From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DDBEC14AD20; Thu, 18 Jun 2026 21:07:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781816839; cv=none; b=Io+tICpCnxDBr//IEE8GE7dJnkqdfQiu3+E1HW9uu97uYbsmH1IFtkQ/9bxFpq9D7d1OYoj24fZ2IZty/EhmREX804BvCs/r5Uw3fFmbUPaAHafCoR3S6PkXWCY+TlHYH84Hp3zvBs9AKGaMJ7s/0LSMj6pqXlGlnf1V7DQjgwQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781816839; c=relaxed/simple; bh=P8ahyLEUSY/L+t9kwvK/Sx6Q5A/PLuVM9mZGX59bw28=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=StP0+rGJhz+sga54TWavD/ZQ4fziksakm2Q80hX0G4X0O3fGb+E4AghDm0P5m/c7PnwiGvyQn1msLKefMJCFDxWGU/OjIFX8lgntEzh/Q/zlshjfssifFuTidUsLHidQdFwvKQcw4USWiG2Cl62s/BC3aJpGFAT98sZNZh9g/aA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EMcXFAty; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EMcXFAty" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7690C1F000E9; Thu, 18 Jun 2026 21:07:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781816838; bh=aKcLcoZc0XbVzuHW4tda6GVNppvLxUk/gwYmWzl7I5E=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=EMcXFAtyzSsNQyMRTvmswqzR7TyPIw3JlPF7ox/H40xevTpNXWy7pGoIjsCEI6xf/ thIFDhQ/Vyw5TM5ujiSWtbsyuf72sYpCGIXXtjQdMx1qGMKLc69RRmUipG+hsIQoA0 4UpMJyRXbNkMGK8GuC+uU4eS9/W788gto4jNVnywGjPC/Fm9mFEN9dQKyJO+48NZog pYQP4jQcWiu7idC1eUdSZq0cJJmIaQXWOaNslyDyoCYVbHhw70JragMXbYoChjWjMC ZouNe1jU1Nh8OUvJr8M5AbQV+mcylLVzAfEoiBPtpmM3x/KMyupSG7BBdXdm78t2mv 1z1qRP9+pJN5A== Message-ID: <9bbaa053-ea06-4b36-98ba-dc487a28964e@kernel.org> Date: Thu, 18 Jun 2026 23:07:13 +0200 Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: + userfaultfd-prevent-registration-of-special-vmas.patch added to mm-hotfixes-unstable branch To: Linus Torvalds , Andrew Morton Cc: mm-commits@vger.kernel.org, vladimirelitokarev@gmail.com, viro@zeniv.linux.org.uk, stable@vger.kernel.org, peterx@redhat.com, oleg@redhat.com, jack@suse.cz, brauner@kernel.org, rppt@kernel.org References: <20260618183442.BBCD71F000E9@smtp.kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/18/26 21:25, Linus Torvalds wrote: > On Thu, 18 Jun 2026 at 11:34, Andrew Morton wrote: >> >> Since VM_SPECIAL includes VM_DONTEXPAND which is set but hugetlb, exclude >> hugetlb VMAs from the check for VM_SPECIAL. > > This seems bogus. > > If somebody sets DONTEXPAND, then that mapping *is* special, and > userfaultfd should not mess with it. Right, it's one kind of special. > > It feels like hugetlbfs is just wrong to do this. > > This has caused problems before, see MADV_DODUMP which has that same > "hugetlb doesn't follow the rules" check. > > What exactly is it that hugetlbfs wants that DONTEXPAND thing to be? I tried digging into the history: hugetlb doesn't support VMA merging (or VMA expansion through mremap), apparently because of hugetlb reservation interactions. Just nasty. So I guess for userfaultd at least the VMA mergability/expansion doesn't matter. Hopefully. > > That said, I don't like the VM_SPECIAL bit mask all that much, because > it doesn't specify *what* kind of "special" it is. Right. It's documented as "Special vmas that are non-mergable, non-mlock()able." It's a bit weird that we throw both things into one bin but ok. And we use it in other context, like #define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB) Maybe we should rename and possibly split that up, like VM_NO_MLOCK VM_NO_VMA_MERGE And then have some generic "there are really special things mapped in here" VM_HAS_SPECIAL_MAPPINGS This might become a bigger (overdue?) audit, because I wouldn't immediately be able to tell why MLOCK would have a problem with VM_DONTEXPAND. VMA_REMAP_FLAGS is also odd, because it talks about "Physically remapped pages are special" but then also includes VM_DONTEXPAND. Any suggestion? -- Cheers, David