linux-kernel - Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP

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Message-ID: <20250613142903.GL1174925@nvidia.com>
Date: Fri, 13 Jun 2025 11:29:03 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Zi Yan <ziy@...dia.com>, Alex Mastro <amastro@...com>,
	David Hildenbrand <david@...hat.com>,
	Nico Pache <npache@...hat.com>
Subject: Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP_FIXED
 mappings

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