linux-kernel - Re: [PATCHSET v4 0/5] pagemap: make useable for non-privilege users

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Message-ID: <CALYGNiPidOy47E5ArW28ny4m+nTN5P8gAjYZtdC2_C1nV-yomQ@mail.gmail.com>
Date:	Tue, 14 Jul 2015 23:15:41 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Mark Williamson <mwilliamson@...o-software.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCHSET v4 0/5] pagemap: make useable for non-privilege users

On Tue, Jul 14, 2015 at 9:52 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Tue, 14 Jul 2015 18:37:34 +0300 Konstantin Khlebnikov <khlebnikov@...dex-team.ru> wrote:
>
>> This patchset makes pagemap useable again in the safe way (after row hammer
>> bug it was made CAP_SYS_ADMIN-only). This patchset restores access for
>> non-privileged users but hides PFNs from them.
>
> Documentation/vm/pagemap.txt hasn't been updated to describe these
> privilege issues?

Will do. Too much time passed between versions, I planned but forgot about that.

>
>> Also it adds bit 'map-exlusive' which is set if page is mapped only here:
>> it helps in estimation of working set without exposing pfns and allows to
>> distinguish CoWed and non-CoWed private anonymous pages.
>>
>> Second patch removes page-shift bits and completes migration to the new
>> pagemap format: flags soft-dirty and mmap-exlusive are available only
>> in the new format.
>
> I'm not really seeing a description of the new format in these
> changelogs.  Precisely what got removed, what got added and which
> capabilities change the output in what manner?

Now pfn (bits 0-54) is zero if task who opened pagemap has no
CAP_SYS_ADMIN (system-wide).

in v2 format page-shift (bits 55-60) now used for flags:
55 - soft-dirty (added for checkpoint-restore, I guess)
56 - mmap-exclusive (added in last patch)
57-60 - free for use

I'll document the history of these changes.

>
>
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