On Thu, May 17, 2018 at 12:56 PM, Matthew Wilcox <wi...@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 <wi...@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 <mawil...@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 <mawil...@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 <schwidef...@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.
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