Re: [PATCH 3/4] mm/mprotect: un-inline folio_pte_batch_flags()

Pedro Falcato
2026-03-20T11:00:03Z

On Fri, Mar 20, 2026 at 10:36:59AM +0000, Lorenzo Stoakes (Oracle) wrote:
> On Thu, Mar 19, 2026 at 10:41:54PM +0100, David Hildenbrand (Arm) wrote:
> >
> > >
> > > This is all seems VERY delicate, and subject to somebody else coming along and
> > > breaking it/causing some of these noinline/__always_inline invocations to make
> > > things far worse.
> > >
> > > I also reserve the right to seriously rework this pile of crap software.
> > >
> > > I'd rather we try to find less fragile ways to optimise!
> > >
> > > Maybe there's some steps that are bigger wins than others?
> >
> > What we can do is, collect similar folio_pte_batch_*() variants and
> > centralize them in mm/utils.c.
>
> I'm not sure that addresses any of the comments above?
>
> Putting logic specific to components of mm away from where those components
> are and into mm/util.c seems like a complete regression in terms of
> fragility and code separation.
>
> And for what reason would you want to do that? To force a noinline of an
> inline and people 'just have to know' that's why you randomly separated the
> two?
>
> Doesn't sound appealing overall.
>
> I'd rather we find a way to implement the batching such that it doesn't
> exhibit bad inlining decisions in the first place.

Yes, you make a good point. At the end of the day (taking change_protection()
as the example at hand here), after these changes:
change_protection()
loop over p4ds
loop over puds
loop over pmds
loop over ptes
nr_ptes = loop over ptes and find out how many we have
if (making write) {
loop over nr_ptes
loop over ptes and find out how many are anonexclusive or not, in a row
loop over ptes and set them
} else {
loop over ptes and set them
}

which the compiler FWIW tries to inline it all into one function, but then
does a poor job at figuring things out. And the CPU gets confused too. It
was frankly shocking how much performance I could squeeze out of a

if (nr_ptes == 1) {
/* don't bother with the loops and the funny logic */
}

I would not be surprised if the other syscalls have similar problems.

>
> I mean mprotect_folio_batch() having !folio, !folio_test_large() checks
> only there is already silly, we should have a _general_ function that does
> optimisations like that.
>
> Isn't the issue rather than folio_pte_batch_flags() shouldn't be an inline
> function in internal.h but rather a function in mm/util.c?
>
> >
> > For
> >
> > nr_ptes = mprotect_folio_pte_batch(folio, pte, oldpte,
> > max_nr_ptes, /* flags = */ 0)
> >
> > We might just be able to use folio_pte_batch()?
>
> Yup.
>
> >
> > For the other variant (soft-dirt+write) we'd have to create a helper like
> >
> > folio_pte_batch_sd_w() [better name suggestion welcome]
> >
> > That will reduce the code footprint overall I guess.
>
> I mean yeah that's a terrible name so obviously it'd have to be something
> better.
>
> But again, this seems pretty stupid, now we're writing a bunch of duplicate
> per-case code to force noinline because we made the central function inline
> no?

Yeah.

>
> That's also super fragile, because an engineer might later decide that
> pattern is horrible and fix it, and we regress this.
>
> But I mean overall, is the perf here really all that important? Are people
> really that dependent on mprotect() et al. performing brilliantly fast?

Obviously no one truly depends on mprotect, but I believe in fast primitives :)

> Couldn't we do this with any mm interface and end up making efforts that
> degrade code quality, increase fragility for dubious benefit?

Yes, which is why I don't want to degrade code quality :) It would be ideal to
find something that works both ways. Per my description of the existing code
above, you can tell that it's neither fast, nor beautiful :p

--
Pedro