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:

quoted

On Mon, Aug 3, 2020 at 5:01 AM Catalin Marinas [off-list ref] wrote:

quoted

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

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