Re: [PATCH v5 00/26] userfaultfd-wp: Support shmem and hugetlbfs

Peter Xu
2021-07-19T20:38:03Z

On Mon, Jul 19, 2021 at 09:21:18PM +0200, David Hildenbrand wrote:
> Hi Peter,

Hi, David,

>
> I just stumbled over copy_page_range() optimization
>
> /*
> * Don't copy ptes where a page fault will fill them correctly.
> * Fork becomes much lighter when there are big shared or private
> * readonly mappings. The tradeoff is that copy_page_range is more
> * efficient than faulting.
> */
> if (!(src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP)) &&
> !src_vma->anon_vma)
> return 0;
>
> IIUC, that means you'll not copy the WP bits for shmem and,
> therefore, lose them during fork.

Good point.

I think the fix shouldn't be hard - we can also skip this if dst_vma->vm_flags
has VM_UFFD_WP set (that means UFFD_FEATURE_EVENT_FORK is enabled too). But
I'll check a bit into page copy later to make sure it works (maybe I'll add a
small test case too).

Thanks!

--
Peter Xu