Re: [PATCH] arm64/mm: don't WARN when alloc/free-ing device private pages

[John Hubbard <jhubbard@nvidia.com>]

On 4/10/23 00:39, John Hubbard wrote:
>> pfn_to_page(x) for values 0xc00_0000 < x < 0x1000_0000 will produce a
>> kernel VA that points outside the region set aside for the vmemmap.
>> This region is currently unused, but that will likely change soon.
>>
> 
> I tentatively think I'm in this case right now. Because there is no wrap
> around happening in my particular config, which is CONFIG_ARM64_VA_BITS
> == 48, and PAGE_SIZE == 4KB and sizeof(struct page) == 64 (details
> below).
> 

Correction, actually it *is* wrapping around, and ending up as a bogus
user space address, as you said it would when being above the range:

page_to_pfn(0xffffffffaee00000):  0x0000000ffec38000


> It occurs to me that ZONE_DEVICE and (within that category) device
> private page support need only support rather large setups. On x86, it
> requires 64-bit. And on arm64, from what I'm learning after a day or so
> of looking around and comparing, I think we must require at least 48 bit
> VA support. Otherwise there's just no room for things.

I'm still not sure of how to make room, but working on it.


thanks,
-- 
John Hubbard
NVIDIA