linux-kernel - Re: [PATCH 5/5] mm: migrate: return -ENOSYS if THP migration is unsupported

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Message-ID: <A842D483-D9F2-4347-9A4F-1939769BC831@nvidia.com>
Date:   Fri, 6 Nov 2020 15:17:26 -0500
From:   Zi Yan <ziy@...dia.com>
To:     Yang Shi <shy828301@...il.com>
CC:     <mhocko@...e.com>, <songliubraving@...com>, <mgorman@...e.de>,
        <jack@...e.cz>, <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] mm: migrate: return -ENOSYS if THP migration is
 unsupported

On 3 Nov 2020, at 8:03, Yang Shi wrote:

> In the current implementation unmap_and_move() would return -ENOMEM if
> THP migration is unsupported, then the THP will be split.  If split is
> failed just exit without trying to migrate other pages.  It doesn't make
> too much sense since there may be enough free memory to migrate other
> pages and there may be a lot base pages on the list.
>
> Return -ENOSYS to make consistent with hugetlb.  And if THP split is
> failed just skip and try other pages on the list.
>
> Just skip the whole list and exit when free memory is really low.
>
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/migrate.c | 62 ++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 46 insertions(+), 16 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8f6a61c9274b..b3466d8c7f03 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1172,7 +1172,7 @@ static int unmap_and_move(new_page_t get_new_page,
>  	struct page *newpage = NULL;
>
>  	if (!thp_migration_supported() && PageTransHuge(page))
> -		return -ENOMEM;
> +		return -ENOSYS;
>
>  	if (page_count(page) == 1) {
>  		/* page was freed from under us. So we are done. */
> @@ -1376,6 +1376,20 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  	return rc;
>  }
>
> +static inline int try_split_thp(struct page *page, struct page *page2,
> +				struct list_head *from)
> +{
> +	int rc = 0;
> +
> +	lock_page(page);
> +	rc = split_huge_page_to_list(page, from);
> +	unlock_page(page);
> +	if (!rc)
> +		list_safe_reset_next(page, page2, lru);

This does not work as expected, right? After macro expansion, we have
page2 = list_next_entry(page, lru). Since page2 is passed as a pointer, the change
does not return back the caller. You need to use the pointer to page2 here.

> +
> +	return rc;
> +}
> +
>  /*
>   * migrate_pages - migrate the pages specified in a list, to the free pages
>   *		   supplied as the target for the page migration
> @@ -1445,24 +1459,40 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  						reason, &ret_pages);
>
>  			switch(rc) {
> +			/*
> +			 * THP migration might be unsupported or the
> +			 * allocation could've failed so we should
> +			 * retry on the same page with the THP split
> +			 * to base pages.
> +			 *
> +			 * Head page is retried immediately and tail
> +			 * pages are added to the tail of the list so
> +			 * we encounter them after the rest of the list
> +			 * is processed.
> +			 */
> +			case -ENOSYS:
> +				/* THP migration is unsupported */
> +				if (is_thp) {
> +					if (!try_split_thp(page, page2, from)) {
> +						nr_thp_split++;
> +						goto retry;
> +					}
> +
> +					nr_thp_failed++;
> +					nr_failed += nr_subpages;
> +					break;
> +				}
> +
> +				/* Hugetlb migration is unsupported */
> +				nr_failed++;
> +				break;
>  			case -ENOMEM:
>  				/*
> -				 * THP migration might be unsupported or the
> -				 * allocation could've failed so we should
> -				 * retry on the same page with the THP split
> -				 * to base pages.
> -				 *
> -				 * Head page is retried immediately and tail
> -				 * pages are added to the tail of the list so
> -				 * we encounter them after the rest of the list
> -				 * is processed.
> +				 * When memory is low, don't bother to try to migrate
> +				 * other pages, just exit.

The comment does not match the code below. For THPs, the code tries to split the THP
and migrate the base pages if the split is successful.

>  				 */
>  				if (is_thp) {
> -					lock_page(page);
> -					rc = split_huge_page_to_list(page, from);
> -					unlock_page(page);
> -					if (!rc) {
> -						list_safe_reset_next(page, page2, lru);
> +					if (!try_split_thp(page, page2, from)) {
>  						nr_thp_split++;
>  						goto retry;
>  					}
> @@ -1490,7 +1520,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				break;
>  			default:
>  				/*
> -				 * Permanent failure (-EBUSY, -ENOSYS, etc.):
> +				 * Permanent failure (-EBUSY, etc.):
>  				 * unlike -EAGAIN case, the failed page is
>  				 * removed from migration page list and not
>  				 * retried in the next outer loop.
> -- 
> 2.26.2


—
Best Regards,
Yan Zi

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