powerpc/thp: Fix crash on mremap

This patch fix the below crash

NIP [c00000000004cee4] .__hash_page_thp+0x2a4/0x440
LR [c0000000000439ac] .hash_page+0x18c/0x5e0
...
Call Trace:
[c000000736103c40] [00001ffffb000000] 0x1ffffb000000(unreliable)
[437908.479693] [c000000736103d50] [c0000000000439ac] .hash_page+0x18c/0x5e0
[437908.479699] [c000000736103e30] [c00000000000924c] .do_hash_page+0x4c/0x58

On ppc64 we use the pgtable for storing the hpte slot information and
store address to the pgtable at a constant offset (PTRS_PER_PMD) from
pmd. On mremap, when we switch the pmd, we need to withdraw and deposit
the pgtable again, so that we find the pgtable at PTRS_PER_PMD offset
from new pmd.

We also want to move the withdraw and deposit before the set_pmd so
that, when page fault find the pmd as trans huge we can be sure that
pgtable can be located at the offset.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 5 insertions, 9 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 95d1acb0f3d2..5d80c53b87cb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1502,19 +1502,15 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
                         spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
                 pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
                 VM_BUG_ON(!pmd_none(*new_pmd));
-                set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
-                if (new_ptl != old_ptl) {
-                        pgtable_t pgtable;
 
-                        /*
-                         * Move preallocated PTE page table if new_pmd is on
-                         * different PMD page table.
-                         */
+                if (pmd_move_must_withdraw(new_ptl, old_ptl)) {
+                        pgtable_t pgtable;
                         pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
                         pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
-
-                        spin_unlock(new_ptl);
                 }
+                set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
+                if (new_ptl != old_ptl)
+                        spin_unlock(new_ptl);
                 spin_unlock(old_ptl);
         }
 out: