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

Alexander Gordeev
2025-09-09T14:39:55Z

On Tue, Sep 09, 2025 at 03:49:46PM +0200, Kevin Brodsky wrote:
> On 09/09/2025 13:54, David Hildenbrand wrote:
> > On 09.09.25 13:45, Alexander Gordeev wrote:
> >> On Tue, Sep 09, 2025 at 12:09:48PM +0200, David Hildenbrand wrote:
> >>> On 09.09.25 11:40, Alexander Gordeev wrote:
> >>>> On Tue, Sep 09, 2025 at 11:07:36AM +0200, David Hildenbrand wrote:
> >>>>> On 08.09.25 09:39, Kevin Brodsky wrote:
> >>>>>> arch_{enter,leave}_lazy_mmu_mode() currently have a stateless API
> >>>>>> (taking and returning no value). This is proving problematic in
> >>>>>> situations where leave() needs to restore some context back to its
> >>>>>> original state (before enter() was called). In particular, this
> >>>>>> makes it difficult to support the nesting of lazy_mmu sections -
> >>>>>> leave() does not know whether the matching enter() call occurred
> >>>>>> while lazy_mmu was already enabled, and whether to disable it or
> >>>>>> not.
> >>>>>>
> >>>>>> This patch gives all architectures the chance to store local state
> >>>>>> while inside a lazy_mmu section by making enter() return some value,
> >>>>>> storing it in a local variable, and having leave() take that value.
> >>>>>> That value is typed lazy_mmu_state_t - each architecture defining
> >>>>>> __HAVE_ARCH_ENTER_LAZY_MMU_MODE is free to define it as it sees fit.
> >>>>>> For now we define it as int everywhere, which is sufficient to
> >>>>>> support nesting.
> >>>> ...
> >>>>>> {
> >>>>>> + lazy_mmu_state_t lazy_mmu_state;
> >>>>>> ...
> >>>>>> - arch_enter_lazy_mmu_mode();
> >>>>>> + lazy_mmu_state = arch_enter_lazy_mmu_mode();
> >>>>>> ...
> >>>>>> - arch_leave_lazy_mmu_mode();
> >>>>>> + arch_leave_lazy_mmu_mode(lazy_mmu_state);
> >>>>>> ...
> >>>>>> }
> >>>>>>
> >>>>>> * In a few cases (e.g. xen_flush_lazy_mmu()), a function knows that
> >>>>>>      lazy_mmu is already enabled, and it temporarily disables it by
> >>>>>>      calling leave() and then enter() again. Here we want to ensure
> >>>>>>      that any operation between the leave() and enter() calls is
> >>>>>>      completed immediately; for that reason we pass
> >>>>>> LAZY_MMU_DEFAULT to
> >>>>>>      leave() to fully disable lazy_mmu. enter() will then
> >>>>>> re-enable it
> >>>>>>      - this achieves the expected behaviour, whether nesting
> >>>>>> occurred
> >>>>>>      before that function was called or not.
> >>>>>>
> >>>>>> Note: it is difficult to provide a default definition of
> >>>>>> lazy_mmu_state_t for architectures implementing lazy_mmu, because
> >>>>>> that definition would need to be available in
> >>>>>> arch/x86/include/asm/paravirt_types.h and adding a new generic
> >>>>>>     #include there is very tricky due to the existing header soup.
> >>>>>
> >>>>> Yeah, I was wondering about exactly that.
> >>>>>
> >>>>> In particular because LAZY_MMU_DEFAULT etc resides somewehere
> >>>>> compeltely
> >>>>> different.
> >>>>>
> >>>>> Which raises the question: is using a new type really of any
> >>>>> benefit here?
> >>>>>
> >>>>> Can't we just use an "enum lazy_mmu_state" and call it a day?
> >>>>
> >>>> I could envision something completely different for this type on s390,
> >>>> e.g. a pointer to a per-cpu structure. So I would really ask to stick
> >>>> with the current approach.
>
> This is indeed the motivation - let every arch do whatever it sees fit.
> lazy_mmu_state_t is basically an opaque type as far as generic code is
> concerned, which also means that this API change is the first and last
> one we need (famous last words, I know).
>
> I mentioned in the cover letter that the pkeys-based page table
> protection series [1] would have an immediate use for lazy_mmu_state_t.
> In that proposal, any helper writing to pgtables needs to modify the
> pkey register and then restore it. To reduce the overhead, lazy_mmu is
> used to set the pkey register only once in enter(), and then restore it
> in leave() [2]. This currently relies on storing the original pkey
> register value in thread_struct, which is suboptimal and most
> importantly doesn't work if lazy_mmu sections nest. With this series, we
> could instead store the pkey register value in lazy_mmu_state_t
> (enlarging it to 64 bits or more).
>
> I also considered going further and making lazy_mmu_state_t a pointer as
> Alexander suggested - more complex to manage, but also a lot more flexible.
>
> >>> 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.

Thanks!