID: References: <20260617194059.2529406-1-rppt@kernel.org> <5a993689-f730-406d-8515-8bb6025cc851@kernel.org> <41ef0dce-e973-4947-b5e3-150fdb07f1a6@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <41ef0dce-e973-4947-b5e3-150fdb07f1a6@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CB21F1C0009 X-Stat-Signature: 8snrt3se1bqus6m5kpqguni3ky9apg63 X-HE-Tag: 1781774520-187911 X-HE-Meta: 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 Opd0H2t0 Ece+iMBwsi3huxkZ5Yn+YidS2/D9ilRWXQ4bJT4Y2yFepflBJNgX+RcnKVA5PM7k+ZRVMsYU3KKjRgNQ8YssB/pSXEvCAsEyrYPljQ9wZvuX3PK6HA/2LDyKeJN68SQeGSDMvNLpzFrdl5NtkiwxLfrNZWjJWoLNRgnQPtofPP4V6v87wyq88XlaPyL9anUr0OrExSMs+aROa/5GPgoitZoT9OLZT6p4lRt7eWadCcUmTZ7mM2SnUhIwIN6yu9XOFdY5H Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 18, 2026 at 10:47:19AM +0200, David Hildenbrand (Arm) wrote: > On 6/18/26 10:43, Mike Rapoport wrote: > > On Thu, Jun 18, 2026 at 11:34:12AM +0300, Mike Rapoport wrote: > >> On Thu, Jun 18, 2026 at 10:19:17AM +0200, David Hildenbrand (Arm) wrote: > >>> > >>> I'm sure you considered VM_SPECIAL, which additionally includes VM_DONTEXPAND. > >>> > >>> Would that be better, or what was the reason to allow VM_DONTEXPAND? > >> > >> By itself VM_DONTEXPAND won't matter, as uffd can't resize a VMA. > >> But thinking more about it, it's better to make vma_can_userfault() more > >> restrictive and just use VM_SPECIAL. > > > > Ah, hugetlb sets VM_DONTEXPAND, so it must me excluded to allow uffd with > > hugetlb. > > It would probably be cleaner to just allow hugetlb, and then check for > VM_SPECIAL if not hugetlb. Cleaner in what sense? Will be uglier for sure, just take a look at vma_can_userfault(). > -- > Cheers, > David -- Sincerely yours, Mike.