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

[David Hildenbrand (Arm)]

>> +		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@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> ---
>  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