Re: [PATCH v1 2/5] userfaultfd: introduce access-likely mode for common operations

[Peter Xu <peterx@redhat.com>]

On Wed, Jun 22, 2022 at 11:50:35AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Using a PTE on x86 with cleared access-bit (aka young-bit)
> takes ~600 cycles more than when the access bit is set. At the same
> time, setting the access-bit for memory that is not used (e.g.,
> prefetched) can introduce greater overheads, as the prefetched memory is
> reclaimed later than it should be.
> 
> Userfaultfd currently does not set the access-bit (excluding the
> huge-pages case). Arguably, it is best to let the user control whether
> the access bit should be set or not. The expected use is to request
> userfaultfd to set the access-bit when the copy/wp operation is done to
> resolve a page-fault, and not to set the access-bit when the memory is
> prefetched.
> 
> Introduce UFFDIO_[op]_ACCESS_LIKELY to enable userspace to request the
> young bit to be set.
> 
> 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: David Hildenbrand <david@redhat.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Hmm.. is the hugetlb code overlooked (for both of the hints), or maybe I
missed it?  Do we need to cover them too?  Thanks,

-- 
Peter Xu