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@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. > > 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
