[PATCH v2 2/2] mm/mm_init: decouple page checking and init_on_{alloc, free}

[Joshua Hahn]

init_on_alloc and init_on_free protect the kernel by initializing
allocated and freed pages to 0 on allocation time / deletion.
Commit 700d2e9a36b93601270c1e15550acde2521386c5 ("mm, page_alloc: reduce
page alloc/free sanity checks") removed page checking from hot pcp
drain and refill paths, and instead coupled it with CONFIG_DEBUG_VM,
debug_pagealloc, page poisoning, and init_on_{alloc, free}.

As the commit suggests, the first three turn the kernel into a debug
kernel, while the last hardens the kernel against leaking sensitive memory.
While enabling page checking is relatively low-cost and tying it
together with page initialization is not unreasonable, it does feel like
a bit of a side-effect, rather than an obvious consequence.

With page checking now pulled out as a boot time parameter that can be
set independently, let's decouple page checking and init_on_alloc and
init_on_free.

As a direct side effect, systems that have init_on_alloc or init_on_free
will no longer have page checking enabled by default; they will either
have to pass the check_pages boot parameter, build the kernel with
CONFIG_DEBUG_VM, or enable debug_pagealloc / page poisoning.

Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
---
 mm/mm_init.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 01d46efc42b4..59636d2c0178 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -2566,19 +2566,15 @@ static void __init mem_debugging_and_hardening_init(void)
 		_init_on_free_enabled_early = false;
 	}
 
-	if (_init_on_alloc_enabled_early) {
-		want_check_pages = true;
+	if (_init_on_alloc_enabled_early)
 		static_branch_enable(&init_on_alloc);
-	} else {
+	else
 		static_branch_disable(&init_on_alloc);
-	}
 
-	if (_init_on_free_enabled_early) {
-		want_check_pages = true;
+	if (_init_on_free_enabled_early)
 		static_branch_enable(&init_on_free);
-	} else {
+	else
 		static_branch_disable(&init_on_free);
-	}
 
 	if (IS_ENABLED(CONFIG_KMSAN) &&
 	    (_init_on_alloc_enabled_early || _init_on_free_enabled_early))
-- 
2.47.3