mm/page_alloc: fix race condition in unaccepted memory handling

The page allocator tracks the number of zones that have unaccepted memory
using static_branch_enc/dec() and uses that static branch in hot paths to
determine if it needs to deal with unaccepted memory.

Borislav and Thomas pointed out that the tracking is racy: operations on
static_branch are not serialized against adding/removing unaccepted pages
to/from the zone.

Sanity checks inside static_branch machinery detects it:

WARNING: CPU: 0 PID: 10 at kernel/jump_label.c:276 __static_key_slow_dec_cpuslocked+0x8e/0xa0

The comment around the WARN() explains the problem:

	/*
	 * Warn about the '-1' case though; since that means a
	 * decrement is concurrent with a first (0->1) increment. IOW
	 * people are trying to disable something that wasn't yet fully
	 * enabled. This suggests an ordering problem on the user side.
	 */

The effect of this static_branch optimization is only visible on
microbenchmark.

Instead of adding more complexity around it, remove it altogether.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Fixes: dcdfdd40fa82 ("mm: Add support for unaccepted memory")
Link: https://lore.kernel.org/all/20250506092445.GBaBnVXXyvnazly6iF@fat_crate.local
Reported-by: Borislav Petkov <[email protected]>
Tested-by: Borislav Petkov (AMD) <[email protected]>
Reported-by: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Brendan Jackman <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: <[email protected]>	[6.5+]
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions