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

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Message-ID: <b103edb7-c41b-4a5b-9d9f-9690c5b25eb7@redhat.com>
Date: Fri, 9 Aug 2024 18:20:06 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 Oscar Salvador <osalvador@...e.de>, Jason Gunthorpe <jgg@...dia.com>,
 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 09.08.24 18:08, Peter Xu wrote:
> Pfnmaps can always be identified with special bits in the ptes/pmds/puds.
> However that's unnecessary if the vma is stable, and when it's mapped under
> VM_PFNMAP | VM_IO.
> 
> Instead of adding similar checks in all the levels for huge pfnmaps, let
> folio_walk_start() fail even earlier for these mappings.  It's also
> something gup-slow already does, so make them match.
> 
> Cc: David Hildenbrand <david@...hat.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/pagewalk.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index cd79fb3b89e5..fd3965efe773 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -727,6 +727,11 @@ struct folio *folio_walk_start(struct folio_walk *fw,
>   	p4d_t *p4dp;
>   
>   	mmap_assert_locked(vma->vm_mm);
> +
> +	/* It has no folio backing the mappings at all.. */
> +	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
> +		return NULL;
> +

That is in general not what we want, and we still have some places that 
wrongly hard-code that behavior.

In a MAP_PRIVATE mapping you might have anon pages that we can happily walk.

vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO, 
vm_normal_page_pud()] should be able to identify PFN maps and reject 
them, no?

-- 
Cheers,

David / dhildenb