Re: [PATCH 1/3] mm: migrate: Fix the return value of migrate_pages()

[Mike Kravetz <mike.kravetz@oracle.com>]

On 11/7/21 01:57, Baolin Wang wrote:
> As Zi Yan pointed out, the syscall move_pages() can return a non-migrated
> number larger than the number of pages the users tried to migrate, when a
> THP page is failed to migrate. This is confusing for users.
> 
> Since other migration scenarios do not care about the actual non-migrated
> number of pages except the memory compaction migration which will fix in
> following patch. Thus we can change the return value to return the number
> of {normal page, THP, hugetlb} instead to avoid this issue, and the number
> of THP splits will be considered as the number of non-migrated THP, no matter
> how many subpages of the THP are migrated successfully. Meanwhile we should
> still keep the migration counters using the number of normal pages.
> 
> Co-developed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/migrate.c | 63 +++++++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 47 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a11e948..9aafdab 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1428,7 +1428,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
>   * @mode:		The migration mode that specifies the constraints for
>   *			page migration, if any.
>   * @reason:		The reason for page migration.
> - * @ret_succeeded:	Set to the number of pages migrated successfully if
> + * @ret_succeeded:	Set to the number of normal pages migrated successfully if
>   *			the caller passes a non-NULL pointer.
>   *
>   * The function returns after 10 attempts or if no pages are movable any more
> @@ -1436,7 +1436,9 @@ static inline int try_split_thp(struct page *page, struct page **page2,
>   * It is caller's responsibility to call putback_movable_pages() to return pages
>   * to the LRU or free list only if ret != 0.
>   *
> - * Returns the number of pages that were not migrated, or an error code.
> + * Returns the number of {normal page, THP} that were not migrated, or an error code.
> + * The number of THP splits will be considered as the number of non-migrated THP,
> + * no matter how many subpages of the THP are migrated successfully.
>   */
>  int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		free_page_t put_new_page, unsigned long private,
> @@ -1445,6 +1447,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	int retry = 1;
>  	int thp_retry = 1;
>  	int nr_failed = 0;
> +	int nr_failed_pages = 0;
>  	int nr_succeeded = 0;
>  	int nr_thp_succeeded = 0;
>  	int nr_thp_failed = 0;
> @@ -1456,13 +1459,16 @@ 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);
> +	LIST_HEAD(thp_split_pages);
>  	bool nosplit = (reason == MR_NUMA_MISPLACED);
> +	bool no_subpage_counting = false;
>  
>  	trace_mm_migrate_pages_start(mode, reason);
>  
>  	if (!swapwrite)
>  		current->flags |= PF_SWAPWRITE;
>  
> +thp_subpage_migration:
>  	for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
>  		retry = 0;
>  		thp_retry = 0;
> @@ -1511,18 +1517,20 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  			case -ENOSYS:
>  				/* THP migration is unsupported */
>  				if (is_thp) {
> -					if (!try_split_thp(page, &page2, from)) {
> +					nr_thp_failed++;
> +					if (!try_split_thp(page, &page2, &thp_split_pages)) {

Does thp_split_pages need to be initialized before this call?

-- 
Mike Kravetz