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

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

On Wed, Jan 22, 2020 at 09:06:51AM +0100, Michal Hocko wrote:
>On Tue 21-01-20 11:01:30, Yang Shi wrote:
>> 
>> 
>> 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@redhat.com
>> > > > and my proposed workaround was http://lkml.kernel.org/r/20180829145537.GZ10223@dhcp22.suse.cz
>> > > 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@suse.com>
>> > 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@suse.com>
>> 
>> 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.
>
>Yes this patch just lives with the changed semantic and tries to make it
>sensible. So if some page cannot be migrated then we just stop and
>return the number of non migrated pages at the tail of the given array.
>This would make error handling slightly easier because you know that
>count - ret pages of the array can be skipped if ret >= 0.
>

Got some different idea for this.

Replied in the patch thread.

>> 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?
>
>I have tried to preserve the original semantic as possible. As explained
>in the changelog there were quite some discrepancies even before. This
>new one was not really intentional. We have effectively two options
>here. Either somebody really depend on the former semantic and we have
>to fix this or we can relax the semantic as the above patch attempts.
>
>I would be more inclined for the second option as nobody has complained
>about the new semantic except for few ltp tests which do not represent
>real workload. If you have a real usecase then speak up please.
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me