linux-kernel - Re: [PATCH v4 07/12] mm: enable lazy

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Message-ID: <84dab8b1-ea28-4dcc-9721-9aaa6fcd12ba@arm.com>
Date: Fri, 7 Nov 2025 11:13:15 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 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 07/12] mm: enable lazy_mmu sections to nest

On 06/11/2025 16:32, Ritesh Harjani (IBM) wrote:
> Alexander Gordeev <agordeev@...ux.ibm.com> writes:
>
>> On Wed, Nov 05, 2025 at 02:19:03PM +0530, Ritesh Harjani wrote:
>>>> + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is
>>>> + * currently enabled.
>>>>   */
>>>>  #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
>>>>  static inline void lazy_mmu_mode_enable(void)
>>>>  {
>>>> -	arch_enter_lazy_mmu_mode();
>>>> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
>>>> +
>>>> +	VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
>>>> +	/* enable() must not be called while paused */
>>>> +	VM_WARN_ON(state->nesting_level > 0 && !state->active);
>>>> +
>>>> +	if (state->nesting_level++ == 0) {
>>>> +		state->active = true;
>>>> +		arch_enter_lazy_mmu_mode();
>>>> +	}
>>>>  }
>>> Some architectures disables preemption in their
>>> arch_enter_lazy_mmu_mode(). So shouldn't the state->active = true should
>>> happen after arch_enter_lazy_mmu_mode() has disabled preemption()? i.e.
>> Do you have some scenario in mind that could cause an issue?
>>
> No not really. But that's a deviation from what previous arch hooks were
> expecting. Although thinking this through - I don't have any usecase
> where this can be a problem.

Which arch hook expectations are you referring to?

> But let me re-visit some of the code paths on ppc64 lazy mmu... 
>
> Looking at the arch specific usecase I see we always do get_cpu_var()
> for accessing the per-cpu batch array which disables preemption before
> accessing the per-cpu structure.. This per-cpu structure is where we
> batch pte updates...

arch_enter() disables preemption so accesses to per-CPU variables
anywhere in the section shouldn't be an issue either way.

The bigger picture (regarding patch 9) is that what in_lazy_mmu_state()
returns is based on the current task's state (not a per-CPU variable),
and always false while in interrupt. As a result whether preemption is
disabled or not should make no difference, only program order matters.

- Kevin

> For e.g... 
>   
>     arch_enter_lazy_mmu_mode()
>         hpte_need_flush()
>             get_cpu_var()   // this takes care of preempt_disable() 
>             adds vpns to per-cpu batch[i]
>             put_cpu_var()   // 
>     arch_leave_lazy_mmu_mode()
>
>> IOW, what could go wrong if the process is scheduled to another
>> CPU before preempt_disable() is called?
> So from above - I don't think your sequence to update
>    state->active = true 
> before calling arch_enter hook should be a problem.
> Based on above this looks mostly ok to me.
>
> -ritesh