[PATCH v2] mm/hugetlb: Fix kernel crash with hugetlb mremap

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

This fixes the below crash:

kernel BUG at include/linux/mm.h:2373!
cpu 0x5d: Vector: 700 (Program Check) at [c00000003c6e76e0]
    pc: c000000000581a54: pmd_to_page+0x54/0x80
    lr: c00000000058d184: move_hugetlb_page_tables+0x4e4/0x5b0
    sp: c00000003c6e7980
   msr: 9000000000029033
  current = 0xc00000003bd8d980
  paca    = 0xc000200fff610100   irqmask: 0x03   irq_happened: 0x01
    pid   = 9349, comm = hugepage-mremap
kernel BUG at include/linux/mm.h:2373!
[link register   ] c00000000058d184 move_hugetlb_page_tables+0x4e4/0x5b0
[c00000003c6e7980] c00000000058cecc move_hugetlb_page_tables+0x22c/0x5b0 (unreliable)
[c00000003c6e7a90] c00000000053b78c move_page_tables+0xdbc/0x1010
[c00000003c6e7bd0] c00000000053bc34 move_vma+0x254/0x5f0
[c00000003c6e7c90] c00000000053c790 sys_mremap+0x7c0/0x900
[c00000003c6e7db0] c00000000002c450 system_call_exception+0x160/0x2c0

the kernel can't use huge_pte_offset before it set the pte entry because a page table
lookup check for huge PTE bit in the page table to differentiate between a
huge pte entry and a pointer to pte page. A huge_pte_alloc won't mark the
page table entry huge and hence kernel should not use huge_pte_offset after
a huge_pte_alloc.

Fixes: 550a7d60bd5e ("mm, hugepages: add mremap() support for hugepage backed vma")

Cc: stable@vger.kernel.org
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
 mm/hugetlb.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

Changes from v1:
* Add Fixes: tag
* Add Reviewed-by: 

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 61895cc01d09..e57650a9404f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4851,14 +4851,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 }
 
 static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
-			  unsigned long new_addr, pte_t *src_pte)
+			  unsigned long new_addr, pte_t *src_pte, pte_t *dst_pte)
 {
 	struct hstate *h = hstate_vma(vma);
 	struct mm_struct *mm = vma->vm_mm;
-	pte_t *dst_pte, pte;
 	spinlock_t *src_ptl, *dst_ptl;
+	pte_t pte;
 
-	dst_pte = huge_pte_offset(mm, new_addr, huge_page_size(h));
 	dst_ptl = huge_pte_lock(h, mm, dst_pte);
 	src_ptl = huge_pte_lockptr(h, mm, src_pte);
 
@@ -4917,7 +4916,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
 		if (!dst_pte)
 			break;
 
-		move_huge_pte(vma, old_addr, new_addr, src_pte);
+		move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte);
 	}
 	flush_tlb_range(vma, old_end - len, old_end);
 	mmu_notifier_invalidate_range_end(&range);
-- 
2.34.1