Re: [PATCH] dax: Fix use of zero page

From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH] dax: Fix use of zero page
Date: Thu, 17 May 2018 14:48:14 -0600	[thread overview]
Message-ID: <20180517204814.GA31184@linux.intel.com> (raw)
In-Reply-To: <CAPcyv4jkXiNMSqxhAshhS-+cXFnVJFkVAixrjc4Hjn7xOhgMyw@mail.gmail.com>

On Thu, May 17, 2018 at 01:03:48PM -0700, Dan Williams wrote:
> On Thu, May 17, 2018 at 12:56 PM, Matthew Wilcox <willy@infradead.org> wrote:
> > 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.
> 
> Unless I am missing something I think this sounds like 4.18-rc1
> material with a cc: stable. Last I heard no one is really using
> dccssblk + dax, and MIPS has no way to describe pmem outside of
> memmap= which is only a development tool.

Yea, I agree that this is v4.18 material.
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