linux-kernel - Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT

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Message-ID: <7ef927d8-190d-4b22-8ec7-dcb9f5f75dba@redhat.com>
Date: Thu, 28 Aug 2025 23:00:00 +0200
From: David Hildenbrand <david@...hat.com>
To: "Roy, Patrick" <roypat@...zon.co.uk>,
 "seanjc@...gle.com" <seanjc@...gle.com>
Cc: "tabba@...gle.com" <tabba@...gle.com>,
 "ackerleytng@...gle.com" <ackerleytng@...gle.com>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>, "rppt@...nel.org"
 <rppt@...nel.org>, "will@...nel.org" <will@...nel.org>,
 "vbabka@...e.cz" <vbabka@...e.cz>, "Cali, Marco" <xmarcalx@...zon.co.uk>,
 "Kalyazin, Nikita" <kalyazin@...zon.co.uk>,
 "Thomson, Jack" <jackabt@...zon.co.uk>, "Manwaring, Derek"
 <derekmn@...zon.com>
Subject: Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT_MAP

On 28.08.25 11:39, Roy, Patrick wrote:
> Add AS_NO_DIRECT_MAP for mappings where direct map entries of folios are
> set to not present . Currently, mappings that match this description are
> secretmem mappings (memfd_secret()). Later, some guest_memfd
> configurations will also fall into this category.
> 
> Reject this new type of mappings in all locations that currently reject
> secretmem mappings, on the assumption that if secretmem mappings are
> rejected somewhere, it is precisely because of an inability to deal with
> folios without direct map entries, and then make memfd_secret() use
> AS_NO_DIRECT_MAP on its address_space to drop its special
> vma_is_secretmem()/secretmem_mapping() checks.
> 
> This drops a optimization in gup_fast_folio_allowed() where
> secretmem_mapping() was only called if CONFIG_SECRETMEM=y. secretmem is
> enabled by default since commit b758fe6df50d ("mm/secretmem: make it on
> by default"), so the secretmem check did not actually end up elided in
> most cases anymore anyway.
> 
> Use a new flag instead of overloading AS_INACCESSIBLE (which is already
> set by guest_memfd) because not all guest_memfd mappings will end up
> being direct map removed (e.g. in pKVM setups, parts of guest_memfd that
> can be mapped to userspace should also be GUP-able, and generally not
> have restrictions on who can access it).
> 
> Signed-off-by: Patrick Roy <roypat@...zon.co.uk>
> ---
[...]

> +static inline bool vma_is_no_direct_map(const struct vm_area_struct *vma)
> +{
> +	return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping);
> +}
> +

"vma_is_no_direct_map" reads a bit weird.

"vma_has_no_direct_map" or "vma_no_direct_mapping" might be better.

With the comment Mike and Fuad raised, this LGTM.


-- 
Cheers

David / dhildenb