Re: [PATCH] mm: stricter check on THP migration entry

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jun 18, 2022 at 03:32:43PM +0800, Miaohe Lin wrote:
> When VM_BUG_ON check for THP migration entry, the existing code only check
> thp_migration_supported case, but not for !thp_migration_supported case.
> If !thp_migration_supported() and !pmd_present(), the original code may
> dead loop in theory.  To make the VM_BUG_ON check more consistent, we need
> to catch both cases.

This patch makes no sense to me.

> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/memory.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index fee2884481f2..6a9a17c7f58a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5043,10 +5043,9 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  
>  		barrier();
>  		if (unlikely(is_swap_pmd(vmf.orig_pmd))) {
> -			VM_BUG_ON(thp_migration_supported() &&
> -					  !is_pmd_migration_entry(vmf.orig_pmd));
> -			if (is_pmd_migration_entry(vmf.orig_pmd))
> -				pmd_migration_entry_wait(mm, vmf.pmd);
> +			VM_BUG_ON(!thp_migration_supported() ||
> +				  !is_pmd_migration_entry(vmf.orig_pmd));
> +			pmd_migration_entry_wait(mm, vmf.pmd);
>  			return 0;
>  		}
>  		if (pmd_trans_huge(vmf.orig_pmd) || pmd_devmap(vmf.orig_pmd)) {
> -- 
> 2.23.0
> 
>