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

Huang, Ying
2023-01-05T05:52:59Z

Alistair Popple <apopple@nvidia.com> writes:

> Huang Ying <ying.huang@intel.com> writes:
>
>> 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.
>>
>> 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>
>> Cc: Bharata B Rao <bharata@amd.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: haoxin <xhao@linux.alibaba.com>
>> ---
>> mm/migrate.c | 114 ++++++++++++++++++++++++++++++++++++++++++++-------
>> 1 file changed, 99 insertions(+), 15 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ec9263a33d38..bdbe73fe2eb7 100644
>> --- 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++) {
>> + 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++;
>
> This currently isn't relevant for -ENOMEM and I think it would be
> clearer if it was dropped.

OK.

>> + stats->nr_failed_pages += nr_pages;
>
> Makes sense not to continue migration with low memory, but shouldn't we
> add the remaining unmigrated hugetlb folios to stats->nr_failed_pages as
> well? Ie. don't we still have to continue the iteration to to find and
> account for these?

I think nr_failed_pages only counts tried pages. IIUC, it's the
original behavior and behavior for non-hugetlb pages too.

>> + goto out;
>
> Given this is the only use of the out label, and that there is a special
> case for -ENOMEM there anyway I think it would be clearer to return
> directly.

Sounds good. Will do that in next version.

>> + 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;
>> +}
>> +
>> /*
>> * migrate_pages - migrate the folios specified in a list, to the free folios
>> * supplied as the target for the page migration
>> @@ -1437,7 +1518,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> int retry = 1;
>> int large_retry = 1;
>> int thp_retry = 1;
>> - int nr_failed = 0;
>> + int nr_failed;
>> int nr_retry_pages = 0;
>> int nr_large_failed = 0;
>> int pass = 0;
>> @@ -1454,6 +1535,12 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> trace_mm_migrate_pages_start(mode, reason);
>>
>> memset(&stats, 0, sizeof(stats));
>> + rc = migrate_hugetlbs(from, get_new_page, put_new_page, private, mode, reason,
>> + &stats, &ret_folios);
>> + if (rc < 0)
>> + goto out;
>> + nr_failed = rc;
>> +
>> split_folio_migration:
>> for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
>> retry = 0;
>> @@ -1462,30 +1549,28 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> nr_retry_pages = 0;
>>
>> list_for_each_entry_safe(folio, folio2, from, lru) {
>> + if (folio_test_hugetlb(folio)) {
>
> How do we hit this case? Shouldn't migrate_hugetlbs() have already moved
> any hugetlb folios off the from list?

Retried hugetlb folios will be kept in from list.

>> + list_move_tail(&folio->lru, &ret_folios);
>> + continue;
>> + }
>> +
>> /*
>> * Large folio statistics is based on the source large
>> * folio. Capture required information that might get
>> * lost during migration.
>> */
>> - is_large = folio_test_large(folio) && !folio_test_hugetlb(folio);
>> + is_large = folio_test_large(folio);
>> is_thp = is_large && folio_test_pmd_mappable(folio);
>> nr_pages = folio_nr_pages(folio);
>> +
>> cond_resched();
>>
>> - if (folio_test_hugetlb(folio))
>> - rc = unmap_and_move_huge_page(get_new_page,
>> - put_new_page, private,
>> - &folio->page, pass > 2, mode,
>> - reason,
>> - &ret_folios);
>> - else
>> - rc = unmap_and_move(get_new_page, put_new_page,
>> - private, folio, pass > 2, mode,
>> - reason, &ret_folios);
>> + rc = unmap_and_move(get_new_page, put_new_page,
>> + private, folio, pass > 2, mode,
>> + reason, &ret_folios);
>> /*
>> * The rules are:
>> - * Success: non hugetlb folio will be freed, hugetlb
>> - * folio will be put back
>> + * Success: folio will be freed
>> * -EAGAIN: stay on the from list
>> * -ENOMEM: stay on the from list
>> * -ENOSYS: stay on the from list
>> @@ -1512,7 +1597,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> stats.nr_thp_split += is_thp;
>> break;
>> }
>> - /* Hugetlb migration is unsupported */
>> } else if (!no_split_folio_counting) {
>> nr_failed++;
>> }

Best Regards,
Huang, Ying