linux-kernel - Re: [PATCH v4 01/12] mm: introduce AS_NO_DIRECT

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Message-ID: <3dc4bb80-0beb-4bbb-bfd8-47fc096f70e9@redhat.com>
Date: Tue, 25 Feb 2025 17:52:02 +0100
From: David Hildenbrand <david@...hat.com>
To: Patrick Roy <roypat@...zon.co.uk>, rppt@...nel.org, seanjc@...gle.com
Cc: pbonzini@...hat.com, corbet@....net, willy@...radead.org,
 akpm@...ux-foundation.org, song@...nel.org, jolsa@...nel.org,
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Subject: Re: [PATCH v4 01/12] mm: introduce AS_NO_DIRECT_MAP

On 21.02.25 17:07, Patrick Roy 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.
> 
> 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 gfp_t mapping_gfp_mask(struct address_space * mapping)
>   {
>   	return mapping->gfp_mask;
> diff --git a/lib/buildid.c b/lib/buildid.c
> index c4b0f376fb34..80b5d805067f 100644
> --- a/lib/buildid.c
> +++ b/lib/buildid.c
> @@ -65,8 +65,8 @@ static int freader_get_folio(struct freader *r, loff_t file_off)
>   
>   	freader_put_folio(r);
>   
> -	/* reject secretmem folios created with memfd_secret() */
> -	if (secretmem_mapping(r->file->f_mapping))
> +	/* reject secretmem folios created with memfd_secret() or guest_memfd() */
> +	if (secretmem_mapping(r->file->f_mapping) || mapping_no_direct_map(r->file->f_mapping))
>   		return -EFAULT;

Maybe I'm missing it, but why do we have to special-case secretmem with 
that at all anymore?

Couldn't we just let secretmem set AS_NO_DIRECT_MAP as well, and convert 
all/most secretmem specific stuff to check AS_NO_DIRECT_MAP as well?

-- 
Cheers,

David / dhildenb