linux-kernel - Re: [PATCH 1/4] mm: allow guard regions in file-backed and read-only mappings

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Message-ID: <6500a93f-aad1-4b21-a94e-feb493c344a3@redhat.com>
Date: Tue, 18 Feb 2025 17:01:16 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
 "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
 linux-api@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
 Juan Yescas <jyescas@...gle.com>, Kalesh Singh <kaleshsingh@...gle.com>
Subject: Re: [PATCH 1/4] mm: allow guard regions in file-backed and read-only
 mappings

On 13.02.25 19:17, Lorenzo Stoakes wrote:
> There is no reason to disallow guard regions in file-backed mappings -
> readahead and fault-around both function correctly in the presence of PTE
> markers, equally other operations relating to memory-mapped files function
> correctly.
> 
> Additionally, read-only mappings if introducing guard-regions, only
> restrict the mapping further, which means there is no violation of any
> access rights by permitting this to be so.
> 
> Removing this restriction allows for read-only mapped files (such as
> executable files) correctly which would otherwise not be permitted.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>   mm/madvise.c | 8 +-------
>   1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6ecead476a80..e01e93e179a8 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1051,13 +1051,7 @@ static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked)
>   	if (!allow_locked)
>   		disallowed |= VM_LOCKED;
>   
> -	if (!vma_is_anonymous(vma))
> -		return false;
> -
> -	if ((vma->vm_flags & (VM_MAYWRITE | disallowed)) != VM_MAYWRITE)
> -		return false;
> -
> -	return true;
> +	return !(vma->vm_flags & disallowed);
>   }
>   
>   static bool is_guard_pte_marker(pte_t ptent)

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

I assume these markers cannot completely prevent us from allocating 
pages/folios for these underlying file/pageache ranges of these markers 
in case of shmem during page faults, right?

-- 
Cheers,

David / dhildenb