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

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Date:   Fri, 21 Oct 2022 08:28:43 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     <akpm@...ux-foundation.org>, <david@...hat.com>, <ziy@...dia.com>,
        <shy828301@...il.com>, <jingshan@...ux.alibaba.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: gup: Re-pin pages in case of trying several
 times to migrate

Alistair Popple <apopple@...dia.com> writes:

> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
>
>> On 10/20/2022 4:15 PM, Huang, Ying wrote:
>>> Baolin Wang <baolin.wang@...ux.alibaba.com> 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@...ux.alibaba.com>
>>>> ---
>>>>   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().

Yes.  You are right.  The error code of migrate_pages() isn't good for
caller here.

Best Regards,
Huang, Ying