Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Lorenzo Stoakes (Oracle)]

On Tue, Mar 31, 2026 at 03:09:08PM -0600, Nico Pache wrote:
> On Tue, Mar 31, 2026 at 3:04 PM David Hildenbrand (Arm)
> <david@xxxxxxxxxx> wrote:
> >
> > On 3/31/26 22:50, David Hildenbrand (Arm) wrote:
> > > On 3/31/26 22:06, Lorenzo Stoakes (Oracle) wrote:
> > >> On Tue, Mar 31, 2026 at 10:00:58PM +0200, David Hildenbrand (Arm) wrote:
> > >>>
> > >>> Right.
> > >>>
> > >>> The original code used
> > >>>
> > >>>     bool mmap_locked = true;
> > >>>
> > >>> If the fix get squashed, maybe we should revert to that handling to
> > >>> minimize the churn?
> > >>
> > >> Well collapse_single_pmd() and all descendents would need to be updated to
> > >> invert the boolean, not sure it's worth it.
> > >
> > > The code is confusing me.
> > >
> > > In the old code, collapse_single_pmd() was called with "mmap_locked ==
> > > true" and set "mmap_locked=false".
> > >
> > > Now we call it with "mmap_unlocked=false" and it sets "mmap_unlocked=true".
> > >
> > > So far so good.
> > >
> > > However, collapse_scan_pmd() consumed "mmap_locked", now it effectively
> > > consumes "mmap_unlocked".
> >
> > Okay, I'm too confused for today and give up :)
> >
> > FWIW, this is the effective change, and the effective changes to
> > mmap_unlocked and lock_dropped ... confuse me:
>
> basically all the logic was inverted.

Yup.

> >  out_maybelock:
> >         /* Caller expects us to hold mmap_lock on return */
> > -       if (!mmap_locked)
> > +       if (mmap_unlocked) {
> > +               *lock_dropped = false;
>
> Lorenzo, is this correct? At first glance, shouldn't this be
> `locked_dropped = true`?

It is :) you're looking at the old code from your patch maybe?

I include my fix-patch again below to make life easier, but that code is:

+		if (mmap_unlocked) {
 			cond_resched();
 			mmap_read_lock(mm);
-			*lock_dropped = false;
+			mmap_unlocked = false;
+			*lock_dropped = true;

Having one on top of the other is causing the confusion :)

I also asked Claude to check this btw for correctness. I'm pretty confident it's
ok.

Cheers, Lorenzo

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