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 23:44:04 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	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>,
	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 11:33 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> 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"?

This's no longer true. After recent fixes for "anon_vma endless growing" new vma
might reuse old anon_vma from grandparent vma.

>
> 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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