linux-kernel - Re: [PATCH v2 2/7] mm: introduce local state for lazy

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Message-ID: <250835cd-f07a-4b8a-bc01-ace24b407efc@arm.com>
Date: Wed, 10 Sep 2025 18:11:54 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: David Hildenbrand <david@...hat.com>, 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 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,
 Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH v2 2/7] mm: introduce local state for lazy_mmu sections

On 09/09/2025 16:38, Alexander Gordeev wrote:
>>>>> Would that integrate well with LAZY_MMU_DEFAULT etc?
>>>> Hmm... I though the idea is to use LAZY_MMU_* by architectures that
>>>> want to use it - at least that is how I read the description above.
>>>>
>>>> It is only kasan_populate|depopulate_vmalloc_pte() in generic code
>>>> that do not follow this pattern, and it looks as a problem to me.
>> This discussion also made me realise that this is problematic, as the
>> LAZY_MMU_{DEFAULT,NESTED} macros were meant only for architectures'
>> convenience, not for generic code (where lazy_mmu_state_t should ideally
>> be an opaque type as mentioned above). It almost feels like the kasan
>> case deserves a different API, because this is not how enter() and
>> leave() are meant to be used. This would mean quite a bit of churn
>> though, so maybe just introduce another arch-defined value to pass to
>> leave() for such a situation - for instance,
>> arch_leave_lazy_mmu_mode(LAZY_MMU_FLUSH)?
> What about to adjust the semantics of apply_to_page_range() instead?
>
> It currently assumes any caller is fine with apply_to_pte_range() to
> enter the lazy mode. By contrast, kasan_(de)populate_vmalloc_pte() are
> not fine at all and must leave the lazy mode. That literally suggests
> the original assumption is incorrect.
>
> We could change int apply_to_pte_range(..., bool create, ...) to e.g.
> apply_to_pte_range(..., unsigned int flags, ...) and introduce a flag
> that simply skips entering the lazy mmu mode.

This is pretty much what Ryan proposed [1r] some time ago, although for
a different purpose (avoiding nesting). There wasn't much appetite for
it then, but I agree that this would be a more logical way to go about it.

- Kevin

[1r]
https://lore.kernel.org/all/20250530140446.2387131-4-ryan.roberts@arm.com/