mm: allow arch code to control the user page table ceiling

On architectures where a pgd entry may be shared between user and kernel
(e.g.  ARM+LPAE), freeing page tables needs a ceiling other than 0.
This patch introduces a generic USER_PGTABLES_CEILING that arch code can
override.  It is the responsibility of the arch code setting the ceiling
to ensure the complete freeing of the page tables (usually in
pgd_free()).

[catalin.marinas@arm.com: commit log; shift_arg_pages(), asm-generic/pgtables.h changes]
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: <stable@vger.kernel.org>	[3.3+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 14 insertions, 4 deletions

diff --git a/fs/exec.c b/fs/exec.c
index a96a4885bbbf..87e731f020fb 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -613,7 +613,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
                  * when the old and new regions overlap clear from new_end.
                  */
                 free_pgd_range(&tlb, new_end, old_end, new_end,
-                        vma->vm_next ? vma->vm_next->vm_start : 0);
+                        vma->vm_next ? vma->vm_next->vm_start : USER_PGTABLES_CEILING);
         } else {
                 /*
                  * otherwise, clean from old_start; this is done to not touch
@@ -622,7 +622,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
                  * for the others its just a little faster.
                  */
                 free_pgd_range(&tlb, old_start, old_end, new_end,
-                        vma->vm_next ? vma->vm_next->vm_start : 0);
+                        vma->vm_next ? vma->vm_next->vm_start : USER_PGTABLES_CEILING);
         }
         tlb_finish_mmu(&tlb, new_end, old_end);
 
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index bfd87685fc1f..a59ff51b0166 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -7,6 +7,16 @@
 #include <linux/mm_types.h>
 #include <linux/bug.h>
 
+/*
+ * On almost all architectures and configurations, 0 can be used as the
+ * upper ceiling to free_pgtables(): on many architectures it has the same
+ * effect as using TASK_SIZE.  However, there is one configuration which
+ * must impose a more careful limit, to avoid freeing kernel pgtables.
+ */
+#ifndef USER_PGTABLES_CEILING
+#define USER_PGTABLES_CEILING   0UL
+#endif
+
 #ifndef __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
 extern int ptep_set_access_flags(struct vm_area_struct *vma,
                                  unsigned long address, pte_t *ptep,
diff --git a/mm/mmap.c b/mm/mmap.c
index b2c363f7ae54..288958f05f1c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2302,7 +2302,7 @@ static void unmap_region(struct mm_struct *mm,
         update_hiwater_rss(mm);
         unmap_vmas(&tlb, vma, start, end);
         free_pgtables(&tlb, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
-                                 next ? next->vm_start : 0);
+                                 next ? next->vm_start : USER_PGTABLES_CEILING);
         tlb_finish_mmu(&tlb, start, end);
 }
 
@@ -2682,7 +2682,7 @@ void exit_mmap(struct mm_struct *mm)
         /* Use -1 here to ensure all VMAs in the mm are unmapped */
         unmap_vmas(&tlb, vma, 0, -1);
 
-        free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, 0);
+        free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
         tlb_finish_mmu(&tlb, 0, -1);
 
         /*