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

[Huan Yang <link@vivo.com>]

HI Christoph

在 2025/3/18 16:33, Christoph Hellwig 写道:
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> 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.
OK, I learn it more.
>
> So you'll need a vmap for folios to support this use case.
May can't
>
>>> 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.

A folio may be 2MB or more large 1GB, what if we only need a little, 1M or 512MB, can vmap based on folio can solve it?

Normally, can offer 4k-page based array map it. But consider HVO, can't. That's why wanto base on pfn.

Thank you

Huan Yang

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