linux-kernel - Re: [PATCH 5/5] vfio/type1: Use mapping page mask for pfnmaps

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Message-ID: <20250217123320.051fad67.alex.williamson@redhat.com>
Date: Mon, 17 Feb 2025 12:33:20 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterx@...hat.com,
 mitchell.augustin@...onical.com, clg@...hat.com, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Subject: Re: [PATCH 5/5] vfio/type1: Use mapping page mask for pfnmaps

On Fri, 14 Feb 2025 19:46:22 +0000
Matthew Wilcox <willy@...radead.org> wrote:

> On Wed, Feb 05, 2025 at 04:17:21PM -0700, Alex Williamson wrote:
> > +			if (is_invalid_reserved_pfn(*pfn)) {
> > +				unsigned long epfn;
> > +
> > +				epfn = (((*pfn << PAGE_SHIFT) + ~pgmask + 1)
> > +					& pgmask) >> PAGE_SHIFT;
> > +				ret = min_t(int, npages, epfn - *pfn);  
> 
> You've really made life hard for yourself by passing around a page mask
> instead of an order (ie 0/PMD_ORDER/PUD_ORDER).  Why not:
> 
> 				epfn = round_up(*pfn + 1, 1 << order);
> 
> Although if you insist on passing around a mask, this could be:
> 
> 				unsigned long sz = (~pgmask >> PAGE_SHIFT) + 1;
> 				unsigned long epfn = round_up(*pfn + 1, sz)
> 

Hey Willy!

I was wishing I had an order, but I didn't want to mangle
follow_pfnmap_start() and follow_pfnmap_setup() too much.  Currently
the latter is doing:

	args->pfn = pfn_base + ((args->address & ~addr_mask) >> PAGE_SHIFT);

If follow_pfnmap_start() passed an order, this would need to change to
something equivalent to:

	args->pfn = pfn_base + ((args->address >> PAGE_SHIFT) & ((1UL << order) - 1));

Looks pretty ugly as well, so maybe I'm just shifting the ugliness
around, or maybe someone can spot a more elegant representation.
Thanks,

Alex