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

David Hildenbrand (Arm)
2026-03-31T14:13:47Z

>> + 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 ...

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?

--
Cheers,

David