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

Zi Yan
2022-09-21T16:30:24Z

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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

--
Best Regards,
Yan, Zi

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