[PATCH 2/2] ring-buffer: Do not check si_mem_available() during SYSTEM_BOOTING

[Amit Machhiwal <amachhiw@linux.ibm.com>]

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.

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,
defer_init() leaves only a single section per node (e.g.  16 MiB with 64
KB pages) initialized up-front.  On kernels with large static binary
footprints (such as debug configurations enabling PAGE_OWNER,
DEBUG_PAGEALLOC, KFENCE, or SLUB_DEBUG), the static kernel image and
early core initialisations (SLUB caches, vmalloc, static ftrace records)
consume virtually all managed pages in this initial pool.

At T=0.000000, watermarks have not yet been established
(totalreserve_pages = 0), so si_mem_available() returns the raw free
page count (often 0-1 pages).  When the snapshot buffer or global trace
buffer attempts to allocate 2 sub-pages, si_mem_available() returns <
nr_pages and prematurely aborts with -ENOMEM.

This failure is false: if the page allocation were actually attempted
via alloc_pages_node(), the page allocator would trigger
deferred_grow_zone() on demand to initialise additional deferred memory
sections.  Checking si_mem_available() before attempting the allocation
short-circuits this on-demand growth.

Skip the si_mem_available() check when system_state == SYSTEM_BOOTING.
Once the system transitions past early boot and page_alloc_init_late()
initialises all deferred memory, si_mem_available() accurately reflects
system-wide free memory and the check operates as intended for runtime
allocations.

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/
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 kernel/trace/ring_buffer.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 04bb94c29f58..a9f82e2f8fad 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2452,9 +2452,15 @@ 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.
+	 *
+	 * Skip this check during early boot: with CONFIG_DEFERRED_STRUCT_PAGE_INIT,
+	 * NR_FREE_PAGES only reflects the initial non-deferred pool at this
+	 * stage. si_mem_available() returns a false negative while actual
+	 * allocations succeed by growing the zone on demand via
+	 * deferred_grow_zone().
 	 */
 	i = si_mem_available();
-	if (i < nr_pages)
+	if (system_state != SYSTEM_BOOTING && i < nr_pages)
 		return -ENOMEM;
 
 	/*
-- 
2.54.0 (Apple Git-157)