linux-kernel - Re: [PATCH v4 11/12] x86/xen: use lazy_mmu

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Message-ID: <216d54f1-334f-4600-9ecb-f7788b1abd7d@arm.com>
Date: Tue, 4 Nov 2025 11:28:42 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>, 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 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 11/12] x86/xen: use lazy_mmu_state when
 context-switching

On 03/11/2025 19:23, David Hildenbrand (Red Hat) wrote:
> On 03.11.25 19:29, Kevin Brodsky wrote:
>> On 03/11/2025 16:15, David Hildenbrand (Red Hat) wrote:
>>> On 29.10.25 11:09, Kevin Brodsky wrote:
>>>> [...]
>>>>
>>>> @@ -437,7 +436,7 @@ static void xen_end_context_switch(struct
>>>> task_struct *next)
>>>>          xen_mc_flush();
>>>>        leave_lazy(XEN_LAZY_CPU);
>>>> -    if (test_and_clear_ti_thread_flag(task_thread_info(next),
>>>> TIF_LAZY_MMU_UPDATES))
>>>> +    if (next->lazy_mmu_state.active)
>>>
>>> This is nasty. If in_lazy_mmu_mode() is not sufficient, we will want
>>> to have a separate helper that makes it clear what the difference
>>> between both variants is.
>>
>> in_lazy_mmu_mode() operates on current, but here we're operating on a
>> different task. The difference is more fundamental than just passing a
>> task_struct * or not: in_lazy_mmu_mode() is about whether we're
>> currently in lazy MMU mode, i.e. not paused and not in interrupt
>> context. A task that isn't scheduled is never in lazy MMU mode -
>> lazy_mmu_state.active is just the saved state to be restored when
>> scheduled again.
>>
>> My point here is that we could have a helper for this use-case, but it
>> should not be used in other situations (at least not on current). Maybe
>> __task_lazy_mmu_active(task)? I do wonder if accessing lazy_mmu_state
>> directly isn't expressing the intention well enough though (checking the
>> saved state).
>
>
> Likely there should be a
>
> /**
>  * task_lazy_mmu_active - test whether the lazy-mmu mode is active for a
>  *              task
>  * @task: ...
>  *
>  * The lazy-mmu mode is active if a task has lazy-mmu mode enabled and
>  * currently not paused.
>  */
> static inline bool task_lazy_mmu_active(struct task_struct *task)
> {
>     return task->lazy_mmu_state.active;
> }
>
> /**
>  * in_lazy_mmu_mode() - test whether current is in lazy-mmu mode
>  *
>  * Test whether the current task is in lazy-mmu mode: whether the
>  * interrupts are enabled and the lazy-mmu mode is active for the
>  * current task.
>  */
>  static inline bool in_lazy_mmu_mode(void)
>  {
> +    if (in_interrupt())
> +        return false;
> +
>      return task_lazy_mmu_active(current);
>  }
>
>
> Something like that. Maybe we can find better terminology.

That's probably the clearest yes, will make the change. I can't think of
more self-documenting names, spelling out the difference in the comments
is likely the best we can do.

- Kevin