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

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Message-ID: <af4414b6-617c-4dc8-bddc-3ea00d1f6f3b@redhat.com>
Date: Fri, 24 Oct 2025 15:23:39 +0200
From: David Hildenbrand <david@...hat.com>
To: Kevin Brodsky <kevin.brodsky@....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

>>> + * 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! :)

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.

> 
> 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 :)

[...]

>>> +static inline bool in_lazy_mmu_mode(void)
>>
>> So these functions will reveal the actual arch state, not whether
>> _enabled() was called.
>>
>> As I can see in later patches, in interrupt context they are also
>> return "not in lazy mmu mode".
> 
> Yes - the idea is that a task is in lazy MMU mode if it enabled it and
> is in process context. The mode is never enabled in interrupt context.
> This has always been the intention, but it wasn't formalised until patch
> 12 (except on arm64).

Okay, thanks for clarifying.

-- 
Cheers

David / dhildenb