[PATCH v3 2/2] migrate: add migrate_entry_wait_huge()

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

When we have a page fault for the address which is backed by a hugepage
under migration, the kernel can't wait correctly and do busy looping on
hugepage fault until the migration finishes.
This is because pte_offset_map_lock() can't get a correct migration entry
or a correct page table lock for hugepage.
This patch introduces migration_entry_wait_huge() to solve this.

Note that the caller, hugetlb_fault(), gets the pointer to the "leaf"
entry with huge_pte_offset() inside which all the arch-dependency of
the page table structure are. So migration_entry_wait_huge() and
__migration_entry_wait() are free from arch-dependency.

ChangeLog v3:
 - use huge_pte_lockptr

ChangeLog v2:
 - remove dup in migrate_entry_wait_huge()

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: stable@vger.kernel.org # 2.6.35
---
 include/linux/swapops.h |  3 +++
 mm/hugetlb.c            |  2 +-
 mm/migrate.c            | 23 ++++++++++++++++++-----
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git v3.10-rc3.orig/include/linux/swapops.h v3.10-rc3/include/linux/swapops.h
index 47ead51..c5fd30d 100644
--- v3.10-rc3.orig/include/linux/swapops.h
+++ v3.10-rc3/include/linux/swapops.h
@@ -137,6 +137,7 @@ static inline void make_migration_entry_read(swp_entry_t *entry)
 
 extern void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 					unsigned long address);
+extern void migration_entry_wait_huge(struct mm_struct *mm, pte_t *pte);
 #else
 
 #define make_migration_entry(page, write) swp_entry(0, 0)
@@ -148,6 +149,8 @@ static inline int is_migration_entry(swp_entry_t swp)
 static inline void make_migration_entry_read(swp_entry_t *entryp) { }
 static inline void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 					 unsigned long address) { }
+static inline void migration_entry_wait_huge(struct mm_struct *mm,
+					pte_t *pte) { }
 static inline int is_write_migration_entry(swp_entry_t entry)
 {
 	return 0;
diff --git v3.10-rc3.orig/mm/hugetlb.c v3.10-rc3/mm/hugetlb.c
index 8e1af32..d91a438 100644
--- v3.10-rc3.orig/mm/hugetlb.c
+++ v3.10-rc3/mm/hugetlb.c
@@ -2877,7 +2877,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (ptep) {
 		entry = huge_ptep_get(ptep);
 		if (unlikely(is_hugetlb_entry_migration(entry))) {
-			migration_entry_wait(mm, (pmd_t *)ptep, address);
+			migration_entry_wait_huge(mm, ptep);
 			return 0;
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
 			return VM_FAULT_HWPOISON_LARGE |
diff --git v3.10-rc3.orig/mm/migrate.c v3.10-rc3/mm/migrate.c
index 6f2df6e..64ff118 100644
--- v3.10-rc3.orig/mm/migrate.c
+++ v3.10-rc3/mm/migrate.c
@@ -204,15 +204,14 @@ static void remove_migration_ptes(struct page *old, struct page *new)
  * get to the page and wait until migration is finished.
  * When we return from this function the fault will be retried.
  */
-void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
-				unsigned long address)
+static void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
+				spinlock_t *ptl)
 {
-	pte_t *ptep, pte;
-	spinlock_t *ptl;
+	pte_t pte;
 	swp_entry_t entry;
 	struct page *page;
 
-	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
+	spin_lock(ptl);
 	pte = *ptep;
 	if (!is_swap_pte(pte))
 		goto out;
@@ -240,6 +239,20 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 	pte_unmap_unlock(ptep, ptl);
 }
 
+void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
+				unsigned long address)
+{
+	spinlock_t *ptl = pte_lockptr(mm, pmd);
+	pte_t *ptep = pte_offset_map(pmd, address);
+	__migration_entry_wait(mm, ptep, ptl);
+}
+
+void migration_entry_wait_huge(struct mm_struct *mm, pte_t *pte)
+{
+	spinlock_t *ptl = huge_pte_lockptr(mm, pte);
+	__migration_entry_wait(mm, pte, ptl);
+}
+
 #ifdef CONFIG_BLOCK
 /* Returns true if all buffers are successfully locked */
 static bool buffer_migrate_lock_buffers(struct buffer_head *head,
-- 
1.7.11.7

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