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Date:   Wed, 29 Jul 2020 17:12:54 +0300
From:   Mike Rapoport <rppt@linux.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Justin He <Justin.He@arm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Dave Jiang <dave.jiang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steve Capper <Steve.Capper@arm.com>,
        Mark Rutland <Mark.Rutland@arm.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Anshuman Khandual <Anshuman.Khandual@arm.com>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
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        Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
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Subject: Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem
 alignment
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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.