Re: [PATCH 6/8] mm/khugepaged: unmap pte before releasing vma write lock

[Usama Arif]

On Mon,  6 Jul 2026 09:44:53 -0600 Nico Pache <npache@xxxxxxxxxx> wrote:

> We are currently dropping the anon_vma write lock before unmapping the
> PTE. Although this is safe, due to us still holding the mmap_write_lock,
> its safer and less confusing to switch the order of these two operations.
> 
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Would it not be better to add a comment here why its safe to do so, instead
of extending lock hold period?

I think its more confusing to hold a lock for longer than we should.

> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c4ea2dc1591b..3c6f1254deca 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1548,10 +1548,10 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
>  
>  	result = SCAN_SUCCEED;
>  out_up_write:
> -	if (anon_vma_locked)
> -		anon_vma_unlock_write(vma->anon_vma);
>  	if (pte)
>  		pte_unmap(pte);
> +	if (anon_vma_locked)
> +		anon_vma_unlock_write(vma->anon_vma);
>  	mmap_write_unlock(mm);
>  out_nolock:
>  	if (folio)
> -- 
> 2.54.0
> 
>