Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP_FIXED mappings

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Jun 13, 2025 at 09:41:11AM -0400, Peter Xu wrote:

> +	/* Choose the alignment */
> +	if (IS_ENABLED(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP) && phys_len >= PUD_SIZE) {
> +		ret = mm_get_unmapped_area_aligned(file, addr, len, phys_addr,
> +						   flags, PUD_SIZE, 0);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	if (phys_len >= PMD_SIZE) {
> +		ret = mm_get_unmapped_area_aligned(file, addr, len, phys_addr,
> +						   flags, PMD_SIZE, 0);
> +		if (ret)
> +			return ret;
> +	}

Hurm, we have contiguous pages now, so PMD_SIZE is not so great, eg on
4k ARM with we can have a 16*2M=32MB contiguity, and 16k ARM uses
contiguity to get a 32*16k=1GB option.

Forcing to only align to the PMD or PUD seems suboptimal..

> +fallback:
> +	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);

Why not put this into mm_get_unmapped_area_vmflags() and get rid of
thp_get_unmapped_area_vmflags() too?

Is there any reason the caller should have to do a retry?

Jason