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

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

On Fri, Oct 16, 2026 at 07:12:47PM +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.  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 <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 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

Hello,

due to other conflicting changes this no longer applies.

Will you send a rebase?

Thanks

Michal