mm, mempolicy: fix up gup usage in lookup_node

mainline inclusion
from mainline-v5.8-rc1
commit 2d3a36a4
category: bugfix
bugzilla: 47439
CVE: NA
---------------------------

ba841078 ("mm/mempolicy: Allow lookup_node() to handle fatal signal")
has added a special casing for 0 return value because that was a possible
gup return value when interrupted by fatal signal.  This has been fixed by
ae46d2aa ("mm/gup: Let __get_user_pages_locked() return -EINTR for
fatal signal") in the mean time so ba841078 can be reverted.

This patch however doesn't go all the way to revert it because the check
for 0 is wrong and confusing here.  Firstly it is inherently unsafe to
access the page when get_user_pages_locked returns 0 (aka no page
returned).

Fortunatelly this will not happen because get_user_pages_locked will not
return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not
the case here.  Document this potential error code in gup code while we
are at it.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Link: http://lkml.kernel.org/r/20200421071026.18394-1-mhocko@kernel.org


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

 Conflicts:
	mm/gup.c
[wangxiongfeng: conflicts in comments ]
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Reviewed-by: Kefeng  Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>