linux-kernel - Re: [GIT PULL] VFIO updates for v6.17-rc1

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Message-ID: <20250805132558.GA365447@nvidia.com>
Date: Tue, 5 Aug 2025 10:25:58 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lizhe.67@...edance.com" <lizhe.67@...edance.com>
Subject: Re: [GIT PULL] VFIO updates for v6.17-rc1

On Tue, Aug 05, 2025 at 04:00:53PM +0300, Linus Torvalds wrote:
> On Tue, 5 Aug 2025 at 10:47, David Hildenbrand <david@...hat.com> wrote:
> >
> > The concern is rather false positives, meaning, you want consecutive
> > PFNs (just like within a folio), but -- because the stars aligned --
> > you get consecutive "struct page" that do not translate to consecutive PFNs.
> 
> So I don't think that can happen with a valid 'struct page', because
> if the 'struct page's are in different sections, they will have been
> allocated separately too.

This is certainly true for the CONFIG_SPARSEMEM_VMEMMAP case, but in
the other cases I thought we end up with normal allocations for struct
page? This is what David was talking about.

So then we are afraid of this:

  a = kvmalloc_array(nelms_a);
  b = kvmalloc_array(nelms_b);

  assert(a + nelms_a != b)

I thought this was possible with our allocator, especially vmemmap?

David, there is another alternative to prevent this, simple though a
bit wasteful, just allocate a bit bigger to ensure the allocation
doesn't end on an exact PAGE_SIZE boundary?

Jason