On Thu, Oct 22, 2026 at 11:48:17PM +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: [email protected]
> Reported-by: Michal Suchánek <[email protected]>
> Closes: https://lore.kernel.org/all/[email protected]/
> Link: https://lore.kernel.org/all/[email protected]/
> Signed-off-by: Amit Machhiwal <[email protected]>
It is usually good practice to not use in-reply-to to link the series between
each other.
> ---
> 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 | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 9c4e40847e43..8e5a59a7b69b 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -2463,10 +2463,27 @@ 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.
This is quite a massive comment for something simple: When
CONFIG_DEFERRED_STRUCT_PAGE_INIT, si_mem_available() will reflect the actual
available memory only after SYSTEM_FREEING_INITMEM.
> */
> i = si_mem_available();
> +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> + if (system_state >= SYSTEM_FREEING_INITMEM &&
> + i < (nr_pages << subbuf_order))
> + return -ENOMEM;
> +#else
> if (i < (nr_pages << subbuf_order))
> return -ENOMEM;
> +#endif
Could we avoid the double i < (nr_pages << subbuf_order) with something like
if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) &&
system_state < SYSTEM_FREEING_INITMEM)
i = LONG_MAX;
>
> user_thread = current->mm != NULL;
> }
>
> base-commit: 7be4bc52fd9e0e55dc9a2e0232b89427d2c4b614
> --
> 2.54.0 (Apple Git-157)
>
--
Vincent
