mm: revert 0def08e3 ("mm/mempolicy.c: check return code of check_range")

Revert commit 0def08e3acc2 because check_range can't fail in
migrate_to_node with considering current usecases.

Quote from Johannes

: I think it makes sense to revert.  Not because of the semantics, but I
: just don't see how check_range() could even fail for this callsite:
:
: 1. we pass mm->mmap->vm_start in there, so we should not fail due to
:    find_vma()
:
: 2. we pass MPOL_MF_DISCONTIG_OK, so the discontig checks do not apply
:    and so can not fail
:
: 3. we pass MPOL_MF_MOVE | MPOL_MF_MOVE_ALL, the page table loops will
:    continue until addr == end, so we never fail with -EIO

And I added a new VM_BUG_ON for checking migrate_to_node's future usecase
which might pass to MPOL_MF_STRICT.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vasiliy Kulikov <segooon@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 3d64b369180d..0b78fb9ea65b 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -946,15 +946,18 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
         nodemask_t nmask;
         LIST_HEAD(pagelist);
         int err = 0;
-        struct vm_area_struct *vma;
 
         nodes_clear(nmask);
         node_set(source, nmask);
 
-        vma = check_range(mm, mm->mmap->vm_start, mm->task_size, &nmask,
+        /*
+         * This does not "check" the range but isolates all pages that
+         * need migration.  Between passing in the full user address
+         * space range and MPOL_MF_DISCONTIG_OK, this call can not fail.
+         */
+        VM_BUG_ON(!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)));
+        check_range(mm, mm->mmap->vm_start, mm->task_size, &nmask,
                         flags | MPOL_MF_DISCONTIG_OK, &pagelist);
-        if (IS_ERR(vma))
-                return PTR_ERR(vma);
 
         if (!list_empty(&pagelist)) {
                 err = migrate_pages(&pagelist, new_node_page, dest,