[v2 PATCH] mm: thp: check total_mapcount instead of page_mapcount

[Yang Shi]

When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
return false positive for PTE-mapped THP since page_mapcount() is used
to check if the THP is unmapped, but it just checks compound mapount and
head page's mapcount.  If the THP is PTE-mapped and head page is not
mapped, it may return false positive.

Use total_mapcount() instead of page_mapcount() for try_to_unmap() and
do so for the VM_BUG_ON_PAGE in split_huge_page_to_list as well.

This changed the semantic of try_to_unmap(), but I don't see there is
any usecase that expects try_to_unmap() just unmap one subpage of a huge
page.  So using page_mapcount() seems like a bug.

[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@xxxxxxxxxxxx/

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
v2: Removed dead code and updated the comment of try_to_unmap() per Zi
    Yan.

 mm/huge_memory.c | 11 +----------
 mm/rmap.c        | 10 ++++++----
 2 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b25deaa..3b08b9ba1578 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2348,7 +2348,6 @@ static void unmap_page(struct page *page)
 		ttu_flags |= TTU_SPLIT_FREEZE;
 
 	unmap_success = try_to_unmap(page, ttu_flags);
-	VM_BUG_ON_PAGE(!unmap_success, page);
 }
 
 static void remap_page(struct page *page, unsigned int nr)
@@ -2718,7 +2717,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 	}
 
 	unmap_page(head);
-	VM_BUG_ON_PAGE(compound_mapcount(head), head);
+	VM_BUG_ON_PAGE(total_mapcount(head), head);
 
 	/* block interrupt reentry in xa_lock and spinlock */
 	local_irq_disable();
@@ -2758,14 +2757,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 		__split_huge_page(page, list, end);
 		ret = 0;
 	} else {
-		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
-			pr_alert("total_mapcount: %u, page_count(): %u\n",
-					mapcount, count);
-			if (PageTail(page))
-				dump_page(head, NULL);
-			dump_page(page, "total_mapcount(head) > 0");
-			BUG();
-		}
 		spin_unlock(&ds_queue->split_queue_lock);
 fail:		if (mapping)
 			xa_unlock(&mapping->i_pages);
diff --git a/mm/rmap.c b/mm/rmap.c
index 693a610e181d..f52825b1330d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1742,12 +1742,14 @@ static int page_not_mapped(struct page *page)
 }
 
 /**
- * try_to_unmap - try to remove all page table mappings to a page
- * @page: the page to get unmapped
+ * try_to_unmap - try to remove all page table mappings to a page and the
+ *                compound page it belongs to
+ * @page: the page or the subpages of compound page to get unmapped
  * @flags: action and flags
  *
  * Tries to remove all the page table entries which are mapping this
- * page, used in the pageout path.  Caller must hold the page lock.
+ * page and the compound page it belongs to, used in the pageout path.
+ * Caller must hold the page lock.
  *
  * If unmap is successful, return true. Otherwise, false.
  */
@@ -1777,7 +1779,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
 	else
 		rmap_walk(page, &rwc);
 
-	return !page_mapcount(page) ? true : false;
+	return !total_mapcount(page) ? true : false;
 }
 
 /**
-- 
2.26.2