linux-kernel - Re: [PATCH v4 09/12] powerpc/mm: replace batch->active with in_lazy_mmu

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Message-ID: <e3326a9c-108a-4eb2-b12e-bff2b5edd1d3@arm.com>
Date: Tue, 4 Nov 2025 11:33:03 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, 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 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 09/12] powerpc/mm: replace batch->active with
 in_lazy_mmu_mode()

On 03/11/2025 16:05, David Hildenbrand wrote:
> On 29.10.25 11:09, Kevin Brodsky wrote:
>> A per-CPU batch struct is activated when entering lazy MMU mode; its
>> lifetime is the same as the lazy MMU section (it is deactivated when
>> leaving the mode). Preemption is disabled in that interval to ensure
>> that the per-CPU reference remains valid.
>>
>> The generic lazy_mmu layer now tracks whether a task is in lazy MMU
>> mode. We can therefore use the generic helper in_lazy_mmu_mode()
>> to tell whether a batch struct is active instead of tracking it
>> explicitly.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>
> I suspect you were not able to test this on real HW. Some help from
> the ppc folks would be appreciated.

Indeed, it would be nice to get some testing on ppc HW that actually
uses lazy MMU (!radix_enabled()).

>
> LGTM, but the interaction with pause/resume adds a bit of complication
> on top.

Does it? This series doesn't change when arch_enter() and arch_leave()
are called, batch->active and in_lazy_mmu_mode() should coincide. 

- Kevin