Re: [PATCH v2 1/3] pagemap: add mmap-exclusive bit for marking pages mapped only here

From: Konstantin Khlebnikov
Date: Wed May 13 2015 - 06:59:56 EST

On 12.05.2015 13:40, Kirill A. Shutemov wrote:
On Tue, May 12, 2015 at 12:43:03PM +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)

Probably, worth noting that file-private pages are anon in this context.

You mean there's another kind of CoW pages? Yep, but from the kernel
point of view these pages are the same. Anyway Userspace could look
into /proc/*/maps and see is there any file beyond anon vma.

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