mm/debug_pagealloc: ask users for default setting of debug_pagealloc

Since commit 031bc5743f158 ("mm/debug-pagealloc: make debug-pagealloc
boottime configurable") CONFIG_DEBUG_PAGEALLOC is by default not adding
any page debugging.

This resulted in several unnoticed bugs, e.g.

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

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 there are two stages of overhead.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fe4378fc0ab6..36a0a792f4f8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -478,7 +478,8 @@ 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)
@@ -489,6 +490,9 @@ 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);