Re: [PATCH v4 5/5] pagemap: add mmap-exclusive bit for marking pages mapped only here

Reviewed-by: Mark Williamson <redacted>

On Tue, Jul 21, 2015 at 9:17 AM, Naoya Horiguchi
[off-list ref] wrote:

On Tue, Jul 14, 2015 at 06:37:49PM +0300, Konstantin Khlebnikov wrote:

quoted

This patch sets bit 56 in pagemap if this page is mapped only once.
It allows to detect exclusively used pages without exposing PFN:

present file exclusive state
0       0    0         non-present
1       1    0         file page mapped somewhere else
1       1    1         file page mapped only here
1       0    0         anon non-CoWed page (shared with parent/child)
1       0    1         anon CoWed page (or never forked)

CoWed pages in (MAP_FILE | MAP_PRIVATE) areas are anon in this context.

MMap-exclusive bit doesn't reflect potential page-sharing via swapcache:
page could be mapped once but has several swap-ptes which point to it.
Application could detect that by swap bit in pagemap entry and touch
that pte via /proc/pid/mem to get real information.

Signed-off-by: Konstantin Khlebnikov <redacted>
Requested-by: Mark Williamson [off-list ref]
Link: http://lkml.kernel.org/r/CAEVpBa+_RyACkhODZrRvQLs80iy0sqpdrd0AaP_-tgnX3Y9yNQ@mail.gmail.com

Reviewed-by: Naoya Horiguchi <redacted>