Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

[Yang Shi]

On 1/21/20 12:40 AM, Michal Hocko wrote:
> On Tue 21-01-20 09:44:16, Wei Yang wrote:
> > On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
> > > On Mon 20-01-20 14:06:26, Michal Hocko wrote:
> > > > On Sat 18-01-20 13:26:43, Yang Shi wrote:
> > > > > The do_move_pages_to_node() might return > 0 value, the number of pages
> > > > > that are not migrated, then the value will be returned to userspace
> > > > > directly.  But, move_pages() syscall would just return 0 or errno.  So,
> > > > > we need reset the return value to 0 for such case as what pre-v4.17 did.
> > > > The patch is wrong. migrate_pages returns the number of pages it
> > > > _hasn't_ migrated or -errno. Yeah that semantic sucks but...
> > > > So err != 0 is always an error. Except err > 0 doesn't really provide
> > > > any useful information to the userspace. I cannot really remember what
> > > > was the actual behavior before my rework because there were some gotchas
> > > > hidden there.
> > > OK, so I've double checked. do_move_page_to_node_array would carry the
> > > error code over to do_pages_move and it would store the status stored
> > > in the pm array. It contains page_to_nid(page) so the resulting code
> > > indeed behaves properly before my change and this is a regression. I
> > Thanks, I see the change.
> >
> > > have a very vague recollection that this has been brought up already.
> > > <...looks in notes...>
> > > Found it! The report is
> > > http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@xxxxxxxxxx
> > > and my proposed workaround was http://lkml.kernel.org/r/20180829145537.GZ10223@xxxxxxxxxxxxxx
> > Well, the above two links return 404.
> You are right. They are not archived for some reason. Anyway, the patch
> I was proposing back then is below:
>
> commit cfb88c266b645197135cde2905c2bfc82f6d82a9
> Author: Michal Hocko <mhocko@xxxxxxxx>
> Date:   Wed Nov 14 12:19:09 2018 +0100
>
>      mm: fix do_pages_move error reporting
>      
>      a49bd4d71637 ("mm, numa: rework do_pages_move") has changed the way how
>      we report error to layers above. As the changelog mentioned the semantic
>      was quite unclear previously because the return 0 could mean both
>      success and failure.
>      
>      The above mentioned commit didn't get all the way down to fix this
>      completely because it doesn't report pages that we even haven't
>      attempted to migrate and therefore we cannot simply say that the
>      semantic is:
>      - err < 0 - errno
>      - err >= 0 number of non-migrated pages.
>      
>      Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>      Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks, Michal. But, it looks this patch still could return > 0 value 
(the total number of non-migrated pages, including not even attempted 
pages) too, but the problem we are trying to fix is to make 
do_pages_move() return <= 0 value only since the man page of 
move_pages() doesn't allow return > 0 value.

And, by looking into the old code (v4.16), I spotted another problem. 
The migrate_pages() would store the migration failure error code into 
page_to_node->status. So, When do_move_page_to_node_array() returns > 0 
value, the return value would be reset to 0 and the migration error 
codes for non-migrated pages would be stored into status to return to 
userspace. But, the rework removed this.

I didn't dig into the intention of the rework, is it expected?

> diff --git a/mm/migrate.c b/mm/migrate.c
> index f7e4bfdc13b7..aa53ebc523eb 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1615,8 +1615,16 @@ 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) {
> +			/*
> +			 * Possitive err means the number of failed pages to
> +			 * migrate. Make sure to report the rest of the
> +			 * nr_pages is not migrated as well.
> +			 */
> +			if (err > 0)
> +				err += nr_pages - i - 1;
>   			goto out;
> +		}
>   		if (i > start) {
>   			err = store_status(status, start, current_node, i - start);
>   			if (err)