From: Michal Hocko Date: Wed, 18 Apr 2018 11:07:22 +0200 Subject: [LTP] [RFC PATCH] mm: correct status code which move_pages() returns for zero page In-Reply-To: <7674C632-FE3E-42D2-B19D-32F531617043@cs.rutgers.edu> References: <20180417110615.16043-1-liwang@redhat.com> <20180417130300.GF17484@dhcp22.suse.cz> <20180417141442.GG17484@dhcp22.suse.cz> <20180417190044.GK17484@dhcp22.suse.cz> <7674C632-FE3E-42D2-B19D-32F531617043@cs.rutgers.edu> Message-ID: <20180418090722.GV17484@dhcp22.suse.cz> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: quoted-printable To: ltp@lists.linux.it On Tue 17-04-18 16:09:33, Zi Yan wrote: > On 17 Apr 2018, at 15:00, Michal Hocko wrote: >=20 > > On Tue 17-04-18 22:28:33, Li Wang wrote: > >> On Tue, Apr 17, 2018 at 10:14 PM, Michal Hocko wrote: > >> > >>> On Tue 17-04-18 15:03:00, Michal Hocko wrote: > >>>> On Tue 17-04-18 19:06:15, Li Wang wrote: > >>>> [...] > >>>>> diff --git a/mm/migrate.c b/mm/migrate.c > >>>>> index f65dd69..2b315fc 100644 > >>>>> --- a/mm/migrate.c > >>>>> +++ b/mm/migrate.c > >>>>> @@ -1608,7 +1608,7 @@ static int do_pages_move(struct mm_struct *mm, > >>> nodemask_t task_nodes, > >>>>> continue; > >>>>> > >>>>> err =3D store_status(status, i, err, 1); > >>>>> - if (err) > >>>>> + if (!err) > >>>>> goto out_flush; > >>>> > >>>> This change just doesn't make any sense to me. Why should we bail ou= t if > >>>> the store_status is successul? I am trying to wrap my head around the > >>>> test case. 6b9d757ecafc ("mm, numa: rework do_pages_move") tried to > >>>> explain that move_pages has some semantic issues and the new > >>>> implementation might be not 100% replacement. Anyway I am studying t= he > >>>> test case to come up with a proper fix. > >>> > >>> OK, I get what the test cases does. I've failed to see the subtle > >>> difference between alloc_pages_on_node and numa_alloc_onnode. The lat= er > >>> doesn't faul in anything. > >>> > >>> Why are we getting EPERM is quite not yet clear to me. > >>> add_page_for_migration uses FOLL_DUMP which should return EFAULT on > >>> zero pages (no_page_table()). > >>> > >>> err =3D PTR_ERR(page); > >>> if (IS_ERR(page)) > >>> goto out; > >>> > >>> therefore bails out from add_page_for_migration and store_status shou= ld > >>> store that value. There shouldn't be any EPERM on the way. > >>> > >> > >> Yes, I print the the return value and confirmed the > >> add_page_for_migration()=E2=80=8B > >> do right things for zero page. and after store_status(...) the status = saves > >> -EFAULT. > >> So I did the change above. > > > > OK, I guess I knnow what is going on. I must be overwriting the status > > on the way out by > > > > out_flush: > > /* Make sure we do not overwrite the existing error */ > > err1 =3D do_move_pages_to_node(mm, &pagelist, current_node); > > if (!err1) > > err1 =3D store_status(status, start, current_node, i - start); > > > > This error handling is rather fragile and I was quite unhappy about it > > at the time I was developing it. I have to remember all the details why > > I've done it that way but I would bet my hat this is it. More on this > > tomorrow. >=20 > Hi Michal and Li, >=20 > The problem is that the variable start is not set properly after store_st= atus(), > like the "start =3D i;" after the first store_status(). >=20 > The following patch should fix the problem (it has passed all move_pages = test cases from ltp > on my machine): >=20 > diff --git a/mm/migrate.c b/mm/migrate.c > index f65dd69e1fd1..32afa4723e7f 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1619,6 +1619,8 @@ static int do_pages_move(struct mm_struct *mm, node= mask_t task_nodes, > if (err) > goto out; > } > + /* Move to next page (i+1), after we have saved page stat= us (until i) */ > + start =3D i + 1; > current_node =3D NUMA_NO_NODE; > } > out_flush: >=20 > Feel free to check it by yourselves. Yes, you are right. I never update start if the last page in the range fails and so we overwrite the whole [start, i] range. I wish the code wasn't that ugly and subtle but considering how we can fail in different ways and that we want to batch as much as possible I do not see an easy way. Care to send the patch? I would just drop the comment. Thanks! --=20 Michal Hocko SUSE Labs