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

[Yang Shi]

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.

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