Re: [PATCH v2 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/24/2022 9:56 AM, Huang, Ying wrote:
> Yang Shi <shy828301@gmail.com> writes:
> 
>> On Fri, Oct 21, 2022 at 11:41 AM Andrew Morton
>> <akpm@linux-foundation.org> wrote:
>>>
>>> On Fri, 21 Oct 2022 18:16:23 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
>>>
>>>> When THP migration, if THPs are split and all subpages are migrated successfully
>>>> , the migrate_pages() will still return the number of THP that were not migrated.
>>>> That will confuse the callers of migrate_pages(), for example, which will make
>>>> the longterm pinning failed though all pages are migrated successfully.
>>>>
>>>> Thus we should return 0 to indicate all pages are migrated in this case.
>>>>
>>>
>>> This had me puzzled for a while.  I think this wording is clearer?
>>>
>>> : During THP migration, if THPs are not migrated but they are split and all
>>> : subpages are migrated successfully, migrate_pages() will still return the
>>> : number of THP pages that were not migrated.  This will confuse the callers
>>> : of migrate_pages().  For example, the longterm pinning will failed though
>>> : all pages are migrated successfully.
>>> :
>>> : Thus we should return 0 to indicate that all pages are migrated in this
>>> : case.
>>>
>>> This is a fairly longstanding problem?  No Fixes: we can identify?
>>
>> It doesn't seem like a long standing issue. It seems like commit
>> b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
>> fixed one problem, but introduced this new one IIUC.
>>
>> Before this commit, the code did:
>>
>> nr_failed += retry + thp_retry;
>> rc = nr_failed;
>>
>> But retry and thp_retry were actually reset for each retry until the
>> last one. So as long as there is no permanent migration failure and
>> THP split failure, nr_failed should be 0 IIUC. TBH the code is a
>> little bit hard to follow, please correct me if I'm wrong.
> 
> I think that you are correct.  We can added
> 
> Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")

I think so too. Thanks Yang and Ying for pointing it out.

> 
>>> Did you consider the desirability of a -stable backport?
> 
> I think this can be backport to -stable.

Agree.

Andrew, could you help to add the Fixes tag and cc -stable? Thanks.