[PATCHv2] mm: Fix warning in move_normal_pmd()

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

mremap(2) does not allow source and destination regions to overlap, but
shift_arg_pages() calls move_page_tables() directly and in this case the
source and destination overlap often. It confuses move_normal_pmd():

  WARNING: CPU: 3 PID: 27091 at mm/mremap.c:211 move_page_tables+0x6ef/0x720

move_normal_pmd() expects the destination PMD to be empty, but when
ranges overlap nobody removes PTE page tables on source side.
move_ptes() only removes PTE entries, leaving tables behind.
When the source PMD becomes destination and alignment/size is right we
step onto the warning.

The warning is harmless: kernel correctly fallbacks to handle entries on
per-entry basis.

The fix is to avoid move_normal_pmd() if we see that source and
destination ranges overlap.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions")
Link: https://lore.kernel.org/lkml/20200713025354.GB3644504@google.com/
Reported-and-tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: William Kucharski <william.kucharski@oracle.com>
---
 mm/mremap.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 5dd572d57ca9..340a96a29cbb 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -245,6 +245,26 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 	unsigned long extent, next, old_end;
 	struct mmu_notifier_range range;
 	pmd_t *old_pmd, *new_pmd;
+	bool overlaps;
+
+	/*
+	 * shift_arg_pages() can call move_page_tables() on overlapping ranges.
+	 * In this case we cannot use move_normal_pmd() because destination pmd
+	 * might be established page table: move_ptes() doesn't free page
+	 * table.
+	 */
+	if (old_addr > new_addr) {
+		overlaps = old_addr - new_addr < len;
+	} else {
+		overlaps = new_addr - old_addr < len;
+
+		/*
+		 * We are interating over ranges forward. It means we cannot
+		 * handle overlapping ranges with new_addr > old_addr without
+		 * risking data corruption. Don't do this.
+		 */
+		WARN_ON(overlaps);
+	}
 
 	old_end = old_addr + len;
 	flush_cache_range(vma, old_addr, old_end);
@@ -282,7 +302,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 			split_huge_pmd(vma, old_pmd, old_addr);
 			if (pmd_trans_unstable(old_pmd))
 				continue;
-		} else if (extent == PMD_SIZE) {
+		} else if (!overlaps && extent == PMD_SIZE) {
 #ifdef CONFIG_HAVE_MOVE_PMD
 			/*
 			 * If the extent is PMD-sized, try to speed the move by
-- 
2.26.2