linux-kernel - Re: [PATCH RFC 00/35] mm: remove nth

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Message-ID: <20250822143043.GG1311579@nvidia.com>
Date: Fri, 22 Aug 2025 11:30:43 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: David Hildenbrand <david@...hat.com>
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Subject: Re: [PATCH RFC 00/35] mm: remove nth_page()

On Thu, Aug 21, 2025 at 10:06:26PM +0200, David Hildenbrand wrote:
> As discussed recently with Linus, nth_page() is just nasty and we would
> like to remove it.
> 
> To recap, the reason we currently need nth_page() within a folio is because
> on some kernel configs (SPARSEMEM without SPARSEMEM_VMEMMAP), the
> memmap is allocated per memory section.
> 
> While buddy allocations cannot cross memory section boundaries, hugetlb
> and dax folios can.
> 
> So crossing a memory section means that "page++" could do the wrong thing.
> Instead, nth_page() on these problematic configs always goes from
> page->pfn, to the go from (++pfn)->page, which is rather nasty.
> 
> Likely, many people have no idea when nth_page() is required and when
> it might be dropped.
> 
> We refer to such problematic PFN ranges and "non-contiguous pages".
> If we only deal with "contiguous pages", there is not need for nth_page().
>
> Besides that "obvious" folio case, we might end up using nth_page()
> within CMA allocations (again, could span memory sections), and in
> one corner case (kfence) when processing memblock allocations (again,
> could span memory sections).

I browsed the patches and it looks great to me, thanks for doing this

Jason