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

[David Hildenbrand <david@redhat.com>]

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@oracle.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@redhat.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