[PATCH v4] ring-buffer: Do not check si_mem_available() before deferred init completes

[PATCH v4] ring-buffer: Do not check si_mem_available() before deferred init completes

[Amit Machhiwal]

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.

Use IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) to set i = LONG_MAX during
this window, unconditionally bypassing the threshold check.  On kernels
without deferred init IS_ENABLED() compiles the branch away and 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 <msuchanek@suse.de>
Closes: https://lore.kernel.org/all/arYskzbiaNzBR9MD@kunlun.suse.cz/
Link: https://lore.kernel.org/all/arYskzbiaNzBR9MD@kunlun.suse.cz/
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes since v3:
 - Replace #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT / #else / #endif with
   IS_ENABLED() + i = LONG_MAX to avoid duplicating the threshold check.
   (Vincent Donnefort)
 - Trim the in-code comment to one sentence. (Vincent Donnefort)

Changes since v2:
 - Rebase on top of upstream ring_buffer.c changes from Vincent Donnefort;
   update threshold to i < (nr_pages << subbuf_order). (Michal Suchánek)

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 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c4e40847e43..09c02e838c4d 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2463,8 +2463,16 @@ 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, si_mem_available()
+		 * will reflect the actual available memory only after
+		 * SYSTEM_FREEING_INITMEM.
 		 */
 		i = si_mem_available();
+		if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) &&
+		    system_state < SYSTEM_FREEING_INITMEM)
+			i = LONG_MAX;
+
 		if (i < (nr_pages << subbuf_order))
 			return -ENOMEM;
 

base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
-- 
2.54.0 (Apple Git-157)