Re: [PATCH 5/6] mm: migrate: don't split THP for misplaced NUMA page

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

On Mon, 29 Mar 2021 11:33:11 -0700
Yang Shi <shy828301@gmail.com> wrote:

> The old behavior didn't split THP if migration is failed due to lack of
> memory on the target node.  But the THP migration does split THP, so keep
> the old behavior for misplaced NUMA page migration.
> 
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  mm/migrate.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 86325c750c14..1c0c873375ab 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1444,6 +1444,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	int swapwrite = current->flags & PF_SWAPWRITE;
>  	int rc, nr_subpages;
>  	LIST_HEAD(ret_pages);
> +	bool nosplit = (reason == MR_NUMA_MISPLACED);
>  
>  	if (!swapwrite)
>  		current->flags |= PF_SWAPWRITE;
> @@ -1495,7 +1496,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  			 */
>  			case -ENOSYS:
>  				/* THP migration is unsupported */
> -				if (is_thp) {
> +				if (is_thp && !nosplit) {

This is the "THP migration is unsupported" case, but according to your
description you rather want to change the -ENOMEM case?

Could this be the correct place to trigger THP split for NUMA balancing,
for architectures not supporting THP migration, like s390?

Do I understand it correctly that this change (for -ENOSYS) would
result in always failed THP migrations during NUMA balancing, if THP
migration was not supported?