linux-kernel - Re: [PATCH 08/20] mm: Optimize fullmm TLB flushing

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Date:	Thu, 28 Jun 2012 13:30:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Alex Shi <alex.shi@...el.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Russell King <rmk@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Kyle McMartin <kyle@...artin.ca>,
	James Bottomley <jejb@...isc-linux.org>,
	Chris Zankel <chris@...kel.net>
Subject: Re: [PATCH 08/20] mm: Optimize fullmm TLB flushing

On Thu, 2012-06-28 at 13:19 +0200, Martin Schwidefsky wrote:

> The cpu can create speculative TLB entries, but only if it runs in the
> mode that uses the respective mm. We have two mm's active at the same
> time, the kernel mm (init_mm) and the user mm. While the cpu runs only
> in kernel mode it is not allowed to create TLBs for the user mm.
> While running in user mode it is allowed to speculatively create TLBs.

OK, that's neat.

> Basically we have two special requirements on s390:
> 1) do not modify ptes while attached to another cpu except with the
>    special IPTE / IDTE instructions

Right, and your fullmm case works by doing a global invalidate after all
threads have ceased userspace execution, this allows you to do away with
the IPTE/IDTE instructions since there's no other active cpus on the
userspace mm anymore.


> 2) do a TLB flush before freeing any kind of page table page, s390
>    needs a flush for pud, pmd & pte tables. 

Right, we do that (now)..
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