Re: [PATCH v2] ring-buffer: Do not check si_mem_available() before deferred init completes

During early boot (early_trace_init() called from start_kernel()),
tracing allocates initial ring buffers (temp_buffer, array_buffer, and
snapshot_buffer) for CPU 0.  Later, as secondary CPUs come online during
smp_init(), each CPU's ring buffer is allocated via trace_rb_cpu_prepare().

Before attempting page allocation, __rb_allocate_pages() performs a
heuristic check using si_mem_available() to return early with -ENOMEM if
memory appears insufficient.

However, on kernels built with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y,
NR_FREE_PAGES only reflects the initial non-deferred pool across the entire
period from early_trace_init() through smp_init().  defer_init() leaves
only a single section per node (e.g.  16 MiB with 64 KB pages) initialized
up-front, and on kernels with large static binary footprints (debug
configurations enabling PAGE_OWNER, DEBUG_PAGEALLOC, KFENCE, or SLUB_DEBUG),
early core initialisations (SLUB caches, vmalloc, static ftrace records)
consume virtually all managed pages in this pool.

si_mem_available() therefore returns a false negative (often 0-1 pages)
across both SYSTEM_BOOTING (for the initial CPU 0 buffers) and
SYSTEM_SCHEDULING (for secondary CPU buffers via trace_rb_cpu_prepare()),
causing __rb_allocate_pages() to prematurely abort with -ENOMEM.

This failure is false: the buddy allocator is designed to replenish
ZONE_NORMAL on demand via deferred_grow_zone() when an actual allocation
is attempted.  Checking si_mem_available() before calling alloc_pages_node()
short-circuits this on-demand growth entirely.

deferred_grow_zone() is only permanently disabled after page_alloc_init_late()
completes (it spawns per-node kthreads and blocks until done), which happens
inside kernel_init_freeable() — before system_state is set to
SYSTEM_FREEING_INITMEM.  Skip the si_mem_available() check until
SYSTEM_FREEING_INITMEM, at which point all deferred pages are initialized
and si_mem_available() is accurate.

Wrap the guard in #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT so that on kernels
without deferred init the original check is preserved unchanged.

Fixes: 2a872fa4e9c8 ("ring-buffer: Check if memory is available before allocation")
Cc: stable@vger.kernel.org
Reported-by: Michal Suchánek <redacted>
Closes: https://lore.kernel.org/all/arYskzbiaNzBR9MD@kunlun.suse.cz/ (local)
Link: https://lore.kernel.org/all/arYskzbiaNzBR9MD@kunlun.suse.cz/ (local)
Signed-off-by: Amit Machhiwal <redacted>
---
Changes since v1:
 - Extend the guard to cover SYSTEM_SCHEDULING as well: smp_init() calls
   trace_rb_cpu_prepare() -> __rb_allocate_pages() before page_alloc_init_late()
   completes, so the false negative window spans both SYSTEM_BOOTING and
   SYSTEM_SCHEDULING. Use system_state >= SYSTEM_FREEING_INITMEM as the correct
   boundary.  (Sashiko, Vincent Donnefort)
 - Wrap in #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT to preserve the original
   check unchanged on non-deferred kernels. (Vincent Donnefort)
 - Drop Tested-by: guard condition changed, requesting fresh test.

 kernel/trace/ring_buffer.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 04bb94c29f58..aa877a272c89 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c

@@ -2452,10 +2452,24 @@ static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
 	 * memory. It may not be accurate. But we don't care, we just want
 	 * to prevent doing any allocation when it is obvious that it is
 	 * not going to succeed.
+	 *
+	 * With CONFIG_DEFERRED_STRUCT_PAGE_INIT, NR_FREE_PAGES only reflects
+	 * the initial non-deferred pool during early boot and throughout SMP
+	 * bring-up (smp_init() runs before page_alloc_init_late() completes
+	 * deferred initialization). si_mem_available() returns a false negative
+	 * across this entire window while actual allocations succeed by growing
+	 * the zone on demand via deferred_grow_zone(). Only re-enable the check
+	 * from SYSTEM_FREEING_INITMEM onward, after page_alloc_init_late() has
+	 * finished and si_mem_available() is accurate.
 	 */
 	i = si_mem_available();
+#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
+	if (system_state >= SYSTEM_FREEING_INITMEM && i < nr_pages)
+		return -ENOMEM;
+#else
 	if (i < nr_pages)
 		return -ENOMEM;
+#endif
 
 	/*
 	 * If a user thread allocates too much, and si_mem_available()

base-commit: 2f2f7eeef8b0a1a9b9ca998231419873ccae8d4e