mremap: move_ptes: check pte dirty after its removal

Linus found there still is a race in mremap after commit 5d1904204c99
("mremap: fix race between mremap() and page cleanning").

As described by Linus:
 "the issue is that another thread might make the pte be dirty (in the
  hardware walker, so no locking of ours will make any difference)
  *after* we checked whether it was dirty, but *before* we removed it
  from the page tables"

Fix it by moving the check after we removed it from the page table.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index eff3de3..d4a6e40 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1456,9 +1456,9 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
                new_ptl = pmd_lockptr(mm, new_pmd);
                if (new_ptl != old_ptl)
                        spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
-               if (pmd_present(*old_pmd) && pmd_dirty(*old_pmd))
-                       force_flush = true;
                pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
+               if (pmd_present(pmd) && pmd_dirty(pmd))
+                       force_flush = true;
                VM_BUG_ON(!pmd_none(*new_pmd));
 
                if (pmd_move_must_withdraw(new_ptl, old_ptl) &&

diff --git a/mm/mremap.c b/mm/mremap.c
index 6ccecc0..30d7d24 100644 (file)
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -149,14 +149,18 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
                if (pte_none(*old_pte))
                        continue;
 
+               pte = ptep_get_and_clear(mm, old_addr, old_pte);
                /*
-                * We are remapping a dirty PTE, make sure to
-                * flush TLB before we drop the PTL for the
+                * If we are remapping a dirty PTE, make sure
+                * to flush TLB before we drop the PTL for the
                 * old PTE or we may race with page_mkclean().
+                *
+                * This check has to be done after we removed the
+                * old PTE from page tables or another thread may
+                * dirty it after the check and before the removal.
                 */
-               if (pte_present(*old_pte) && pte_dirty(*old_pte))
+               if (pte_present(pte) && pte_dirty(pte))
                        force_flush = true;
-               pte = ptep_get_and_clear(mm, old_addr, old_pte);
                pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
                pte = move_soft_dirty_pte(pte);
                set_pte_at(mm, new_addr, new_pte, pte);