[PATCH] mm: revert 0def08e3, mm/mempolicy.c: check return code ofcheck_range

[Minchan Kim]

This patch reverts 0def08e3 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 add new VM_BUG_ON for checking migrate_to_node's future usecase
which might pass to MPOL_MF_STRICT.

Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Vasiliy Kulikov <segooon@xxxxxxxxx>
Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/mempolicy.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 3d64b36..9ec87bd 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -946,15 +946,16 @@ 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,
+	/*
+	 * Collect migrate pages and it shoudn't be failed.
+	 */
+	VM_BUG_ON(flags & MPOL_MF_STRICT);
+	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,
-- 
1.7.9.5

-- 
Kind regards,
Minchan Kim
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