Re: [PATCH v2 2/7] mm: introduce local state for lazy_mmu sections

[Alexander Gordeev <agordeev@xxxxxxxxxxxxx>]

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@xxxxxxx/

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!