Re: [PATCH] move_pages.2: Returning positive value is a new error case

[Yang Shi]

On 1/29/20 10:58 AM, Michal Hocko wrote:
> On Thu 30-01-20 02:30:32, Yang Shi wrote:
> > Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
> > the semantic of move_pages() has changed to return the number of
> > non-migrated pages if they were result of a non-fatal reasons (usually a
> > busy page).  This was an unintentional change that hasn't been noticed
> > except for LTP tests which checked for the documented behavior.
> >
> > There are two ways to go around this change.  We can even get back to the
> > original behavior and return -EAGAIN whenever migrate_pages is not able
> > to migrate pages due to non-fatal reasons.  Another option would be to
> > simply continue with the changed semantic and extend move_pages
> > documentation to clarify that -errno is returned on an invalid input or
> > when migration simply cannot succeed (e.g. -ENOMEM, -EBUSY) or the
> > number of pages that couldn't have been migrated due to ephemeral
> > reasons (e.g. page is pinned or locked for other reasons).
> >
> > We decided to keep the second option in kernel because this behavior is in
> > place for some time without anybody complaining and possibly new users
> > depending on it.  Also it allows to have a slightly easier error handling
> > as the caller knows that it is worth to retry when err > 0.
> >
> > Update man pages to reflect the new semantic.
> Thanks a lot! Looks good to me. I would just add a note that it is
> generally recommended to pre-initialize status array by -1 to have a
> reliable way to check which pages have been migrated. The man page says
> "The array contains valid values only if move_pages() did not return an
> error."
>
> I would just add. "Pre initialization of the array to -1 or similar
> value which cannot represent a real numa node could help to identify
> pages that have been migrated".

Sure. Adding this note right after the explanation of positive value 
error seems fine?

> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> > ---
> >   man2/move_pages.2 | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/man2/move_pages.2 b/man2/move_pages.2
> > index 1bf1053..c6cf3f8 100644
> > --- a/man2/move_pages.2
> > +++ b/man2/move_pages.2
> > @@ -164,9 +164,13 @@ returns zero.
> >   .\" do the right thing?
> >   On error, it returns \-1, and sets
> >   .I errno
> > -to indicate the error.
> > +to indicate the error. Or positive value to report the number of
> > +non-migrated pages.
> >   .SH ERRORS
> >   .TP
> > +.B Positive value
> > +The number of non-migrated pages if they were result of a non-fatal
> > +reasons since version 4.17.
> >   .B E2BIG
> >   Too many pages to move.
> >   Since Linux 2.6.29,
> > --
> > 1.8.3.1