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

["Michal Suchánek" <msuchanek@suse.de>]

On Fri, Oct 09, 2026 at 10:05:57PM +0530, Amit Machhiwal wrote:
> 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.

Tested-by: Michal Suchánek <msuchanek@suse.de>

> 
> 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)
> 
>