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

[John Hubbard <jhubbard@nvidia.com>]

On 4/6/23 13:07, Andrew Morton wrote:
> On Wed, 5 Apr 2023 21:05:15 -0700 John Hubbard <jhubbard@nvidia.com> wrote:
> 
>> Although CONFIG_DEVICE_PRIVATE and hmm_range_fault() and related
>> functionality was first developed on x86, it also works on arm64.
>> However, when trying this out on an arm64 system, it turns out that
>> there is a massive slowdown during the setup and teardown phases.
>>
>> This slowdown is due to lots of calls to WARN_ON()'s that are checking
>> for pages that are out of the physical range for the CPU. However,
>> that's a design feature of device private pages: they are specfically
>> chosen in order to be outside of the range of the CPU's true physical
>> pages.
>>
>> ...
>>
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1157,8 +1157,10 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node,
>>  int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>  		struct vmem_altmap *altmap)
>>  {
>> +/* Device private pages are outside of the CPU's physical page range. */
>> +#ifndef CONFIG_DEVICE_PRIVATE
>>  	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
> 
> For a simple expression like this to cause a "massive slowdown", I
> assume the WARN is triggering.  But changelog doesn't mention massive
> dmesg spewage?

Well, it should. Whoever wrote that needs to improve the changelog. :)

> 
> Given Ard's comments, perhaps a switch to WARN_ON_ONCE() would suit?

That would fix up the user-visible problems, which would be very nice.

Meanwhile, I'm trying to sort out whether this really is a false 
positive for arm64.

thanks,
-- 
John Hubbard
NVIDIA