[PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc

[Christian Borntraeger]

since commit 031bc5743f158 ("mm/debug-pagealloc: make debug-pagealloc
boottime configurable") CONFIG_DEBUG_PAGEALLOC is by default a no-op.

This resulted in several unnoticed bugs, e.g.

https://lkml.kernel.org/g/<569F5E29.3090107@xxxxxxxxxx>
or
https://lkml.kernel.org/g/<56A20F30.4050705@xxxxxxxxxx>

as this behaviour change was not even documented in Kconfig.

Let's provide a new Kconfig symbol that allows to change the default
back to enabled, e.g. for debug kernels. This also makes the change
obvious to kernel packagers.

Let's also change the Kconfig description for CONFIG_DEBUG_PAGEALLOC,
to indicate that it is ok to enable this by default.

Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
 mm/Kconfig.debug | 17 +++++++++++++++++
 mm/page_alloc.c  |  6 +++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index 957d3da..4cf1212 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -26,5 +26,22 @@ config DEBUG_PAGEALLOC
 	  that would result in incorrect warnings of memory corruption after
 	  a resume because free pages are not saved to the suspend image.
 
+	  By default this option will be almost for free and can be activated
+	  in distribution kernels. The overhead and the debugging can be enabled
+	  by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc command line
+	  parameter.
+
+	  If unsure say yes.
+
+config DEBUG_PAGEALLOC_ENABLE_DEFAULT
+	bool "Enable debug page memory allocations by default?"
+        default off
+        depends on DEBUG_PAGEALLOC
+        ---help---
+	  Enable debug page memory allocations by default? This value
+	  can be overridden by debug_pagealloc=off|on.
+
+	  If unsure say no.
+
 config PAGE_POISONING
 	bool
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9d666df..933def7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -479,7 +479,8 @@ void prep_compound_page(struct page *page, unsigned int order)
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
 unsigned int _debug_guardpage_minorder;
-bool _debug_pagealloc_enabled __read_mostly;
+bool _debug_pagealloc_enabled __read_mostly
+			= IS_ENABLED(CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT);
 bool _debug_guardpage_enabled __read_mostly;
 
 static int __init early_debug_pagealloc(char *buf)
@@ -490,6 +491,9 @@ static int __init early_debug_pagealloc(char *buf)
 	if (strcmp(buf, "on") == 0)
 		_debug_pagealloc_enabled = true;
 
+	if (strcmp(buf, "off") == 0)
+		_debug_pagealloc_enabled = false;
+
 	return 0;
 }
 early_param("debug_pagealloc", early_debug_pagealloc);
-- 
2.3.0