linux-kernel - Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk

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Message-ID: <20240819121926.GG2032816@nvidia.com>
Date: Mon, 19 Aug 2024 09:19:26 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	Will Deacon <will@...nel.org>, Gavin Shan <gshan@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Zi Yan <ziy@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for
 folio_walk_start()

On Fri, Aug 16, 2024 at 07:56:30PM +0200, David Hildenbrand wrote:

> I think KVM does something nasty: if it something with a "struct page", and
> it's not PageReserved, it would take a reference (if I get
> kvm_pfn_to_refcounted_page()) independent if it's a "normal" or "not normal"
> page -- it essentially ignores the vm_normal_page() information in the page
> tables ...

Oh that's nasty. Nothing should be upgrading the output of the follow
functions to refcounted. That's what GUP is for.

And PFNMAP pages, even if they have struct pages for some reason,
should *NEVER* be refcounted because they are in a PFNMAP VMA. That is
completely against the whole point :\ If they could be safely
refcounted then it would be a MIXEDMAP.

Jason