Re: [PATCH v1 2/4] mm: Expose abnormal new_pte during move_ptes

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 29 May 2025 15:56:48 +0000 Pu Lehui <pulehui@huaweicloud.com> wrote:

> From: Pu Lehui <pulehui@huawei.com>
> 
> When executing move_ptes, the new_pte must be NULL, otherwise it will be
> overwritten by the old_pte, and cause the abnormal new_pte to be leaked.
> In order to make this problem to be more explicit, let's add
> WARN_ON_ONCE when new_pte is not NULL.
> 
> ...
>
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -237,6 +237,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
>  
>  	for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
>  				   new_pte++, new_addr += PAGE_SIZE) {
> +		WARN_ON_ONCE(!pte_none(*new_pte));
> +
>  		if (pte_none(ptep_get(old_pte)))
>  			continue;
>  

We now have no expectation that this will trigger, yes?  It's a sanity
check that patch [1/4] is working?  Perhaps VM_WARN_ON_ONCE() would be
more appropriate.  And maybe even a comment:

	/* temporary, remove this one day */