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

[Yang Shi]

On 1/22/20 7:27 PM, Wei Yang wrote:
> On Thu, Jan 23, 2020 at 07:38:51AM +0800, 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.
> First, we want to change the semantic of move_pages(2). The return value
> indicates the number of pages we didn't managed to migrate?

This is my understanding.

> Second, the return value from migrate_pages() doesn't mean the number of pages
> we failed to migrate. For example, one -ENOMEM is returned on the first page,
> migrate_pages() would return 1. But actually, no page successfully migrated.

This would not happen at all since migrate_pages() would just return 
-ENOMEM instead of a positive value, right?

> Third, even the migrate_pages() return the exact non-migrate page, we are not
> sure those non-migrated pages are at the tail of the list. Because in the last
> case in migrate_pages(), it just remove the page from list. It could be a page
> in the middle of the list. Then, in userspace, how the return value be
> leveraged to determine the valid status? Any page in the list could be the
> victim.

I think this problem has been discussed in another thread. Yes, the 
status may have non-valid value, but it is supposed to have valid value 
iff move_pages() return 0. Positive value is an error case, so the 
validity of status is not guaranteed.

> Sounds we need to think about this carefully.
>
> > 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>
> > ---
> > 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