mm, debug_pagealloc: don't rely on static keys too early

Commit 96a2b03f281d ("mm, debug_pagelloc: use static keys to enable
debugging") has introduced a static key to reduce overhead when
debug_pagealloc is compiled in but not enabled.  It relied on the
assumption that jump_label_init() is called before parse_early_param()
as in start_kernel(), so when the "debug_pagealloc=on" option is parsed,
it is safe to enable the static key.

However, it turns out multiple architectures call parse_early_param()
earlier from their setup_arch().  x86 also calls jump_label_init() even
earlier, so no issue was found while testing the commit, but same is not
true for e.g.  ppc64 and s390 where the kernel would not boot with
debug_pagealloc=on as found by our QA.

To fix this without tricky changes to init code of multiple
architectures, this patch partially reverts the static key conversion
from 96a2b03f281d.  Init-time and non-fastpath calls (such as in arch
code) of debug_pagealloc_enabled() will again test a simple bool
variable.  Fastpath mm code is converted to a new
debug_pagealloc_enabled_static() variant that relies on the static key,
which is enabled in a well-defined point in mm_init() where it's
guaranteed that jump_label_init() has been called, regardless of
architecture.

[sfr@canb.auug.org.au: export _debug_pagealloc_enabled_early]
  Link: http://lkml.kernel.org/r/20200106164944.063ac07b@canb.auug.org.au
Link: http://lkml.kernel.org/r/20191219130612.23171-1-vbabka@suse.cz
Fixes: 96a2b03f281d ("mm, debug_pagelloc: use static keys to enable debugging")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Qian Cai <cai@lca.pw>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 24 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 409be5ec7e2c..d047bf7d8fd4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -694,34 +694,27 @@ void prep_compound_page(struct page *page, unsigned int order)
 #ifdef CONFIG_DEBUG_PAGEALLOC
 unsigned int _debug_guardpage_minorder;
 
-#ifdef CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT
-DEFINE_STATIC_KEY_TRUE(_debug_pagealloc_enabled);
-#else
+bool _debug_pagealloc_enabled_early __read_mostly
+			= IS_ENABLED(CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT);
+EXPORT_SYMBOL(_debug_pagealloc_enabled_early);
 DEFINE_STATIC_KEY_FALSE(_debug_pagealloc_enabled);
-#endif
 EXPORT_SYMBOL(_debug_pagealloc_enabled);
 
 DEFINE_STATIC_KEY_FALSE(_debug_guardpage_enabled);
 
 static int __init early_debug_pagealloc(char *buf)
 {
-	bool enable = false;
-
-	if (kstrtobool(buf, &enable))
-		return -EINVAL;
-
-	if (enable)
-		static_branch_enable(&_debug_pagealloc_enabled);
-
-	return 0;
+	return kstrtobool(buf, &_debug_pagealloc_enabled_early);
 }
 early_param("debug_pagealloc", early_debug_pagealloc);
 
-static void init_debug_guardpage(void)
+void init_debug_pagealloc(void)
 {
 	if (!debug_pagealloc_enabled())
 		return;
 
+	static_branch_enable(&_debug_pagealloc_enabled);
+
 	if (!debug_guardpage_minorder())
 		return;
 
@@ -1186,7 +1179,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	 */
 	arch_free_page(page, order);
 
-	if (debug_pagealloc_enabled())
+	if (debug_pagealloc_enabled_static())
 		kernel_map_pages(page, 1 << order, 0);
 
 	kasan_free_nondeferred_pages(page, order);
@@ -1207,7 +1200,7 @@ static bool free_pcp_prepare(struct page *page)
 
 static bool bulkfree_pcp_prepare(struct page *page)
 {
-	if (debug_pagealloc_enabled())
+	if (debug_pagealloc_enabled_static())
 		return free_pages_check(page);
 	else
 		return false;
@@ -1221,7 +1214,7 @@ static bool bulkfree_pcp_prepare(struct page *page)
  */
 static bool free_pcp_prepare(struct page *page)
 {
-	if (debug_pagealloc_enabled())
+	if (debug_pagealloc_enabled_static())
 		return free_pages_prepare(page, 0, true);
 	else
 		return free_pages_prepare(page, 0, false);
@@ -1973,10 +1966,6 @@ void __init page_alloc_init_late(void)
 
 	for_each_populated_zone(zone)
 		set_zone_contiguous(zone);
-
-#ifdef CONFIG_DEBUG_PAGEALLOC
-	init_debug_guardpage();
-#endif
 }
 
 #ifdef CONFIG_CMA
@@ -2106,7 +2095,7 @@ static inline bool free_pages_prezeroed(void)
  */
 static inline bool check_pcp_refill(struct page *page)
 {
-	if (debug_pagealloc_enabled())
+	if (debug_pagealloc_enabled_static())
 		return check_new_page(page);
 	else
 		return false;
@@ -2128,7 +2117,7 @@ static inline bool check_pcp_refill(struct page *page)
 }
 static inline bool check_new_pcp(struct page *page)
 {
-	if (debug_pagealloc_enabled())
+	if (debug_pagealloc_enabled_static())
 		return check_new_page(page);
 	else
 		return false;
@@ -2155,7 +2144,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
 	set_page_refcounted(page);
 
 	arch_alloc_page(page, order);
-	if (debug_pagealloc_enabled())
+	if (debug_pagealloc_enabled_static())
 		kernel_map_pages(page, 1 << order, 1);
 	kasan_alloc_pages(page, order);
 	kernel_poison_pages(page, 1 << order, 1);