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

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Date:   Mon, 10 Apr 2023 19:48:16 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ard Biesheuvel <ardb@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mark Rutland <mark.rutland@....com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Feiyang Chen <chenfeiyang@...ngson.cn>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH] arm64/mm: don't WARN when alloc/free-ing device private
 pages

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