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

[Dan Williams]

On Tue, Aug 18, 2015 at 9:55 AM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote:
> On Mon, Aug 17, 2015 at 05:46:43PM -0700, Dan Williams wrote:
>> On Mon, Aug 17, 2015 at 2:45 PM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote:
>> > On Fri, Aug 14, 2015 at 07:11:27PM -0700, Dan Williams wrote:
>> >> Although it does not offer perfect protection if device memory is at a
>> >> physically lower address than RAM, skipping the update of these
>> >> variables does seem to be what we want.  For example /dev/mem would
>> >> fail to allow write access to persistent memory if it fails a
>> >> valid_phys_addr_range() check.  Since /dev/mem does not know how to
>> >> write to PMEM in a reliably persistent way, it should not treat a
>> >> PMEM-pfn like RAM.
>> >
>> > So i attach is a patch that should keep ZONE_DEVICE out of consideration
>> > for the buddy allocator. You might also want to keep page reserved and not
>> > free inside the zone, you could replace the generic_online_page() using
>> > set_online_page_callback() while hotpluging device memory.
>> >
>>
>> Hmm, are we already protected by the fact that ZONE_DEVICE is not
>> represented in the GFP_ZONEMASK?
>
> Yeah seems you right, high_zoneidx (which is derive using gfp_zone()) will
> always limit which zones are considered. I thought that under memory presure
> it would go over all of the zonelist entry and eventualy consider the device
> zone. But it doesn't seems to be that way.
>
> Keeping the device zone out of the zonelist might still be a good idea, if
> only to avoid pointless iteration for the page allocator. Unless someone can
> think of a reason why this would be bad.
>

The other question I have is whether disabling ZONE_DMA is a realistic
tradeoff for enabling ZONE_DEVICE?  I.e. can ZONE_DMA default to off
going forward, lose some ISA device support, or do we need to figure
out how to enable > 4 zones.
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