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Subject: Re: [PATCH 11/20] mm, s390: Convert to use generic mmu_gather
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu, 28 Jun 2012 00:13:19 +0200
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Cc: linux-arch@vger.kernel.org, linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>, akpm@linux-foundation.org, Linus Torvalds <torvalds@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. Dadhania" <nikunj@linux.vnet.ibm.com>, Konrad Rzeszutek Wilk <konrad@darnok.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, David Miller <davem@davemloft.net>, Russell King <rmk@arm.linux.org.uk>, Catalin Marinas <catalin.marinas@arm.com>, Chris Metcalf <cmetcalf@tilera.com>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Tony Luck <tony.luck@intel.com>, Paul Mundt <lethal@linux-sh.org>, Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>, Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>, Ralf Baechle <ralf@linux-mips.org>, Kyle McMartin <kyle@mcmartin.ca>, James Bottomley <jejb@parisc-linux.org>, Chris Zankel <chris@zankel.net>

On Wed, 2012-06-27 at 23:15 +0200, Peter Zijlstra wrote:
>=20
> S390 doesn't need a TLB flush after ptep_get_and_clear_full() and
> before __tlb_remove_page() because its ptep_get_and_clear*() family
> already does a full TLB invalidate. Therefore force it to use
> tlb_fast_mode.=20

On that.. ptep_get_and_clear() says:

/*                                                                         =
                   =20
 * This is hard to understand. ptep_get_and_clear and ptep_clear_flush     =
                   =20
 * both clear the TLB for the unmapped pte. The reason is that             =
                   =20
 * ptep_get_and_clear is used in common code (e.g. change_pte_range)       =
                   =20
 * to modify an active pte. The sequence is                                =
                   =20
 *   1) ptep_get_and_clear                                                 =
                   =20
 *   2) set_pte_at                                                         =
                   =20
 *   3) flush_tlb_range                                                    =
                   =20
 * On s390 the tlb needs to get flushed with the modification of the pte   =
                   =20
 * if the pte is active. The only way how this can be implemented is to    =
                   =20
 * have ptep_get_and_clear do the tlb flush. In exchange flush_tlb_range   =
                   =20
 * is a nop.                                                               =
                   =20
 */=20

I think there is another way, arch_{enter,leave}_lazy_mmu_mode() seems
to wrap these sites so you can do as SPARC64 and PPC do and batch
through there.

That should save a number of TLB invalidates..


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