Re: [PATCH 1/2] mm: gup: Re-pin pages in case of trying several times to migrate

[Alistair Popple]

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

> On 10/20/2022 4:15 PM, Huang, Ying wrote:
>> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>>
>>> The migrate_pages() will return the number of {normal page, THP, hugetlb}
>>> that were not migrated, or an error code. That means it can still return
>>> the number of failure count, though the pages have been migrated
>>> successfully with several times re-try.
>> If my understanding were correct, if pages are migrated successfully
>> after several times re-tries, the return value will be 0.  There's one
>> possibility for migrate_pages() to return non-zero but all pages are
>> migrated.  That is, when THP is split and all subpages are migrated
>> successfully.
>
> Yeah, that's the case I tested. Thanks for pointing out. I'll re-write my
> incorrect commit message next time.

This is confusing to me. So users of move_page() will see an
unsuccessful migration even when all subpages were migrated? Seems like
we should fix the return code of migrate_pages() for this case where all
subpages were successfully migrated.

>>
>>> So we should not use the return value of migrate_pages() to determin
>>> if there are pages are failed to migrate. Instead we can validate the
>>> 'movable_page_list' to see if there are pages remained in the list,
>>> which are failed to migrate. That can mitigate the failure of longterm
>>> pinning.
>> Another choice is to use a special return value for split THP + success
>> migration.  But I'm fine to use list_empty(return_pages).
>
> OK. Using list_empty(return_pages) looks more simple.
>
>>
>>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>>> ---
>>>   mm/gup.c | 7 ++++---
>>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/gup.c b/mm/gup.c
>>> index 5182aba..bd8cfcd 100644
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -1914,9 +1914,10 @@ static int migrate_longterm_unpinnable_pages(
>>>   			.gfp_mask = GFP_USER | __GFP_NOWARN,
>>>   		};
>>>   -		if (migrate_pages(movable_page_list, alloc_migration_target,
>>> -				  NULL, (unsigned long)&mtc, MIGRATE_SYNC,
>>> -				  MR_LONGTERM_PIN, NULL)) {
>>> +		ret = migrate_pages(movable_page_list, alloc_migration_target,
>>> +				    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
>>> +				    MR_LONGTERM_PIN, NULL);
>>> +		if (ret < 0 || !list_empty(movable_page_list)) {
>> It seems that !list_empty() is sufficient here.
>
> OK. Drop the 'ret < 0'
>
>>>   			ret = -ENOMEM;
>> Why change the error code?  I don't think it's a good idea to do that.
>
> The GUP need a -errno for failure or partial success when migration, and we can
> not return the number of pages failed to migrate. So returning -ENOMEM seems
> suitable for both cases?

Seem reasonable to me. migrate_pages() might return -EAGAIN which would
cause everything to be re-pinned and tried again which is not what you
want here. See the comment at the start of
check_and_migrate_movable_pages().