[PATCH] thp: move preallocated PTE page table on move_huge_pmd()

["Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>]

Andrey Wagin reported crash on VM_BUG_ON() in pgtable_pmd_page_dtor()
with fallowing backtrace:

  [<ffffffff8119427f>] free_pgd_range+0x2bf/0x410
  [<ffffffff8119449e>] free_pgtables+0xce/0x120
  [<ffffffff8119b900>] unmap_region+0xe0/0x120
  [<ffffffff811a0036>] ? move_page_tables+0x526/0x6b0
  [<ffffffff8119d6a9>] do_munmap+0x249/0x360
  [<ffffffff811a0304>] move_vma+0x144/0x270
  [<ffffffff811a07e9>] SyS_mremap+0x3b9/0x510
  [<ffffffff8172d512>] system_call_fastpath+0x16/0x1b

The crash can be reproduce with this test case:

  #define _GNU_SOURCE
  #include <sys/mman.h>
  #include <stdio.h>
  #include <unistd.h>

  #define MB (1024 * 1024UL)
  #define GB (1024 * MB)

  int main(int argc, char **argv)
  {
	char *p;
	int i;

	p = mmap((void *) GB, 10 * MB, PROT_READ | PROT_WRITE,
			MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
	for (i = 0; i < 10 * MB; i += 4096)
		p[i] = 1;
	mremap(p, 10 * MB, 10 * MB, MREMAP_FIXED | MREMAP_MAYMOVE, 2 * GB);
	return 0;
  }

Due to split PMD lock, we now store preallocated PTE tables for THP
pages per-PMD table.  It means we need to move them to other PMD table
if huge PMD moved there.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Andrey Vagin <avagin@xxxxxxxxxx>
---
 mm/huge_memory.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bccd5a628ea6..33a5dc492810 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1481,8 +1481,18 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
 		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)
+		if (new_ptl != old_ptl) {
+			pgtable_t pgtable;
+
+			/*
+			 * Move preallocated PTE page table if new_pmd is on
+			 * different PMD page table.
+			 */
+			pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
+			pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
+
 			spin_unlock(new_ptl);
+		}
 		spin_unlock(old_ptl);
 	}
 out:
-- 
1.8.4.4

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