Re: [PATCH V2] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

[Li Xinhai <lixinhai.lxh@gmail.com>]

On 12/11/21 11:10 PM, Li Xinhai wrote:
> When BUG_ON check for THP migration entry, the exsiting code only check
> thp_migration_supported case, but not for !thp_migration_supported case.
> To make the BUG_ON check consistent, we need catch both cases.
> 
> Move the BUG_ON check one step eariler, because if the bug happen we
> should know it instead of depend on FOLL_MIGRATION been used by caller.
> 
> Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
> check, the existing code don't help to avoid useless locking within
> pmd_migration_entry_wait(), so remove that check.
> 
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> ---
V1->V2:
Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
for it.

>   mm/gup.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 2c51e9748a6a..94d0e586ca0b 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>   	}
>   retry:
>   	if (!pmd_present(pmdval)) {
> +		/*
> +		 * Should never reach here, if thp migration is not supported;
> +		 * Otherwise, it must be a thp miration entry.
> +		 */
> +		VM_BUG_ON(!thp_migration_supported() ||
> +				  !is_pmd_migration_entry(pmdval));
> +
>   		if (likely(!(flags & FOLL_MIGRATION)))
>   			return no_page_table(vma, flags);
> -		VM_BUG_ON(thp_migration_supported() &&
> -				  !is_pmd_migration_entry(pmdval));
> -		if (is_pmd_migration_entry(pmdval))
> -			pmd_migration_entry_wait(mm, pmd);
> +
> +		pmd_migration_entry_wait(mm, pmd);
>   		pmdval = READ_ONCE(*pmd);
>   		/*
>   		 * MADV_DONTNEED may convert the pmd to null because
>