On Tue, Aug 04, 2020 at 04:27:50PM +0100, Catalin Marinas wrote: > On Mon, Aug 03, 2020 at 05:50:32PM -0700, Peter Collingbourne wrote: > > On Mon, Aug 3, 2020 at 5:01 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > > I think this would work even for the arm64 MTE (though I haven't tried): > > > use memfd_create() to get such file descriptor, mmap() it as MAP_SHARED > > > to populate the initial pattern, mmap() it as MAP_PRIVATE for any > > > subsequent mapping that needs to be copied-on-write. > > > > That would require a separate mmap() (i.e. separate VMA) for each > > page, no? That sounds like it could be expensive both in terms of VMAs > > and the number of mmap syscalls required (i.e. N/PAGE_SIZE). You could > > decrease these costs by increasing the size of the memfd files to more > > than a page, but that would also increase the amount of memory > > required for the reference pages. > > I think I get it now. You'd like a multiple page mmap() to be covered by > a single reference page. The memfd trick wouldn't give you this without > multiple mmap() calls, one for each page. That's why I suggested a special file descriptor that would give the same page on any access. We can piggy back on memfd infrastrucure or create a new interface. -- Kirill A. Shutemov
