linux-kernel - Re: [PATCH] mm/vmalloc: fix mischeck pfn valid in vmap

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Message-ID: <20250318083330.GB18902@lst.de>
Date: Tue, 18 Mar 2025 09:33:30 +0100
From: Christoph Hellwig <hch@....de>
To: Huan Yang <link@...o.com>
Cc: Christoph Hellwig <hch@....de>, akpm@...ux-foundation.org,
	bingbu.cao@...ux.intel.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, lorenzo.stoakes@...cle.com,
	opensource.kernel@...o.com, rppt@...nel.org, ryan.roberts@....com,
	urezki@...il.com, ziy@...dia.com, vivek.kasireddy@...el.com
Subject: Re: [PATCH] mm/vmalloc: fix mischeck pfn valid in vmap_pfns

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.