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

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Date:   Tue, 30 Mar 2021 16:42:07 +0200
From:   Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     mgorman@...e.de, kirill.shutemov@...ux.intel.com, ziy@...dia.com,
        mhocko@...e.com, ying.huang@...el.com, hughd@...gle.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, borntraeger@...ibm.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Subject: Re: [PATCH 5/6] mm: migrate: don't split THP for misplaced NUMA
 page

On Mon, 29 Mar 2021 11:33:11 -0700
Yang Shi <shy828301@...il.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@...il.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?