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

[Michal Hocko <mhocko@kernel.org>]

On Wed 22-01-20 09:26:48, Yang Shi wrote:
> 
> 
> On 1/22/20 12:06 AM, 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.
> 
> OK, I see. Returning > 0 value sounds more straightforward for userspace
> error handling.
> 
> BTW, we should update manpage to reflect the semantic change to indicate > 0
> return value as an error case.

Absolutely. 
 
> > > 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.
> 
> No, I don't have any real usecase. And, I tend to agree the most users may
> not care the reason of migration failure at all. Returning the number of
> non-migrated pages seems more straightforward.
> 
> I agree we could stick with the new semantic and fix the return value as
> what your patch did. I'm going to rebase your patch on top of Wei Yang's
> cleanup if you don't mind.
 
Go ahead. Thanks a lot!

-- 
Michal Hocko
SUSE Labs