[PATCH v2 1/3] mm/page_table_check: Do WARN_ON instead of BUG_ON

[Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>]

Currently, page_table_check when detects errors panics the kernel. Instead,
print a warning as it is more useful compared to unconditionally crashing
the machine.

However, once a warning is detected, the counting of page_table_check
becomes unbalanced, therefore,  disable its activity until the next boot.

In case of where machine hardening requires a more secure environment, it
is still possible to crash machine on page_table_check errors via
panic_on_warn sysctl option.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 mm/page_table_check.c | 37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 93ec7690a0d8..ad4447e999f8 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -22,6 +22,12 @@ static bool __page_table_check_enabled __initdata =
 DEFINE_STATIC_KEY_TRUE(page_table_check_disabled);
 EXPORT_SYMBOL(page_table_check_disabled);
 
+#define PAGE_TABLE_CHECK_WARN(v)						\
+	do {									\
+		if (WARN_ON_ONCE(v))						\
+			static_branch_enable(&page_table_check_disabled);	\
+	} while (false)
+
 static int __init early_page_table_check_param(char *buf)
 {
 	return kstrtobool(buf, &__page_table_check_enabled);
@@ -50,7 +56,8 @@ struct page_ext_operations page_table_check_ops = {
 
 static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
 {
-	BUG_ON(!page_ext);
+	PAGE_TABLE_CHECK_WARN(!page_ext);
+
 	return (void *)(page_ext) + page_table_check_ops.offset;
 }
 
@@ -72,18 +79,18 @@ static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
-	BUG_ON(PageSlab(page));
+	PAGE_TABLE_CHECK_WARN(PageSlab(page));
 	anon = PageAnon(page);
 
 	for (i = 0; i < pgcnt; i++) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
 		if (anon) {
-			BUG_ON(atomic_read(&ptc->file_map_count));
-			BUG_ON(atomic_dec_return(&ptc->anon_map_count) < 0);
+			PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->file_map_count));
+			PAGE_TABLE_CHECK_WARN(atomic_dec_return(&ptc->anon_map_count) < 0);
 		} else {
-			BUG_ON(atomic_read(&ptc->anon_map_count));
-			BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
+			PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->anon_map_count));
+			PAGE_TABLE_CHECK_WARN(atomic_dec_return(&ptc->file_map_count) < 0);
 		}
 		page_ext = page_ext_next(page_ext);
 	}
@@ -110,18 +117,18 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
-	BUG_ON(PageSlab(page));
+	PAGE_TABLE_CHECK_WARN(PageSlab(page));
 	anon = PageAnon(page);
 
 	for (i = 0; i < pgcnt; i++) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
 		if (anon) {
-			BUG_ON(atomic_read(&ptc->file_map_count));
-			BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);
+			PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->file_map_count));
+			PAGE_TABLE_CHECK_WARN(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);
 		} else {
-			BUG_ON(atomic_read(&ptc->anon_map_count));
-			BUG_ON(atomic_inc_return(&ptc->file_map_count) < 0);
+			PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->anon_map_count));
+			PAGE_TABLE_CHECK_WARN(atomic_inc_return(&ptc->file_map_count) < 0);
 		}
 		page_ext = page_ext_next(page_ext);
 	}
@@ -137,15 +144,15 @@ void __page_table_check_zero(struct page *page, unsigned int order)
 	struct page_ext *page_ext;
 	unsigned long i;
 
-	BUG_ON(PageSlab(page));
+	PAGE_TABLE_CHECK_WARN(PageSlab(page));
 
 	page_ext = page_ext_get(page);
-	BUG_ON(!page_ext);
+	PAGE_TABLE_CHECK_WARN(!page_ext);
 	for (i = 0; i < (1ul << order); i++) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
-		BUG_ON(atomic_read(&ptc->anon_map_count));
-		BUG_ON(atomic_read(&ptc->file_map_count));
+		PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->anon_map_count));
+		PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->file_map_count));
 		page_ext = page_ext_next(page_ext);
 	}
 	page_ext_put(page_ext);
-- 
2.41.0.487.g6d72f3e995-goog