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Subject: Re: [PATCH 08/20] mm: Optimize fullmm TLB flushing
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 28 Jun 2012 13:30:48 +0200
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To: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>, akpm@linux-foundation.org, Rik van Riel <riel@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@kernel.dk>, Alex Shi <alex.shi@intel.com>, "Nikunj A.
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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.=20

Right, we do that (now)..

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