linux-kernel - Re: [PATCH v7 03/12] mm: introduce AS_NO_DIRECT

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Message-ID: <c8259ec7-e31d-4771-96f9-e2fb6b573e85@redhat.com>
Date: Thu, 25 Sep 2025 12:25:33 +0200
From: David Hildenbrand <david@...hat.com>
To: Patrick Roy <patrick.roy@...pus.lmu.de>
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Subject: Re: [PATCH v7 03/12] mm: introduce AS_NO_DIRECT_MAP

On 24.09.25 17:10, Patrick Roy wrote:
> From: Patrick Roy <roypat@...zon.co.uk>
> 
> 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).
> 
> Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Signed-off-by: Patrick Roy <roypat@...zon.co.uk>
> ---

I enjoy seeing secretmem special-casing in common code go away.

[...]

>   
>   	/*
> @@ -2763,18 +2761,10 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
>   		reject_file_backed = true;
>   
>   	/* We hold a folio reference, so we can safely access folio fields. */
> -
> -	/* secretmem folios are always order-0 folios. */
> -	if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
> -		check_secretmem = true;
> -
> -	if (!reject_file_backed && !check_secretmem)
> -		return true;
> -

Losing that optimization is not too bad I guess.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb