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, 2 insertions, 2 deletions

diff --git a/mm/slab.c b/mm/slab.c
index f1e1840af533..a89633603b2d 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1416,7 +1416,7 @@ static void kmem_rcu_free(struct rcu_head *head)
 #if DEBUG
 static bool is_debug_pagealloc_cache(struct kmem_cache *cachep)
 {
-	if (debug_pagealloc_enabled() && OFF_SLAB(cachep) &&
+	if (debug_pagealloc_enabled_static() && OFF_SLAB(cachep) &&
 		(cachep->size % PAGE_SIZE) == 0)
 		return true;
 
@@ -2008,7 +2008,7 @@ int __kmem_cache_create(struct kmem_cache *cachep, slab_flags_t flags)
 	 * to check size >= 256. It guarantees that all necessary small
 	 * sized slab is initialized in current slab initialization sequence.
 	 */
-	if (debug_pagealloc_enabled() && (flags & SLAB_POISON) &&
+	if (debug_pagealloc_enabled_static() && (flags & SLAB_POISON) &&
 		size >= 256 && cachep->object_size > cache_line_size()) {
 		if (size < PAGE_SIZE || size % PAGE_SIZE == 0) {
 			size_t tmp_size = ALIGN(size, PAGE_SIZE);