Re: [PATCH] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure

[Dev Jain <dev.jain@arm.com>]

On 01/05/26 1:27 pm, Ye Liu wrote:
> From: Ye Liu <liuye@kylinos.cn>
> 
> __khugepaged_enter() sets MMF_VM_HUGEPAGE before allocating the
> corresponding mm_slot.  If mm_slot_alloc() fails, the function
> returns with the flag set but without inserting the mm into the
> khugepaged tracking structures.
> 
> This leaves the mm in an inconsistent state: it is marked as
> registered (MMF_VM_HUGEPAGE set), but will never be scanned by
> khugepaged.  Future attempts to register the mm are skipped since
> khugepaged_enter_vma() checks the flag and returns early.
> 
> Fix this by clearing MMF_VM_HUGEPAGE when mm_slot_alloc() fails,
> restoring the ability to retry registration later.
> 
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---
>  mm/khugepaged.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7d48d4fbd5f3..60ab7c1b61dd 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -559,8 +559,10 @@ void __khugepaged_enter(struct mm_struct *mm)
>  		return;
>  
>  	slot = mm_slot_alloc(mm_slot_cache);
> -	if (!slot)
> +	if (!slot) {
> +		mm_flags_clear(MMF_VM_HUGEPAGE, mm);
>  		return;
> +	}

You could just move the test_and_set() after this no?
So if slot allocation fails then MMF_VM_HUGEPAGE is never set.

Fixes tag is probably required but not sure about Ccing stable.
>  
>  	spin_lock(&khugepaged_mm_lock);
>  	mm_slot_insert(mm_slots_hash, mm, slot);