linux-kernel - Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

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Date:	Wed, 29 Apr 2015 13:33:54 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Mark Williamson <mwilliamson@...o-software.com>,
	Mark Seaborn <mseaborn@...omium.org>,
	kernel list <linux-kernel@...r.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linux API <linux-api@...r.kernel.org>,
	Finn Grimwood <fgrimwood@...o-software.com>,
	Daniel James <djames@...o-software.com>
Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage

On Wed, Apr 29, 2015 at 12:36 PM, Kirill A. Shutemov
<kirill@...temov.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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