Re: [PATCH v1 4/5] userfaultfd: zero access/write hints

[Peter Xu <peterx@redhat.com>]

On Wed, Jun 22, 2022 at 11:50:37AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> When userfaultfd provides a zeropage in response to ioctl, it provides a
> readonly alias to the zero page. If the page is later written (which is
> the likely scenario), page-fault occurs and the page-fault allocator
> allocates a page and rewires the page-tables.
> 
> This is an expensive flow for cases in which a page is likely be written
> to. Users can use the copy ioctl to initialize zero page (by copying
> zeros), but this is also wasteful.
> 
> Allow userfaultfd users to efficiently map initialized zero-pages that
> are writable. IF UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY is provided would map
> a clear page instead of an alias to the zero page.
> 
> For consistency, introduce also UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Acked-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu