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

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Message-ID: <80be36e5-d6e1-4b37-a1ca-47e92ac21b02-agordeev@linux.ibm.com>
Date: Thu, 11 Sep 2025 14:06:50 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Kevin Brodsky <kevin.brodsky@....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 Wed, Sep 10, 2025 at 06:11:54PM +0200, Kevin Brodsky wrote:

Hi Kevin,

> 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/

May be I missing the point, but I read it as an opposition to the whole
series in general and to the way apply_to_pte_range() would be altered
in particular:

 static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 				     unsigned long addr, unsigned long end,
 				     pte_fn_t fn, void *data, bool create,
-				     pgtbl_mod_mask *mask)
+				     pgtbl_mod_mask *mask, bool lazy_mmu)

The idea of instructing apply_to_page_range() to skip the lazy mmu mode
was not countered. Quite opposite, Liam suggested exactly the same:

<quote>
Could we do something like the pgtbl_mod_mask or zap_details and pass
through a struct or one unsigned int for create and lazy_mmu?

These wrappers are terrible for readability and annoying for argument
lists too.

Could we do something like the pgtbl_mod_mask or zap_details and pass
through a struct or one unsigned int for create and lazy_mmu?

At least we'd have better self-documenting code in the wrappers.. and if
we ever need a third boolean, we could avoid multiplying the wrappers
again.
<quote>

Thanks!