From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-nvdimm-bounces@lists.01.org>
Received: from bombadil.infradead.org (bombadil.infradead.org
 [IPv6:2607:7c80:54:e::133])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (No client certificate requested)
 by ml01.01.org (Postfix) with ESMTPS id 107F2207E4E09
 for <linux-nvdimm@lists.01.org>; Thu, 17 May 2018 12:56:57 -0700 (PDT)
Date: Thu, 17 May 2018 12:56:56 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] dax: Fix use of zero page
Message-ID: <20180517195656.GH26718@bombadil.infradead.org>
References: <20180517183711.GE26718@bombadil.infradead.org>
 <CAPcyv4jXaEMW-41yU8eu11-g53apPUX3hg70aQCuwot6kyApUw@mail.gmail.com>
MIME-Version: 1.0
Content-Disposition: inline
In-Reply-To: <CAPcyv4jXaEMW-41yU8eu11-g53apPUX3hg70aQCuwot6kyApUw@mail.gmail.com>
List-Unsubscribe: <https://lists.01.org/mailman/options/linux-nvdimm>,
 <mailto:linux-nvdimm-request@lists.01.org?subject=unsubscribe>
List-Archive: <http://lists.01.org/pipermail/linux-nvdimm/>
List-Post: <mailto:linux-nvdimm@lists.01.org>
List-Help: <mailto:linux-nvdimm-request@lists.01.org?subject=help>
List-Subscribe: <https://lists.01.org/mailman/listinfo/linux-nvdimm>,
 <mailto:linux-nvdimm-request@lists.01.org?subject=subscribe>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Errors-To: linux-nvdimm-bounces@lists.01.org
Sender: "Linux-nvdimm" <linux-nvdimm-bounces@lists.01.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-nvdimm <linux-nvdimm@lists.01.org>
List-ID: <linux-nvdimm@lists.01.org>

On Thu, May 17, 2018 at 12:32:07PM -0700, Dan Williams wrote:
> On Thu, May 17, 2018 at 11:37 AM, Matthew Wilcox <willy@infradead.org> wrote:
> >
> > I plucked this patch from my XArray work.  It seems self-contained enough
> > that it could go into the DAX tree for merging this cycle.
> >
> > From 8cb56f4ba36af38814ca7b8ba030a66384e59a21 Mon Sep 17 00:00:00 2001
> > From: Matthew Wilcox <mawilcox@microsoft.com>
> > Date: Thu, 29 Mar 2018 22:41:18 -0400
> > Subject: [PATCH] dax: Fix use of zero page
> >
> > Use my_zero_pfn instead of ZERO_PAGE, and pass the vaddr to it so it
> > works on MIPS and s390.
> >
> > Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
> 
> I'm being thick and / or lazy, what's the user visible effect of this fix?

For s390 it appears to be a performance issue:

Author: Martin Schwidefsky <schwidefsky@de.ibm.com>
Date:   Mon Oct 25 16:10:07 2010 +0200

    [S390] zero page cache synonyms
    
    If the zero page is mapped to virtual user space addresses that differ
    only in bit 2^12 or 2^13 we get L1 cache synonyms which can affect
    performance. Follow the mips model and use multiple zero pages to avoid
    the synonyms.

MIPS' use of multiple ZERO_PAGEs predates git history.  Given the
history of MIPS' caches behaving in incredibly weird ways, I'd assume
that getting this wrong results in miniature black holes forming and/or
the CPU calculating the largest prime number.
_______________________________________________
Linux-nvdimm mailing list
Linux-nvdimm@lists.01.org
https://lists.01.org/mailman/listinfo/linux-nvdimm