linux-kernel - Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk

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Message-ID: <d25912b5-cda0-4173-befe-8a16209b1c66@redhat.com>
Date: Thu, 29 Aug 2024 17:10:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Gavin Shan <gshan@...hat.com>, Catalin Marinas <catalin.marinas@....com>,
 x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Paolo Bonzini <pbonzini@...hat.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Alistair Popple <apopple@...dia.com>,
 kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 Sean Christopherson <seanjc@...gle.com>, Oscar Salvador <osalvador@...e.de>,
 Jason Gunthorpe <jgg@...dia.com>, Borislav Petkov <bp@...en8.de>,
 Zi Yan <ziy@...dia.com>, Axel Rasmussen <axelrasmussen@...gle.com>,
 Yan Zhao <yan.y.zhao@...el.com>, Will Deacon <will@...nel.org>,
 Kefeng Wang <wangkefeng.wang@...wei.com>,
 Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk_start()

>>>
>>> If you prefer vm_normal_page_pud() to be defined and check pud_special()
>>> there, I can do that.  But again, I don't yet see how that can make a
>>> functional difference considering the so far very limited usage of the
>>> special bit, and wonder whether we can do that on top when it became
>>> necessary (and when we start to have functional requirement of such).
>>
>> I hope my explanation why pte_special() even exists and how it is used makes
>> it clearer.
>>
>> It's not that much code to handle it like pte_special(), really. I don't
>> expect you to teach GUP-slow about vm_normal_page() etc.
> 
> One thing I can do here is I move the pmd_special() check into the existing
> vm_normal_page_pmd(), then it'll be a fixup on top of this patch:
> 
> ===8<===
> diff --git a/mm/memory.c b/mm/memory.c
> index 288f81a8698e..42674c0748cb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -672,11 +672,10 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>   {
>   	unsigned long pfn = pmd_pfn(pmd);
>   
> -	/*
> -	 * There is no pmd_special() but there may be special pmds, e.g.
> -	 * in a direct-access (dax) mapping, so let's just replicate the
> -	 * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
> -	 */
> +	/* Currently it's only used for huge pfnmaps */
> +	if (unlikely(pmd_special(pmd)))
> +		return NULL;


Better.

I'd appreciate a vm_normal_page_pud(), but I guess I have to be the one 
cleaning up the mess after you.

-- 
Cheers,

David / dhildenb