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

[Baolin Wang]

On 10/24/2022 9:56 AM, Huang, Ying wrote:
> 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()")

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.