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

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Message-ID: <ZtDCs3rJz9OXzFkm@x1n>
Date: Thu, 29 Aug 2024 14:49:23 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...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()

On Thu, Aug 29, 2024 at 05:10:15PM +0200, David Hildenbrand wrote:
> > > > 
> > > > 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.

I'll prepare either a fixup patch for above, or repost if there're more
changes.

Again, please leave explicit comment please.

As I mentioned, to me vm_normal_page_pud() currently should only contain
pud_special() check, as most of the things in pmd/pte don't seem to apply.

I don't feel strongly to add that wrapper yet in this case, but if you can
elaborate what you're suggesting otherwise, it may help me to understand
what you're looking for, then I can try to address them.

Or if you prefer some cleanup patch by yourself, please go ahead.

-- 
Peter Xu