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

["Huang, Ying" <ying.huang@intel.com>]

Zi Yan <ziy@nvidia.com> writes:

> 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.

OK.  Will do that.

Best Regards,
Huang, Ying

>> +			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