Subject: Re: [PATCH v4 06/10] powerpc/mm: Use generic_get_unmapped_area() and call it from arch_get_unmapped_area()

From: Michael Ellerman <[email-masked]>
Date: Tue, 14 Dec 2021 00:02:32 +1100

Christophe Leroy <[email-masked]> writes:
> Use the generic version of arch_get_unmapped_area() which
> is now available at all time instead of its copy
> radix__arch_get_unmapped_area()
>
> Instead of setting mm->get_unmapped_area() to either
> arch_get_unmapped_area() or generic_get_unmapped_area(),
> always set it to arch_get_unmapped_area() and call
> generic_get_unmapped_area() from there when radix is enabled.
>
> Do the same with radix__arch_get_unmapped_area_topdown()
>
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/mm/mmap.c | 127 ++---------------------------------------
>  1 file changed, 6 insertions(+), 121 deletions(-)
>
> diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
> index 9b0d6e395bc0..46781d0103d1 100644
> --- a/arch/powerpc/mm/mmap.c
> +++ b/arch/powerpc/mm/mmap.c
> @@ -81,115 +81,15 @@ static inline unsigned long mmap_base(unsigned long rnd,
>  }
>  
>  #ifdef HAVE_ARCH_UNMAPPED_AREA
> -#ifdef CONFIG_PPC_RADIX_MMU
> -/*
> - * Same function as generic code used only for radix, because we don't need to overload
> - * the generic one. But we will have to duplicate, because hash select
> - * HAVE_ARCH_UNMAPPED_AREA
> - */
> -static unsigned long
> -radix__arch_get_unmapped_area(struct file *filp, unsigned long addr,
> -			     unsigned long len, unsigned long pgoff,
> -			     unsigned long flags)
> -{
> -	struct mm_struct *mm = current->mm;
> -	struct vm_area_struct *vma;
> -	int fixed = (flags & MAP_FIXED);
> -	unsigned long high_limit;
> -	struct vm_unmapped_area_info info;
> -
> -	high_limit = DEFAULT_MAP_WINDOW;
> -	if (addr >= high_limit || (fixed && (addr + len > high_limit)))
> -		high_limit = TASK_SIZE;
> -
> -	if (len > high_limit)
> -		return -ENOMEM;

There are some differences in the above vs the generic code, the generic
arch_get_unmapped_area_topdown() in mm/mmap.c does:

	const unsigned long mmap_end = arch_get_mmap_end(addr);

	if (len > mmap_end - mmap_min_addr)
		return -ENOMEM;

	if (flags & MAP_FIXED)
		return addr;

Our current code adjusts high_limit for fixed mappings that span above
the default map window. We added that logic in:

  35602f82d0c7 ("powerpc/64s/hash: Allow MAP_FIXED allocations to cross 128TB boundary")

That means a fixed mapping that crosses the 128T boundary will be
allowed by our code.

On the other hand the generic code will allow a fixed mapping to cross
the 128T boundary, but only if the size of the mapping is < ~128T.

(The actual size limit is (128T - mmap_min_addr), which is usually 4K or
64K, but is adjustable.)

It's unlikely that any apps are doing fixed mappings larger than 128T
that cross the 128T boundary, but I think we need to allow it. 128T
seems like a lot, but is not compared to the entire 4PB address space.

So I think we need to fix that in the generic code.

The easiest option is probably to pass flags to arch_get_mmap_end(), and
then the arches can decide whether to adjust the return value based on
flags.

Then there's also the extra check we have here:

> -	if (fixed) {
> -		if (addr > high_limit - len)
> -			return -ENOMEM;
> -		return addr;
> -	}

I think we can drop that when converting to the generic version, the
only case in which it matters is when high_limit == TASK_SIZE, and
get_unmapped_area() already does that check after calling us:

	if (addr > TASK_SIZE - len)
		return -ENOMEM;

cheers