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

[Wei Yang <richardw.yang@linux.intel.com>]

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@redhat.com
>and my proposed workaround was http://lkml.kernel.org/r/20180829145537.GZ10223@dhcp22.suse.cz

Well, the above two links return 404.

>
>> If you want to fix this properly then you have to query node status of
>> each page unmigrated when migrate_pages fails with > 0. This would be
>> easier if the fix is done on the latest cleanup posted to the list which
>> consolidates all do_move_pages_to_node and store_status calls to a
>> single function.
>
>Sorry forgot to put a reference to the patch: http://lkml.kernel.org/r/20200119030636.11899-5-richardw.yang@linux.intel.com
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me