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

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Message-ID: <cdd9bc60-96d4-4f19-86c3-dcf598ccbd92-agordeev@linux.ibm.com>
Date: Fri, 12 Sep 2025 16:05:07 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: Kevin Brodsky <kevin.brodsky@....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 Fri, Sep 12, 2025 at 03:02:15PM +0200, David Hildenbrand wrote:
> How would that work with nesting? I feel like there is a fundamental problem
> with nesting with what you describe but I might be wrong.

My picture is - flush on each lazy_mmu_disable(), pause on lazy_mmu_pause()
and honour only top-level arch_enter_lazy_mmu_mode_pte(mm, start, end, ptep) 
context on all nested levels.

In theory (and if I got it right, you leave the door open for this possibility)
every (mm, start, end, ptep) context could be stored for each nesting level
(as an opaque arch-specific data?).

But I do not really expect it ever, since arch_enter_lazy_mmu_mode_pte()
is only to be called in PTE walkers that never span more than one page
table and follow the pattern:

	ptep = pte_offset_map_lock(...);
	arch_enter_lazy_mmu_mode_pte(mm, start, end, ptep);

	for (...; ptep++) {
		/*
		 * set_pte(ptep, ...) or something
		 */
	}

	arch_leave_lazy_mmu_mode();                                             
	pte_unmap_unlock(...);                                         

As result, the lazy mmu mode is only "bound" to a single PTE table on s390,
while arch_enter_lazy_mmu_mode() is going to stay NOP.

So when you say you feel a fundamental problem - what that could be?

> David / dhildenb

Thanks!