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

[David Hildenbrand <david@redhat.com>]

On 05.08.25 13:49, Jason Gunthorpe wrote:
> On Tue, Aug 05, 2025 at 09:47:18AM +0200, David Hildenbrand wrote:
>>> There was discussion here[1] where David Hildenbrand and Jason
>>> Gunthorpe suggested this should be in common code and I believe there
>>> was some intent that this would get reused.  I took this as
>>> endorsement from mm folks.  This can certainly be pulled back into
>>> subsystem code.
>>
>> Yeah, we ended up here after trying to go the folio-way first, but then
>> realizing that code that called GUP shouldn't have to worry about
>> folios, just to detect consecutive pages+PFNs.
>>
>> I think this helper will can come in handy even in folio context.
>> I recall pointing Joanne at it in different fuse context.
> 
> The scatterlist code should use it also, it is doing the same logic.
> 
>> 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.
> 
> I wonder if we can address that from the other side and prevent the
> memory code from creating a bogus contiguous struct page in the first
> place so that struct page contiguity directly reflects physical
> contiguity?

Well, if we could make CONFIG_SPARSEMEM_VMEMMAP the only sparsemem 
option ... :) But I recall it's not that easy (e.g., 32bit).

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().

So if the starts align, the "struct page" of the memory of two memory 
sections are contiguous, although the memory sections are not contiguous.

Just imagine memory holes.

Also, I am not sure if that is really a problem worth solving right now. 
If a helper on !CONFIG_SPARSEMEM_VMEMMAP is a little slower on some 
operations, like nth_page(), I really don't particularly care.

Eventually, !CONFIG_SPARSEMEM_VMEMMAP will go away in some distant 
future and we will all be happy.

-- 
Cheers,

David / dhildenb