linux-kernel - Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment

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Date:   Wed, 29 Jul 2020 17:12:54 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Justin He <Justin.He@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Dave Jiang <dave.jiang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steve Capper <Steve.Capper@....com>,
        Mark Rutland <Mark.Rutland@....com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Anshuman Khandual <Anshuman.Khandual@....com>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Ira Weiny <ira.weiny@...el.com>, Kaly Xin <Kaly.Xin@....com>
Subject: Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem
 alignment

On Wed, Jul 29, 2020 at 03:03:04PM +0200, David Hildenbrand wrote:
> On 29.07.20 15:00, Mike Rapoport wrote:
> > On Wed, Jul 29, 2020 at 11:35:20AM +0200, David Hildenbrand wrote:
> >>>  
> >>> There is still large gap with ARM64_64K_PAGES, though.
> >>>
> >>> As for SPARSEMEM without VMEMMAP, are there actual benefits to use it?
> >>
> >> I was asking myself the same question a while ago and didn't really find
> >> a compelling one.
> > 
> > Memory overhead for VMEMMAP is larger, especially for arm64 that knows
> > how to free empty parts of the memory map with "classic" SPARSEMEM.
> 
> You mean the hole punching within section memmap? (which is why their
> pfn_valid() implementation is special)

Yes, arm (both 32 and 64) do this. And for smaller systems with a few
memory banks this is very reasonable to trade slight (if any) slowdown
in pfn_valid() for several megs of memory.
 
> (I do wonder why that shouldn't work with VMEMMAP, or is it simply not
> implemented?)
 
It's not implemented. There was a patch [1] recently to implement this. 

[1] https://lore.kernel.org/lkml/20200721073203.107862-1-liwei213@huawei.com/

> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.