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

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Wed, Apr 01, 2026 at 09:05:39AM +0200, David Hildenbrand (Arm) wrote:
> On 3/31/26 23:49, Nico Pache wrote:
> > On Tue, Mar 31, 2026 at 3:35 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >>
> >> On Tue, 31 Mar 2026 22:00:58 +0200 "David Hildenbrand (Arm)" <david@kernel.org> 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?
> >>>
> >>> (this is not in stable, right?)
> >>
> >> "mm/khugepaged: unify khugepaged and madv_collapse with
> >> collapse_single_pmd(): is presently in mm-stable.
> >>
> >> Doing a big rebase&rework isn't the done thing, I believe.  So queue it
> >> afterwards, add the Fixes: and tolerate the minor runtime bisection
> >> hole.  It ends up looking like a later hotfix.
> >>
> >>
> >> Below is what I added to mm-unstable.  I added a note-to-self to check
> >> in on Nico's question at
> >> https://lore.kernel.org/all/CAA1CXcCYLGQBy9nT-MSTPVK=XgWWtD6Af-QiA_6pXtM_2pybPg@mail.gmail.com/T/#u
> >>
> >>
> >> From: "Lorenzo Stoakes (Oracle)" <ljs@kernel.org>
> >> Subject: mm/khugepaged: fix issue with tracking lock
> >> Date: Tue, 31 Mar 2026 13:11:18 +0100
> >>
> >> 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.
> >>
> >> Link: https://lkml.kernel.org/r/7760c811-e100-4d40-9217-0813c28314be@lucifer.local
> >> Fixes: 330f3758a3bc ("mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()")
> >> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> >> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> >> Reviewed-by: Nico Pache <npache@redhat.com>
> >> ...
> >> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> >> ---
> >>
> >>  mm/khugepaged.c |   12 ++++++++----
> >>  1 file changed, 8 insertions(+), 4 deletions(-)
> >>
> >> --- a/mm/khugepaged.c~mm-khugepaged-fix-issue-with-tracking-lock
> >> +++ a/mm/khugepaged.c
> >> @@ -2828,6 +2828,7 @@ int madvise_collapse(struct vm_area_stru
> >>         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_stru
> >>         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_stru
> >>                         hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
> >>                 }
> >>
> >> -               result = collapse_single_pmd(addr, vma, lock_dropped, cc);
> >> +               result = collapse_single_pmd(addr, vma, &mmap_unlocked, cc);
> >>
> >>                 switch (result) {
> >>                 case SCAN_SUCCEED:
> >> @@ -2893,8 +2895,10 @@ int madvise_collapse(struct vm_area_stru
> >>
> >>  out_maybelock:
> >>         /* Caller expects us to hold mmap_lock on return */
> >> -       if (*lock_dropped)
> >> +       if (mmap_unlocked) {
> >> +               *lock_dropped = true;
> >
> > Hmm this was =false in David's example and = true here? Now i am also
> > confused hahaha. It seems Lorenzo's code is correct. I think i
> > responded to my own code changes, when I thought David was replying to
> > Lorenzo's.
>
> I think I messed up that in my posting when manually applying :)
>
> So this here is correct.

Yeah the fix-patch on top made it all confusing :)

FWIW :P I also had claude do its thing to check it on this, and it gave the
thumbs up.

>
> --
> Cheers,
>
> David

Thanks, Lorenzo