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

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Message-ID: <CAEVpBaLQx-8Hxjav5OO0xntDs=2wEXJwLXXW3FuNE0yCeBjBeQ@mail.gmail.com>
Date:	Thu, 30 Apr 2015 19:45:06 +0100
From:	Mark Williamson <mwilliamson@...o-software.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Konstantin Khlebnikov <koct9i@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

>> Something like this (see patch in attachment)
>
> THP is not covered.
>
> Any comments on kcmp() idea?

It seems like a modified kcmp() would also be a valid approach but, as
you noted, probably speed-limited for our purposes.  As you say, there
is the option of a vector-oriented equivalent.  It seems like a
generally nice facility to have available in the kernel but my
suspicion is that it wouldn't be optimal for us...

My thinking is that using soft-dirty might give us the best
performance on platforms where it's available.  We're only using
fork() as a cunning/hacky way of tracking what pages change;
soft-dirty would allow us to avoid that too, whereas using kcmp()
would still require the forking overhead.

Does that make sense, or have I missed something?

Thanks,
Mark
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