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

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On Tue, Jul 14, 2015 at 06:37:49PM +0300, Konstantin Khlebnikov wrote:
> 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 <khlebnikov@xxxxxxxxxxxxxx>
> Requested-by: Mark Williamson <mwilliamson@xxxxxxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/CAEVpBa+_RyACkhODZrRvQLs80iy0sqpdrd0AaP_-tgnX3Y9yNQ@xxxxxxxxxxxxxx

Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
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