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

[Huang, Ying]

Yang Shi <shy828301@xxxxxxxxx> writes:

> On Fri, Oct 21, 2022 at 11:41 AM Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> On Fri, 21 Oct 2022 18:16:23 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> 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()")

>> Did you consider the desirability of a -stable backport?

I think this can be backport to -stable.

Best Regards,
Huang, Ying