Re: [PATCH 1/1] Fix zero copy I/O on __get_user_pages allocated pages

[David Hildenbrand]

On 07.05.25 17:41, Pantelis Antoniou wrote:

Hi,

> Recent updates to net filesystems enabled zero copy operations,
> which require getting a user space page pinned.
>
> This does not work for pages that were allocated via __get_user_pages
> and then mapped to user-space via remap_pfn_rage.

Right. Because the struct page of a VM_PFNMAP *must not be touched*. It 
has to be treated like it doesn't exist.

> remap_pfn_range_internal() will turn on VM_IO | VM_PFNMAP vma bits.
> VM_PFNMAP in particular mark the pages as not having struct_page
> associated with them, which is not the case for __get_user_pages()
>
> This in turn makes any attempt to lock a page fail, and breaking
> I/O from that address range.
>
> This patch address it by special casing pages in those VMAs and not
> calling vm_normal_page() for them.
>
> Signed-off-by: Pantelis Antoniou <p.antoniou@xxxxxxxxxxxxxxxxxxx>
> ---
>   mm/gup.c | 22 ++++++++++++++++++----
>   1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 84461d384ae2..e185c18c0c81 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -833,6 +833,20 @@ static inline bool can_follow_write_pte(pte_t pte, struct page *page,
>   	return !userfaultfd_pte_wp(vma, pte);
>   }
>   
> +static struct page *gup_normal_page(struct vm_area_struct *vma,
> +		unsigned long address, pte_t pte)
> +{
> +	unsigned long pfn;
> +
> +	if (vma->vm_flags & (VM_MIXEDMAP | VM_PFNMAP)) {
> +		pfn = pte_pfn(pte);
> +		if (!pfn_valid(pfn) || is_zero_pfn(pfn) || pfn > highest_memmap_pfn)
> +			return NULL;
> +		return pfn_to_page(pfn);
> +	}
> +	return vm_normal_page(vma, address, pte);

I enjoy seeing vm_normal_page() checks in GUP code.

I don't enjoy seeing what you added before that :)

If vm_normal_page() tells you "this is not a normal", then we should not 
touch it. There is one exception: the shared zeropage.


So, unfortunately, this is wrong.

--
Cheers,

David / dhildenb