Re: [RFC PATCH 1/7] x86, mm: ZONE_DEVICE for "device memory"

[Jerome Glisse]

On Fri, Aug 14, 2015 at 02:52:15PM -0700, Dan Williams wrote:
> On Fri, Aug 14, 2015 at 2:37 PM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote:
> > On Wed, Aug 12, 2015 at 11:50:05PM -0400, Dan Williams wrote:
> >> While pmem is usable as a block device or via DAX mappings to userspace
> >> there are several usage scenarios that can not target pmem due to its
> >> lack of struct page coverage. In preparation for "hot plugging" pmem
> >> into the vmemmap add ZONE_DEVICE as a new zone to tag these pages
> >> separately from the ones that are subject to standard page allocations.
> >> Importantly "device memory" can be removed at will by userspace
> >> unbinding the driver of the device.
> >>
> >> Having a separate zone prevents allocation and otherwise marks these
> >> pages that are distinct from typical uniform memory.  Device memory has
> >> different lifetime and performance characteristics than RAM.  However,
> >> since we have run out of ZONES_SHIFT bits this functionality currently
> >> depends on sacrificing ZONE_DMA.
> >>
> >> arch_add_memory() is reorganized a bit in preparation for a new
> >> arch_add_dev_memory() api, for now there is no functional change to the
> >> memory hotplug code.
> >>
> >> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >> Cc: Rik van Riel <riel@xxxxxxxxxx>
> >> Cc: Mel Gorman <mgorman@xxxxxxx>
> >> Cc: linux-mm@xxxxxxxxx
> >> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> ---
> >>  arch/x86/Kconfig       |   13 +++++++++++++
> >>  arch/x86/mm/init_64.c  |   32 +++++++++++++++++++++-----------
> >>  include/linux/mmzone.h |   23 +++++++++++++++++++++++
> >>  mm/memory_hotplug.c    |    5 ++++-
> >>  mm/page_alloc.c        |    3 +++
> >>  5 files changed, 64 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> >> index b3a1a5d77d92..64829b17980b 100644
> >> --- a/arch/x86/Kconfig
> >> +++ b/arch/x86/Kconfig
> >> @@ -308,6 +308,19 @@ config ZONE_DMA
> >>
> >>         If unsure, say Y.
> >>
> >> +config ZONE_DEVICE
> >> +     bool "Device memory (pmem, etc...) hotplug support" if EXPERT
> >> +     default !ZONE_DMA
> >> +     depends on !ZONE_DMA
> >> +     help
> >> +       Device memory hotplug support allows for establishing pmem,
> >> +       or other device driver discovered memory regions, in the
> >> +       memmap. This allows pfn_to_page() lookups of otherwise
> >> +       "device-physical" addresses which is needed for using a DAX
> >> +       mapping in an O_DIRECT operation, among other things.
> >> +
> >> +       If FS_DAX is enabled, then say Y.
> >> +
> >>  config SMP
> >>       bool "Symmetric multi-processing support"
> >>       ---help---
> >> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> >> index 3fba623e3ba5..94f0fa56f0ed 100644
> >> --- a/arch/x86/mm/init_64.c
> >> +++ b/arch/x86/mm/init_64.c
> [..]
> >> @@ -701,11 +694,28 @@ int arch_add_memory(int nid, u64 start, u64 size)
> >>       ret = __add_pages(nid, zone, start_pfn, nr_pages);
> >>       WARN_ON_ONCE(ret);
> >>
> >> -     /* update max_pfn, max_low_pfn and high_memory */
> >> -     update_end_of_memory_vars(start, size);
> >> +     /*
> >> +      * Update max_pfn, max_low_pfn and high_memory, unless we added
> >> +      * "device memory" which should not effect max_pfn
> >> +      */
> >> +     if (!is_dev_zone(zone))
> >> +             update_end_of_memory_vars(start, size);
> >
> > What is the rational for not updating max_pfn, max_low_pfn, ... ?
> >
> 
> The idea is that this memory is not meant to be available to the page
> allocator and should not count as new memory capacity.  We're only
> hotplugging it to get struct page coverage.

But this sounds bogus to me to rely on max_pfn to stay smaller than
first_dev_pfn.  For instance you might plug a device that register
dev memory and then some regular memory might be hotplug, effectively
updating max_pfn to a value bigger than first_dev_pfn.

Also i do not think that the buddy allocator use max_pfn or max_low_pfn
to consider page/zone for allocation or not.

Cheers,
Jérôme
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