Re: [PATCH 2/8] migrate_pages: separate hugetlb folios migration

[Huang, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, 27 Dec 2022 08:28:53 +0800 Huang Ying <ying.huang@xxxxxxxxx> wrote:
>
>> This is a preparation patch to batch the folio unmapping and moving
>> for the non-hugetlb folios.  Based on that we can batch the TLB
>> shootdown during the folio migration and make it possible to use some
>> hardware accelerator for the folio copying.
>> 
>> In this patch the hugetlb folios and non-hugetlb folios migration is
>> separated in migrate_pages() to make it easy to change the non-hugetlb
>> folios migration implementation.
>> 
>> ...
>>
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1404,6 +1404,87 @@ struct migrate_pages_stats {
>>  	int nr_thp_split;
>>  };
>>  
>> +static int migrate_hugetlbs(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,
>> +			    struct migrate_pages_stats *stats,
>> +			    struct list_head *ret_folios)
>> +{
>> +	int retry = 1;
>> +	int nr_failed = 0;
>> +	int nr_retry_pages = 0;
>> +	int pass = 0;
>> +	struct folio *folio, *folio2;
>> +	int rc = 0, nr_pages;
>> +
>> +	for (pass = 0; pass < 10 && retry; pass++) {
>
> Why 10?

This is inherited from the original max pass number from
migrate_pages().  Which is introduced in commit 49d2e9cc4544 ("[PATCH]
Swap Migration V5: migrate_pages() function").  From the code and commit
message, I don't find out why.  I guess that we need some magic number
anyway.

Now, because the magic number is used in 2 places (migrate_pages() and
migrate_hugetlbs()), it's better to define it as a constant macro?

>> +		retry = 0;
>> +		nr_retry_pages = 0;
>> +
>> +		list_for_each_entry_safe(folio, folio2, from, lru) {
>> +			if (!folio_test_hugetlb(folio))
>> +				continue;
>> +
>> +			nr_pages = folio_nr_pages(folio);
>> +
>> +			cond_resched();
>> +
>> +			rc = unmap_and_move_huge_page(get_new_page,
>> +						      put_new_page, private,
>> +						      &folio->page, pass > 2, mode,
>> +						      reason, ret_folios);
>> +			/*
>> +			 * The rules are:
>> +			 *	Success: hugetlb folio will be put back
>> +			 *	-EAGAIN: stay on the from list
>> +			 *	-ENOMEM: stay on the from list
>> +			 *	-ENOSYS: stay on the from list
>> +			 *	Other errno: put on ret_folios list
>> +			 */
>> +			switch(rc) {
>> +			case -ENOSYS:
>> +				/* Hugetlb migration is unsupported */
>> +				nr_failed++;
>> +				stats->nr_failed_pages += nr_pages;
>> +				list_move_tail(&folio->lru, ret_folios);
>> +				break;
>> +			case -ENOMEM:
>> +				/*
>> +				 * When memory is low, don't bother to try to migrate
>> +				 * other folios, just exit.
>> +				 */
>> +				nr_failed++;
>> +				stats->nr_failed_pages += nr_pages;
>> +				goto out;
>> +			case -EAGAIN:
>> +				retry++;
>> +				nr_retry_pages += nr_pages;
>> +				break;
>> +			case MIGRATEPAGE_SUCCESS:
>> +				stats->nr_succeeded += nr_pages;
>> +				break;
>> +			default:
>> +				/*
>> +				 * Permanent failure (-EBUSY, etc.):
>> +				 * unlike -EAGAIN case, the failed folio is
>> +				 * removed from migration folio list and not
>> +				 * retried in the next outer loop.
>> +				 */
>> +				nr_failed++;
>> +				stats->nr_failed_pages += nr_pages;
>> +				break;
>> +			}
>> +		}
>> +	}
>> +out:
>> +	nr_failed += retry;
>> +	stats->nr_failed_pages += nr_retry_pages;
>> +	if (rc != -ENOMEM)
>> +		rc = nr_failed;
>> +
>> +	return rc;
>> +}
>
> The interpretation of the return value of this function is somewhat
> unobvious.
>
> I suggest that this function be fully commented.
>
> Why does a retry contribute to nr_failed.  What is the interpretation
> of nr_failed.  etcetera.

Sure.  Will do that in the next version.

Best Regards,
Huang, Ying