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

[Ard Biesheuvel]

Hello John,

On Thu, 6 Apr 2023 at 06:05, John Hubbard <jhubbard@xxxxxxxxxx> 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.
>

Currently, the vmemmap region is dimensioned to only cover the PFN
range that backs the linear map. So the WARN() seems appropriate here:
you are mapping struct page[] ranges outside of the allocated window,
and afaict, you might actually wrap around and corrupt the linear map
at the start of the kernel VA space like this.


> x86 doesn't have this warning. It only checks that pages are properly
> aligned. I've shown a comparison below between x86 (which works well)
> and arm64 (which has these warnings).
>
> memunmap_pages()
>   pageunmap_range()
>     if (pgmap->type == MEMORY_DEVICE_PRIVATE)
>       __remove_pages()
>         __remove_section()
>           sparse_remove_section()
>             section_deactivate()
>               depopulate_section_memmap()
>                 /* arch/arm64/mm/mmu.c */
>                 vmemmap_free()
>                 {
>                   WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>                   ...
>                 }
>
>                 /* arch/x86/mm/init_64.c */
>                 vmemmap_free()
>                 {
>                   VM_BUG_ON(!PAGE_ALIGNED(start));
>                   VM_BUG_ON(!PAGE_ALIGNED(end));
>                   ...
>                 }
>
> So, the warning is a false positive for this case. Therefore, skip the
> warning if CONFIG_DEVICE_PRIVATE is set.
>

I don't think this is a false positive. We'll need to adjust
VMEMMAP_SIZE to account for this.


> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  arch/arm64/mm/mmu.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 6f9d8898a025..d5c9b611a8d1 100644
> --- 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));
> -
> +#endif
>         if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
>                 return vmemmap_populate_basepages(start, end, node, altmap);
>         else
> @@ -1169,8 +1171,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  void vmemmap_free(unsigned long start, unsigned long end,
>                 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));
> -
> +#endif
>         unmap_hotplug_range(start, end, true, altmap);
>         free_empty_tables(start, end, VMEMMAP_START, VMEMMAP_END);
>  }
> --
> 2.40.0
>