[PATCH v2] mm/mempolicy: support MPOL_MF_STRICT for huge page mapping

[Li Xinhai <lixinhai.lxh@gmail.com>]

MPOL_MF_STRICT is used in mbind() for purposes:
(1) MPOL_MF_STRICT is set alone without MPOL_MF_MOVE or MPOL_MF_MOVE_ALL,
    to check if there is misplaced page and return -EIO;
(2) MPOL_MF_STRICT is set with MPOL_MF_MOVE or MPOL_MF_MOVE_ALL, to check
    if there is misplaced page which is failed to isolate, or page is
    success on isolate but failed to move, and return -EIO.

For non hugepage mapping, (1) and (2) are implemented as expectation.
For hugepage mapping, (1) is not implemented. And in (2), the part about
failed to isolate and report -EIO is not implemented.

This patch implements the missed parts for hugepage mapping. Benefits
with it applied:
- User space can apply same code logic to handle mbind() on hugepage and
  non hugepage mapping;
- Reliably using MPOL_MF_STRICT alone to check whether there is misplaced
  page or not when bind policy on address range, especially for address
  range which contains both hugepage and non hugepage mapping.

Analysis of potential impact to existing users:
- If MPOL_MF_STRICT alone was previously used, hugetlb pages not following
  the memory policy would not cause an EIO error.  After this change,
  hugetlb pages are treated like all other pages. If  MPOL_MF_STRICT alone
  is used and hugetlb pages do not follow memory policy an EIO error will
  be returned.
- For users who using MPOL_MF_STRICT with MPOL_MF_MOVE or
  MPOL_MF_MOVE_ALL, the semantic about some pages could not be moved will
  not be changed by this patch, because failed to isolate and failed to
  move have same effects to users, so their existing code will not be
  impacted.

In mbind man page, the note about 'MPOL_MF_STRICT is ignored on huge page
mappings' can be removed after this patch is applied.

Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: linux-man <linux-man@vger.kernel.org>
---
v1 -> v2:
Revise the commit message(i.e., the first part about impaction) according
to Mike.

 mm/mempolicy.c | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b2920ae..ec897d1 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -557,9 +557,10 @@ static int queue_pages_hugetlb(pte_t *pte, unsigned long hmask,
 			       unsigned long addr, unsigned long end,
 			       struct mm_walk *walk)
 {
+	int ret = 0;
 #ifdef CONFIG_HUGETLB_PAGE
 	struct queue_pages *qp = walk->private;
-	unsigned long flags = qp->flags;
+	unsigned long flags = (qp->flags & MPOL_MF_VALID);
 	struct page *page;
 	spinlock_t *ptl;
 	pte_t entry;
@@ -571,16 +572,44 @@ static int queue_pages_hugetlb(pte_t *pte, unsigned long hmask,
 	page = pte_page(entry);
 	if (!queue_pages_required(page, qp))
 		goto unlock;
+
+	if (flags == MPOL_MF_STRICT) {
+		/*
+		 * STRICT alone means only detecting misplaced page and no
+		 * need to further check other vma.
+		 */
+		ret = -EIO;
+		goto unlock;
+	}
+
+	if (!vma_migratable(walk->vma)) {
+		/*
+		 * Must be STRICT with MOVE*, otherwise .test_walk() have
+		 * stopped walking current vma.
+		 * Detecting misplaced page but allow migrating pages which
+		 * have been queued.
+		 */
+		ret = 1;
+		goto unlock;
+	}
+
 	/* With MPOL_MF_MOVE, we migrate only unshared hugepage. */
 	if (flags & (MPOL_MF_MOVE_ALL) ||
-	    (flags & MPOL_MF_MOVE && page_mapcount(page) == 1))
-		isolate_huge_page(page, qp->pagelist);
+	    (flags & MPOL_MF_MOVE && page_mapcount(page) == 1)) {
+		if (!isolate_huge_page(page, qp->pagelist) &&
+			(flags & MPOL_MF_STRICT))
+			/*
+			 * Failed to isolate page but allow migrating pages
+			 * which have been queued.
+			 */
+			ret = 1;
+	}
 unlock:
 	spin_unlock(ptl);
 #else
 	BUG();
 #endif
-	return 0;
+	return ret;
 }
 
 #ifdef CONFIG_NUMA_BALANCING
-- 
1.8.3.1