Re: [v2 PATCH] mm: move_pages: report the number of non-attempted pages

[Yang Shi]

On 1/27/20 1:55 AM, Michal Hocko wrote:
> On Thu 23-01-20 07:38:51, Yang Shi wrote:
> > Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
> > the semantic of move_pages() was changed to return the number of
> > non-migrated pages (failed to migration) and the call would be aborted
> > immediately if migrate_pages() returns positive value.  But it didn't
> > report the number of pages that we even haven't attempted to migrate.
> > So, fix it by including non-attempted pages in the return value.
> I would rephrased the changelog like this
> "
> Since commit 49bd4d71637 ("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).
>
> This patch implements the second option 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.
> "
>
> > Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> > Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>    [4.17+]
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> With a more clarification, feel free to add
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks. Will post v3 with the rephrased commit log.

> > ---
> > v2: Rebased on top of the latest mainline kernel per Andrew
> >
> >   mm/migrate.c | 24 ++++++++++++++++++++++--
> >   1 file changed, 22 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 86873b6..9b8eb5d 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1627,8 +1627,18 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> >   			start = i;
> >   		} else if (node != current_node) {
> >   			err = do_move_pages_to_node(mm, &pagelist, current_node);
> > -			if (err)
> > +			if (err) {
> > +				/*
> > +				 * Positive err means the number of failed
> > +				 * pages to migrate.  Since we are going to
> > +				 * abort and return the number of non-migrated
> > +				 * pages, so need incude the rest of the
> > +				 * nr_pages that have not attempted as well.
> > +				 */
> > +				if (err > 0)
> > +					err += nr_pages - i - 1;
> >   				goto out;
> > +			}
> >   			err = store_status(status, start, current_node, i - start);
> >   			if (err)
> >   				goto out;
> > @@ -1659,8 +1669,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> >   			goto out_flush;
> >   
> >   		err = do_move_pages_to_node(mm, &pagelist, current_node);
> > -		if (err)
> > +		if (err) {
> > +			if (err > 0)
> > +				err += nr_pages - i - 1;
> >   			goto out;
> > +		}
> >   		if (i > start) {
> >   			err = store_status(status, start, current_node, i - start);
> >   			if (err)
> > @@ -1674,6 +1687,13 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> >   
> >   	/* Make sure we do not overwrite the existing error */
> >   	err1 = do_move_pages_to_node(mm, &pagelist, current_node);
> > +	/*
> > +	 * Don't have to report non-attempted pages here since:
> > +	 *     - If the above loop is done gracefully there is not non-attempted
> > +	 *       page.
> > +	 *     - If the above loop is aborted to it means more fatal error
> > +	 *       happened, should return err.
> > +	 */
> >   	if (!err1)
> >   		err1 = store_status(status, start, current_node, i - start);
> >   	if (!err)
> > --
> > 1.8.3.1