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