On Thu, May 17, 2018 at 12:32:07PM -0700, Dan Williams wrote:
On Thu, May 17, 2018 at 11:37 AM, Matthew Wilcox
> 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(a)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(a)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(a)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
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.