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

[Nico Pache <npache@redhat.com>]

On Tue, Mar 31, 2026 at 8:13 AM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> >> +            if (*lock_dropped) {
> >>                      cond_resched();
> >>                      mmap_read_lock(mm);
> >> -                    mmap_locked = true;
> >> +                    *lock_dropped = false;
> >
> > So this is the bug. 'lock_dropped' needs to record if the lock was _ever_
> > dropped, not if it is _currently_ dropped.
>
> Ah, well spotted. I thought we discussed that at some point during
> review ...

Yes I think we've discussed this before, and IIRC the outcome was, "We
weren't sure why this semantic was in place, or if we truly needed to
track if it was dropped at any point, or simply if it is currently
dropped.". The code is rather confusing, and changed mid-flight during
my series.

Lorenzo asked me to unify this semantic across the two functions to
further simplify readability, but it looks like we indeed needed that
extra tracking.


Cheers,
 -- Nico

>
> Thanks for tackling this!
>
> [...]
>
> >
> > ----8<----
> > From a4dfc7718a15035449f344a0bc7f58e449366405 Mon Sep 17 00:00:00 2001
> > From: "Lorenzo Stoakes (Oracle)" <ljs@kernel.org>
> > Date: Tue, 31 Mar 2026 13:11:18 +0100
> > Subject: [PATCH] mm/khugepaged: fix issue with tracking lock
> >
> > We are incorrectly treating lock_dropped to track both whether the lock is
> > currently held and whether or not the lock was ever dropped.
> >
> > Update this change to account for this.
> >
> > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > ---
> >  mm/khugepaged.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index d21348b85a59..b8452dbdb043 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2828,6 +2828,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >       unsigned long hstart, hend, addr;
> >       enum scan_result last_fail = SCAN_FAIL;
> >       int thps = 0;
> > +     bool mmap_unlocked = false;
> >
> >       BUG_ON(vma->vm_start > start);
> >       BUG_ON(vma->vm_end < end);
> > @@ -2850,10 +2851,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >       for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> >               enum scan_result result = SCAN_FAIL;
> >
> > -             if (*lock_dropped) {
> > +             if (mmap_unlocked) {
> >                       cond_resched();
> >                       mmap_read_lock(mm);
> > -                     *lock_dropped = false;
> > +                     mmap_unlocked = false;
> > +                     *lock_dropped = true;
> >                       result = hugepage_vma_revalidate(mm, addr, false, &vma,
> >                                                       cc);
> >                       if (result  != SCAN_SUCCEED) {
> > @@ -2864,7 +2866,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>
> (in hurry so I might be wrong)
>
> Do we have to handle when collapse_single_pmd() dropped the lock as well?

There are some oddities about madvise that sadly I don't fully
understand. This is one of them. However, I dont believe khugepaged
had these lock_dropped semantics and just tracked the state of the
lock.

>
> --
> Cheers,
>
> David
>