On Wed, Jun 22, 2022 at 11:50:35AM -0700, Nadav Amit wrote: > From: Nadav Amit <namit@xxxxxxxxxx> > > 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@xxxxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> > Cc: Peter Xu <peterx@xxxxxxxxxx> > Cc: David Hildenbrand <david@xxxxxxxxxx> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> > Signed-off-by: Nadav Amit <namit@xxxxxxxxxx> 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
