Re: [PATCH v3 4/7] mm/gup: introduce the FOLL_SAME_FILE GUP flag

[David Hildenbrand]

On 15.04.23 14:09, Lorenzo Stoakes wrote:
> This flag causes GUP to assert that all VMAs within the input range possess
> the same vma->vm_file. If not, the operation fails.
>
> This is part of a patch series which eliminates the vmas parameter from the
> GUP API, implementing the one remaining assertion within the entire kernel
> that requires access to the VMAs associated with a GUP range.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>

[...]

> ---
>   						&start, &nr_pages, i,
> @@ -1595,7 +1603,7 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
>   	 * We want to report -EINVAL instead of -EFAULT for any permission
>   	 * problems or incompatible mappings.
>   	 */
> -	if (check_vma_flags(vma, gup_flags))
> +	if (check_vma_flags(vma, vma->vm_file, gup_flags))
>   		return -EINVAL;

FOLL_SAME_FILE is never set here, just pass NULL instead of vma->vm_file.


As we're not allowing to drop the mmap lock, why can't io_uring simply 
go over all VMAs once, after pinning succeeded, and make sure that the 
files match (or even before pinning)?

In most cases, we're dealing with a single VMA only, it's not like the 
common case is that io_uring pins accross 100s of VMAs.

So I really wonder if the GUP complexity is justified by something. 
(removing the VMAs is certainly a welcome surprise -- as it doesn't make 
any sense when used with FOLL_UNLOCKABLE).

--
Thanks,

David / dhildenb