linux-kernel - Re: [PATCH v3 06/13] mm: introduce generic lazy

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Message-ID: <390e41ae-4b66-40c1-935f-7a1794ba0b71@arm.com>
Date: Fri, 24 Oct 2025 14:13:35 +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 06/13] mm: introduce generic lazy_mmu helpers

On 23/10/2025 21:52, David Hildenbrand wrote:
> On 15.10.25 10:27, Kevin Brodsky wrote:
>> [...]
>>
>> * madvise_*_pte_range() call arch_leave() in multiple paths, some
>>    followed by an immediate exit/rescheduling and some followed by a
>>    conditional exit. These functions assume that they are called
>>    with lazy MMU disabled and we cannot simply use pause()/resume()
>>    to address that. This patch leaves the situation unchanged by
>>    calling enable()/disable() in all cases.
>
> I'm confused, the function simply does
>
> (a) enables lazy mmu
> (b) does something on the page table
> (c) disables lazy mmu
> (d) does something expensive (split folio -> take sleepable locks,
>     flushes tlb)
> (e) go to (a)

That step is conditional: we exit right away if pte_offset_map_lock()
fails. The fundamental issue is that pause() must always be matched with
resume(), but as those functions look today there is no situation where
a pause() would always be matched with a resume().

Alternatively it should be possible to pause(), unconditionally resume()
after the expensive operations are done and then leave() right away in
case of failure. It requires restructuring and might look a bit strange,
but can be done if you think it's justified.

>
> Why would we use enable/disable instead?
>
>>
>> * x86/Xen is currently the only case where explicit handling is
>>    required for lazy MMU when context-switching. This is purely an
>>    implementation detail and using the generic lazy_mmu_mode_*
>>    functions would cause trouble when nesting support is introduced,
>>    because the generic functions must be called from the current task.
>>    For that reason we still use arch_leave() and arch_enter() there.
>
> How does this interact with patch #11? 

It is a requirement for patch 11, in fact. If we called disable() when
switching out a task, then lazy_mmu_state.enabled would (most likely) be
false when scheduling it again.

By calling the arch_* helpers when context-switching, we ensure
lazy_mmu_state remains unchanged. This is consistent with what happens
on all other architectures (which don't do anything about lazy_mmu when
context-switching). lazy_mmu_state is the lazy MMU status *when the task
is scheduled*, and should be preserved on a context-switch.

>
>>
>> Note: x86 calls arch_flush_lazy_mmu_mode() unconditionally in a few
>> places, but only defines it if PARAVIRT_XXL is selected, and we are
>> removing the fallback in <linux/pgtable.h>. Add a new fallback
>> definition to <asm/pgtable.h> to keep things building.
>
> I can see a call in __kernel_map_pages() and
> arch_kmap_local_post_map()/arch_kmap_local_post_unmap().
>
> I guess that is ... harmless/irrelevant in the context of this series?

It should be. arch_flush_lazy_mmu_mode() was only used by x86 before
this series; we're adding new calls to it from the generic layer, but
existing x86 calls shouldn't be affected.

- Kevin