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

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Message-ID: <a18a9b55-b3f0-466f-abc8-39b231c04bb1@redhat.com>
Date: Tue, 5 Aug 2025 15:05:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 "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 14:56, Jason Gunthorpe wrote:
> On Tue, Aug 05, 2025 at 02:41:38PM +0200, David Hildenbrand wrote:
>> On 05.08.25 14:38, Jason Gunthorpe wrote:
>>> On Tue, Aug 05, 2025 at 02:07:49PM +0200, David Hildenbrand wrote:
>>>> I don't see an easy way to guarantee that. E.g., populate_section_memmap
>>>> really just does a kvmalloc_node() and
>>>> __populate_section_memmap()->memmap_alloc() a memblock_alloc().
>>>
>>> Well, it is really easy, if you do the kvmalloc_node and you get the
>>> single unwanted struct page value, then call it again and free the
>>> first one. The second call is guarenteed to not return the unwanted
>>> value because the first call has it allocated.
>>
>> So you want to walk all existing sections to check that? :)
> 
> We don't need to walk, compute the page-1 and carefully run that
> through page_to_pfn algorithm.

You have to test if first_page-1 and last_page+1 are used as the memmap 
for another section.

So yeah, it could be done by some page_to_pfn + pfn_to_page trickery I 
think.

Not that I would like that "allocate until you find something that 
works" approach.

> 
>> That's the kind of approach I would describe with the words Linus used.
> 
> Its some small boot time nastyness, we do this all the time messing up
> the slow path so the fast paths can be simple

nth_page won't go away. If it would go away, I would be sold on almost 
any idea. nth_page is the real problem.

So I don't like the idea of micro-optimizing num_pages_contiguous() by 
adding weird tweaks to the core for that.

-- 
Cheers,

David / dhildenb