Re: [PATCH v2] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure

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

On 06/05/26 6:51 am, 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.
> 
> Fixes: 16618670276a ("mm: khugepaged: avoid pointless allocation for struct mm_slot")
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---
> Changes since v1:
> - Add Fixes tag as suggested by Dev Jain and Lance Yang
> 
>  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;
> +	}

Note that, a racing khugepaged_enter_vma() may back off
when it sees that MMF_VM_HUGEPAGE is set, but then the above
clears the flag after slot alloc failure. So we end up not
registering the mm with khugepaged. But I am sure no one
cares, we are in much big trouble if slot alloc is failing.

Although one could argue the same about this patch, I will
still say it is important to fix "flag is set but not registered
with khugepaged" because that just feels wrong.

Reviewed-by: Dev Jain <dev.jain@arm.com>


>  
>  	spin_lock(&khugepaged_mm_lock);
>  	mm_slot_insert(mm_slots_hash, mm, slot);