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

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

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