Re: [PATCH -V2 5/7] migrate_pages(): fix failure counting for THP on -ENOSYS

Huang, Ying
2022-07-13T08:30:46Z

Oscar Salvador <osalvador@suse.de> writes:

> On Mon, Jul 11, 2022 at 04:49:46PM +0800, Huang Ying wrote:
>> If THP or hugetlbfs page migration isn't supported, unmap_and_move()
>> or unmap_and_move_huge_page() will return -ENOSYS. For THP, splitting
>> will be tried, but if splitting doesn't succeed, the THP will be left
>> in "from" list wrongly. If some other pages are retried, the THP
>> migration failure will counted again. This is fixed via moving the
>> failure THP from "from" to "ret_pages".
>>
>> Another issue of the original code is that the unsupported failure
>> processing isn't consistent between THP and hugetlbfs page. Make them
>> consistent in this patch to make the code easier to be understood too.
>>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Fixes: 5984fabb6e82 ("mm: move_pages: report the number of non-attempted pages")
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Yang Shi <shy828301@gmail.com>
>> ---
>> mm/migrate.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 4bceba143db0..8cce73b7c046 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1192,10 +1192,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>> * tables or check whether the hugepage is pmd-based or not before
>> * kicking migration.
>> */
>> - if (!hugepage_migration_supported(page_hstate(hpage))) {
>> - list_move_tail(&hpage->lru, ret);
>> + if (!hugepage_migration_supported(page_hstate(hpage)))
>> return -ENOSYS;
>> - }
>>
>> if (page_count(hpage) == 1) {
>> /* page was freed from under us. So we are done. */
>> @@ -1392,6 +1390,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> * page 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_pages list then splice to
>> * from list
>> */
>> @@ -1421,6 +1420,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> }
>>
>> nr_failed_pages += nr_subpages;
>> + list_move_tail(&page->lru, &ret_pages);
>
> I must be missing something, but migrate_pages() calls unmap_and_move_huge_page()
> with ret being ret_pages, so
>
> list_move_tail(&hpage->lru, ret) == list_move_tail(&page->lru, &ret_pages)
>
> Yet, you say "This is fixed via moving the failure THP from "from" to "ret_pages"".
> /me confused.

To make it consistent between hugetlb page and THP/normal page, I have
revised the unmap_and_move_huge_page() via deleting the list_move_tail()
there for ENOSYS. After that, we move the hugetlb page and THP/normal
page in the same way in migrate_pages().

Best Regards,
Huang, Ying