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

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Message-ID: <00999740-d762-488a-a946-0c10589df146@redhat.com>
Date: Tue, 5 Aug 2025 15:33:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Cc: 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 05.08.25 15:25, Jason Gunthorpe wrote:
> 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?

:/ in particular doing that through the memblock in sparse_init_nid(), I 
am not so sure that's a good idea.

I prefer Linus' proposal and avoids the one nth_page(), unless any other 
approach can help us get rid of more nth_page() usage -- and I don't 
think your proposal could, right?

-- 
Cheers,

David / dhildenb