Re: [RFC 3/6] mm/migrate_pages: restrict number of pages to migrate in batch

[Zi Yan <ziy@nvidia.com>]

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On 21 Sep 2022, at 2:06, Huang Ying wrote:

> This is a preparation patch to batch the page unmapping and moving
> for the normal pages and THP.
>
> If we had batched the page unmapping, all pages to be migrated would
> be unmapped before copying the contents and flags of the pages.  If
> the number of pages that were passed to migrate_pages() was too large,
> too many pages would be unmapped.  Then, the execution of their
> processes would be stopped for too long time.  For example,
> migrate_pages() syscall will call migrate_pages() with all pages of a
> process.  To avoid this possible issue, in this patch, we restrict the
> number of pages to be migrated to be no more than HPAGE_PMD_NR.  That
> is, the influence is at the same level of THP migration.
>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Matthew Wilcox <willy@infradead.org>
> ---
>  mm/migrate.c | 93 +++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 67 insertions(+), 26 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 4a81e0bfdbcd..1077af858e36 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1439,32 +1439,7 @@ static inline int try_split_thp(struct page *page, struct list_head *split_pages
>  	return rc;
>  }
>
> -/*
> - * migrate_pages - migrate the pages specified in a list, to the free pages
> - *		   supplied as the target for the page migration
> - *
> - * @from:		The list of pages to be migrated.
> - * @get_new_page:	The function used to allocate free pages to be used
> - *			as the target of the page migration.
> - * @put_new_page:	The function used to free target pages if migration
> - *			fails, or NULL if no special handling is necessary.
> - * @private:		Private data to be passed on to get_new_page()
> - * @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 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
> - * because the list has become empty or no retryable pages exist any more.
> - * 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 {normal page, THP, hugetlb} 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,
> +static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  		free_page_t put_new_page, unsigned long private,
>  		enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
>  {
> @@ -1709,6 +1684,72 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	return rc;
>  }
>
> +/*
> + * migrate_pages - migrate the pages specified in a list, to the free pages
> + *		   supplied as the target for the page migration
> + *
> + * @from:		The list of pages to be migrated.
> + * @get_new_page:	The function used to allocate free pages to be used
> + *			as the target of the page migration.
> + * @put_new_page:	The function used to free target pages if migration
> + *			fails, or NULL if no special handling is necessary.
> + * @private:		Private data to be passed on to get_new_page()
> + * @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 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
> + * because the list has become empty or no retryable pages exist any more.
> + * 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 {normal page, THP, hugetlb} 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,
> +		enum migrate_mode mode, int reason, unsigned int *pret_succeeded)
> +{
> +	int rc, rc_gether = 0;
> +	int ret_succeeded, ret_succeeded_gether = 0;
> +	int nr_pages;
> +	struct page *page;
> +	LIST_HEAD(pagelist);
> +	LIST_HEAD(ret_pages);
> +
> +again:
> +	nr_pages = 0;
> +	list_for_each_entry(page, from, lru) {
> +		nr_pages += compound_nr(page);
> +		if (nr_pages > HPAGE_PMD_NR)

It is better to define a new MACRO like NR_MAX_BATCHED_MIGRATION to be
HPAGE_PMD_NR. It makes code easier to understand and change.

> +			break;
> +	}
> +	if (nr_pages > HPAGE_PMD_NR)
> +		list_cut_before(&pagelist, from, &page->lru);
> +	else
> +		list_splice_init(from, &pagelist);
> +	rc = migrate_pages_batch(&pagelist, get_new_page, put_new_page, private,
> +				 mode, reason, &ret_succeeded);
> +	ret_succeeded_gether += ret_succeeded;
> +	list_splice_tail_init(&pagelist, &ret_pages);
> +	if (rc == -ENOMEM) {
> +		rc_gether = rc;
> +		goto out;
> +	}
> +	rc_gether += rc;
> +	if (!list_empty(from))
> +		goto again;
> +out:
> +	if (pret_succeeded)
> +		*pret_succeeded = ret_succeeded_gether;
> +	list_splice(&ret_pages, from);
> +
> +	return rc_gether;
> +}
> +
>  struct page *alloc_migration_target(struct page *page, unsigned long private)
>  {
>  	struct folio *folio = page_folio(page);
> -- 
> 2.35.1


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Best Regards,
Yan, Zi

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