linux-kernel - Re: [PATCH v3 07/13] mm: enable lazy

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Message-ID: <f458fbed-266b-48a5-a6e0-745dc2893a34@arm.com>
Date: Fri, 24 Oct 2025 16:33:21 +0200
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>, "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 v3 07/13] mm: enable lazy_mmu sections to nest

On 24/10/2025 15:23, David Hildenbrand wrote:
>>>> + * currently enabled.
>>>>     */
>>>>    #ifdef CONFIG_ARCH_LAZY_MMU
>>>>    static inline void lazy_mmu_mode_enable(void)
>>>>    {
>>>> -    arch_enter_lazy_mmu_mode();
>>>> +    struct lazy_mmu_state *state = &current->lazy_mmu_state;
>>>> +
>>>> +    VM_BUG_ON(state->count == U8_MAX);
>>>
>>> No VM_BUG_ON() please.
>>
>> I did wonder if this would be acceptable!
>
> Use VM_WARN_ON_ONCE() and let early testing find any such issues.
>
> VM_* is active in debug kernels only either way! :)

That was my intention - I don't think the checking overhead is justified
in production.

>
> If you'd want to handle this in production kernels you'd need
>
> if (WARN_ON_ONCE()) {
>     /* Try to recover */
> }
>
> And that seems unnecessary/overly-complicated for something that
> should never happen, and if it happens, can be found early during testing.

Got it. Then I guess I'll go for a VM_WARN_ON_ONCE() (because indeed
once the overflow/underflow occurs it'll go wrong on every
enable/disable pair).

>
>>
>> What should we do in case of underflow/overflow then? Saturate or just
>> let it wrap around? If an overflow occurs we're probably in some
>> infinite recursion and we'll crash anyway, but an underflow is likely
>> due to a double disable() and saturating would probably allow to
>> recover.
>>
>>>
>>>> +    /* enable() must not be called while paused */
>>>> +    VM_WARN_ON(state->count > 0 && !state->enabled);
>>>> +
>>>> +    if (state->count == 0) {
>>>> +        arch_enter_lazy_mmu_mode();
>>>> +        state->enabled = true;
>>>> +    }
>>>> +    ++state->count;
>>>
>>> Can do
>>>
>>> if (state->count++ == 0) {
>>
>> My idea here was to have exactly the reverse order between enable() and
>> disable(), so that arch_enter() is called before lazy_mmu_state is
>> updated, and arch_leave() afterwards. arch_* probably shouldn't rely on
>> this (or care), but I liked the symmetry.
>
> I see, but really the arch callback should never have to care about that
> value -- unless something is messed up :)

Fair enough, then I can fold those increments/decrements ;)

- Kevin