Re: [PATCH] mm/vmalloc: fix mischeck pfn valid in vmap_pfns

[Christoph Hellwig]

On Tue, Mar 18, 2025 at 04:20:17PM +0800, Huan Yang wrote:
> This prevents us from properly invoking vmap, which is why we have turned to using vmap_pfn instead.
>
> Even if a folio-based vmap is implemented, it still cannot handle mapping multiple folio ranges of physical
>
> memory to vmalloc regions. A range of folio is important, it maybe an offset in memfd, no need entire folio.
>
> So, I still consider vmap_pfn to be the optimal solution for this specific scenario. :)

No, vmap_pfn is entirely for memory not backed by pages or folios,
i.e. PCIe BARs and similar memory.  This must not be mixed with proper
folio backed memory.

So you'll need a vmap for folios to support this use case.

>
>> historically backed by pages and now folios.
>
> So by HVO, it also not backed by pages, only contains folio head, each tail pfn's page struct go away.

And a fully folios based vmap solves that problem.