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[34.140.183.197]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e38031488sm3623513f8f.24.2026.10.23.01.20.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2026 01:20:47 -0700 (PDT) Date: Fri, 23 Oct 2026 09:20:44 +0100 From: Vincent Donnefort To: Amit Machhiwal Cc: Steven Rostedt , Masami Hiramatsu , linux-trace-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Michal =?iso-8859-1?Q?Such=E1nek?= , Mathieu Desnoyers , Madhavan Srinivasan , "Ritesh Harjani (IBM)" , Harsh Prateek Bora , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v3] ring-buffer: Do not check si_mem_available() before deferred init completes Message-ID: References: <20261016134247.70722-1-amachhiw@linux.ibm.com> <20261022181817.49717-1-amachhiw@linux.ibm.com> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Oct 23, 2026 at 09:18:10AM +0100, Vincent Donnefort wrote: > 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: stable@vger.kernel.org > > Reported-by: Michal Suchánek > > 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 > > 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; > or even i = totalram_pages()? > > > > user_thread = current->mm != NULL; > > } > > > > base-commit: 7be4bc52fd9e0e55dc9a2e0232b89427d2c4b614 > > -- > > 2.54.0 (Apple Git-157) > > > > -- > Vincent -- Vincent