Re: [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

["Huang, Ying" <ying.huang@intel.com>]

Li Xinhai <lixinhai.lxh@gmail.com> writes:

> When BUG_ON check for THP migration entry, the exsiting code only check

s/exsiting/existing/

Found some misspelling in the comments too.  Please fix them with some
tool.

Best Regards,
Huang, Ying

> 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 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>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> ---
> V2->V3:
> mention about the dead loop in commit message.
>
> 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