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

[Andrew Morton <akpm@linux-foundation.org>]

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?

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