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Date: Wed, 29 Apr 2015 13:33:54 -0700
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Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage
From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Mark Williamson <mwilliamson@undo-software.com>,
        Mark Seaborn <mseaborn@chromium.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux API <linux-api@vger.kernel.org>,
        Finn Grimwood <fgrimwood@undo-software.com>,
        Daniel James <djames@undo-software.com>
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On Wed, Apr 29, 2015 at 12:36 PM, Kirill A. Shutemov
<kirill@shutemov.name> wrote:
>
> Emm.. I have hard time to understand how writable bit is enough to get
> soft-dirty-alike functionality.

I don't think it is.

For anonymous pages, maybe you can play tricks with comparing the page
'anon_vma' with the vma->anon_vma.

I haven't really thought that through, but does something like

   static inline bool page_is_dirty_in_vma(struct page *page, struct
vm_area_struct *vma)
   {
        struct anon_vma *anon_vma = vma->anon_vma;

        return page->mapping == (void *)anon_vma + PAGE_MAPPING_ANON;
   }

end up working as a "page has been dirtied in this mapping"?

If the page came from another process and hasn't been written to, it
will have the anon_vma pointing to the originalting vma.

I may be high on some bad drugs, though. As mentioned, I didn't really
think this through.

                        Linus
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