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

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

On Wed,  8 Dec 2021 21:39:30 +0800 Li Xinhai <lixinhai.lxh@gmail.com> wrote:

> When BUG_ON check for THP migration entry, we need also consider
> !thp_migration_supported() case, becaouse it is invalid at the code
> context. Now, make this check been covered without adding extra cost.
> 
> Because pmdval instead of *pmd is read, so just call
> pmd_migration_entry_wait().
> 
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> ---
>  mm/gup.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 2c51e9748a6a..49f490bae030 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>  	if (!pmd_present(pmdval)) {
>  		if (likely(!(flags & FOLL_MIGRATION)))
>  			return no_page_table(vma, flags);
> -		VM_BUG_ON(thp_migration_supported() &&
> +		VM_BUG_ON(!thp_migration_supported() ||

I don't get this.  thp_migration_supported() evaluates to a
compile-time constant.  What is the point in doing a runtime BUG based
on a thing which is known at compile-time?

>  				  !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