Re: [PATCH V2 1/6] KVM: ARM: Remove pgtable standard functions from stage-2 page tables

From: Mark Rutland <mark.rutland@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: yuzhao@google.com, Steve.Capper@arm.com, marc.zyngier@arm.com,
	Catalin.Marinas@arm.com, suzuki.poulose@arm.com,
	will.deacon@arm.com, james.morse@arm.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH V2 1/6] KVM: ARM: Remove pgtable standard functions from stage-2 page tables
Date: Mon, 25 Feb 2019 15:49:22 +0000	[thread overview]
Message-ID: <20190225154922.GJ26236@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <7bf0d18e-9714-4da9-a340-10aae5fa9f38@arm.com>

On Mon, Feb 25, 2019 at 07:50:27PM +0530, Anshuman Khandual wrote:
> 
> 
> On 02/25/2019 04:30 PM, Mark Rutland wrote:
> > Hi Anshuman,
> > 
> > On Mon, Feb 25, 2019 at 10:33:54AM +0530, Anshuman Khandual wrote:
> >> ARM64 standard pgtable functions are going to use pgtable_page_[ctor|dtor]
> >> constructs. Certain stage-2 page table allocations are multi order which
> >> cannot be allocated through a generic pgtable function as it does not exist
> >> right now. This prevents all pgtable allocations including multi order ones
> >> in stage-2 from moving into new ARM64 (pgtable_page_[ctor|dtor]) pgtable
> >> functions. Hence remove all generic pgtable allocation function dependency
> >> from stage-2 page tables till there is one multi-order allocation function
> >> available.
> > 
> > I'm a bit confused by this. Which allocations are multi-order?
> > 
> 
> Stage-2 PGD.
> 
> kvm_alloc_stage2_pgd -> alloc_pages_exact
> kvm_free_stage2_pgd -> free_pages_exact
>  
> > Why does that prevent other allcoations from using the regular routines?
> 
> At present both stage-2 PGD (kvm_alloc_stage2_pgd -> alloc_pages_exact), PUD|PMD
> (mmu_memory_cache_alloc) allocates directly from buddy allocator but then while
> freeing back stage-2 PGD directly calls buddy allocator via free_pages_exact but
> PUD|PMD get freed with stage2_[pud|pmd]_free which calls pud|pmd_free instead
> of calling free_pages() directly.

If we allocate/free the stage-2 PGD with {alloc,free}_pages_exact(),
then the PGD level is balanced today.

I don't see what that has to do with the other levels of table.

> All of these worked fine because pud|pmd_free called free_pages() directly with
> out going through pgtable_page_dtor(). But now we are changing pud|pmd_free to
> use pgtable_page_dtor() both for user and host kernel page tables. This will
> break stage2 page table (bad page state errors) because the new free path which
> would call pgtable_page_dtor() where as alloc patch never called pgtable_page_ctor().

I'm lost as to how that relates to the alloc/free of the PGD. AFAICT,
that's unrelated to the problem at hand.

What subtlety am I missing?

> To fix this situation either we move all stage-2 page table to use pte_alloc_one()
> and pte_free() which goes through pgtable_page_ctor/dtor cycle or just keep the
> virtualization page tables out of it (stage2/hyp) and remove the only dependency
> which breaks because of these changes. This series went with the second option. 

It sounds to me like this is just a mismatch between the alloc/free
paths for the PUD and PMD levels of table.

IIUC, we allocate those with {pmd,pud}_alloc_one(), and free them with
stage2_{pud,pmd}_free(), which call {pud,pmd}_free().

I would naively expect p?d_alloc_one() to pair with p?d_free(), so that
being a problem is surprising to me.

Thanks,
Mark.

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