[PATCH 3.2 098/125] mm: revert 0def08e3 ("mm/mempolicy.c: check return code of check_range")

[Ben Hutchings]

3.2.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Minchan Kim <minchan@xxxxxxxxxx>

commit 082708072a4250f5c4dbc62065e7af93f5e45646 upstream.

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@xxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Acked-by: 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>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/mempolicy.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -945,15 +945,18 @@ static int migrate_to_node(struct mm_str
 	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,

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