Re: [PATCH 1/2] mm/userfaultfd: prevent poison marker loss

[David Hildenbrand (Arm)]

On 11/7/26 00:51, David Hildenbrand (Arm) wrote:
> Unregistering userfaultfd leaves poison markers in the page tables.
> However, once unregistered, hugetlb PMD tables can be shared again.
> 
> Further, shmem page-table retraction can discard poison markers even
> while registered in missing mode, and fork() can skip copying page
> tables containing poison markers.
> 
> To fix it, add VM_MAYBE_UFFD_POISON and set it before installing poison
> markers. Preserve it across unregistration, splits, merges and fork(),
> just like we do with guard markers.
> 
> Use the new atomic VMA flag to disable PMD sharing and page-table
> retraction, and to force page-table copying during fork().
> 
> Move VM_UFFD_WP to bit 44 and make HAVE_ARCH_USERFAULTFD_WP depend on
> 64BIT, freeing bit 12 for the new flag on both 32-bit and 64-bit systems.
> Expose the flag as "up" in smaps.
> 
> Losing poison markers, while unpleasant, is not really fatal, and the
> use case is rather a niche thing. In addition, we rely on the atomic
> VMA flags infrastructure ... so let's not CC stable.
> 
> Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
> Fixes: 8a13897fb0da ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs")
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---

Ping on this one.

-- 
Cheers,

David