linux-kernel - Re: [PATCH v4 03/12] powerpc/mm: implement arch_flush_lazy_mmu

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Message-ID: <87jz044xn4.ritesh.list@gmail.com>
Date: Wed, 05 Nov 2025 15:19:35 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Kevin Brodsky <kevin.brodsky@....com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Andreas Larsson <andreas@...sler.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	Borislav Petkov <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	David Hildenbrand <david@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
	David Woodhouse <dwmw2@...radead.org>, "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, 
	Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>, 
	Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ryan Roberts <ryan.roberts@....com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>, 
	Will Deacon <will@...nel.org>, Yeoreum Yun <yeoreum.yun@....com>, 
	linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, 
	sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v4 03/12] powerpc/mm: implement arch_flush_lazy_mmu_mode()

Ritesh Harjani (IBM) <ritesh.list@...il.com> writes:

> Kevin Brodsky <kevin.brodsky@....com> writes:
>
>> Upcoming changes to the lazy_mmu API will cause
>> arch_flush_lazy_mmu_mode() to be called when leaving a nested
>> lazy_mmu section.
>>
>> Move the relevant logic from arch_leave_lazy_mmu_mode() to
>> arch_flush_lazy_mmu_mode() and have the former call the latter.
>>
>> Note: the additional this_cpu_ptr() on the
>> arch_leave_lazy_mmu_mode() path will be removed in a subsequent
>> patch.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>>  .../powerpc/include/asm/book3s/64/tlbflush-hash.h | 15 +++++++++++----
>>  1 file changed, 11 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/tlbflush-hash.h b/arch/powerpc/include/asm/book3s/64/tlbflush-hash.h
>> index 146287d9580f..7704dbe8e88d 100644
>> --- a/arch/powerpc/include/asm/book3s/64/tlbflush-hash.h
>> +++ b/arch/powerpc/include/asm/book3s/64/tlbflush-hash.h
>> @@ -41,6 +41,16 @@ static inline void arch_enter_lazy_mmu_mode(void)
>>  	batch->active = 1;
>>  }
>>  
>> +static inline void arch_flush_lazy_mmu_mode(void)
>> +{
>> +	struct ppc64_tlb_batch *batch;
>> +
>> +	batch = this_cpu_ptr(&ppc64_tlb_batch);
>> +
>> +	if (batch->index)
>> +		__flush_tlb_pending(batch);
>> +}
>> +
>
> This looks a bit scary since arch_flush_lazy_mmu_mode() is getting
> called from several of the places in later patches(). 
>
> Although I think arch_flush_lazy_mmu_mode() will only always be called
> in nested lazy mmu case right?
>
> Do you think we can add a VM_BUG_ON(radix_enabled()); in above to make
> sure the above never gets called in radix_enabled() case. 
>
> I am still going over the patch series, but while reviewing this I
> wanted to take your opinion.
>
> Ohh wait.. There is no way of knowing the return value from
> arch_enter_lazy_mmu_mode().. I think you might need a similar check to
> return from arch_flush_lazy_mmu_mode() too, if radix_enabled() is true.
>

Now that I have gone through this series, it seems plaussible that since
lazy mmu mode supports nesting, arch_flush_lazy_mmu_mode() can get
called while the lazy mmu is active due to nesting.. 

That means we should add the radix_enabled() check as I was talking in
above i.e. 

@@ -38,6 +38,9 @@ static inline void arch_flush_lazy_mmu_mode(void)
 {
        struct ppc64_tlb_batch *batch;

+       if (radix_enabled())
+               return;
+
        batch = this_cpu_ptr(&ppc64_tlb_batch);

        if (batch->index)

Correct? Although otherwise also I don't think it should be a problem
because batch->index is only valid during hash, but I still think we can
add above check so that we don't have to call this_cpu_ptr() to check
for batch->index whenever flush is being called.

-ritesh