From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=209.85.128.53 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792743497; b=P53ktzPmcmsif81ac46b8ZByVWFKA3oEB2SQ8BcaSXbgdCQy2eYUEALYN5lEu0L99i3K +4k2WLP+LIEmK9WOSiYGJYH7didtRl53H+LIpiYtiZtowqgjF6TppybsToUY7F/FaxGMy JxAXcaJSZHcee8IEL/Z9/lPk3m4NkF8/jBcBKAU16xmSrL1yUUvEWFH3pnFkisvwI+/nb 7pd6MqPh82qpoTt9bJj6L01SvHFEtNPl4gzyKfBveZJQx0Q6c4TmvwER4+zyFSMIzl4Xg ZJsv27x2zxCsaN9oz4/RdvsJTIgBrQLk8DB787pAWwtNXW8Yq3z0zW58bNv3suWUc+Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792743497; h=DMARC-Filter:Received:DKIM-Signature:X-Google-DKIM-Signature: X-Forwarded-Encrypted:X-Gm-Message-State:X-Gm-Gg:X-Received:Received: Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:Content-Transfer-Encoding: In-Reply-To; bh=5oVeJI18w0YWnW1MyEvLa+/8268RVOyYzuorpmt19/c=; b=fAvLmsD8juj50AvwqXSFsEZ6/62LSxA4hP/ygXArJnISgRAuF775nSGpVWcCxFCqtmif T40qDQshbTnYd8ZGNxXhZ01EosROqyqQl9lBQRrGx0gaBvVZpmXLcv8YAQkYURkiPBtGS l4sR27LvXBqw2q1af2Vau4EToBBUslmsEDBzPMK8EL0fFNUGYCgbKkWRSy2tygZ1RWB5m Iyo5W+kIBZFBdjLnNl6ThxMPC7eAykh6fYAe95Zj9/ICsw7tHSGZ7c14tyrXN3dxI588b 3JOxTysNI8GmLpU6D9YKUAmwN1kM6hnxTkcynH4BDat1j1MXoPZj08JIl8DXYfZds6A== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=google.com; spf=pass smtp.mailfrom=google.com; dkim=pass header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20251104 header.b=RrWV1pl+; arc=none smtp.remote-ip=209.85.128.53 Received: from mail-wm1-f53.google.com (mail-wm1-f53.google.com [209.85.128.53]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3E972606A326 for ; Fri, 23 Oct 2026 08:18:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 3E972606A326 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20251104 header.b=RrWV1pl+ Received: by mail-wm1-f53.google.com with SMTP id 5b1f17b1804b1-4a20bdaf39cso6463545e9.1 for ; Fri, 23 Oct 2026 01:18:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1792743495; x=1793348295; darn=vger.kernel.org; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:from:to:cc:subject:date:message-id:reply-to:content-type; bh=5oVeJI18w0YWnW1MyEvLa+/8268RVOyYzuorpmt19/c=; b=RrWV1pl+3EtmVhDk3j+FSUjGfIGHBGcq03rhpg7WHleDgWouEuAzLqoTyRgDy5fvz+ wF4UwPvIifknkCZJ1PyVasiujDFi1GArPhxOLkP1kt0rlZ76Wimqmbi6lRV7CAe335li nBN6RuojRiEjRPUXQXtFCRmcizH9AjfxZLYTp9mEVgDdPbIRcaS/V2y+lW/YTHj0zgLN 3/8M6w9m9KPXRpZeM767NWZkPimFSwtRCFhSCyNwuaWR9n+Yp67SOJtl9ONCDR3PThcC Hy2hN4n6do3y8bjEGk7YwetK5ZH6Q27eSVvNfCqUcRHWRJNJuRAwCVBgCixX2iBkqZcn E+xg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1792743495; x=1793348295; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:x-gm-gg:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to:content-type; bh=5oVeJI18w0YWnW1MyEvLa+/8268RVOyYzuorpmt19/c=; b=e65FUC1pg0nvo6l00gogIgSSwG81HJCrzpUEbNLvC0Aee9qUMWmumKdR6jFFlnl43i Vp1RQh8LrZnepp5v+r3UYmXkAP2/it2cfjpqqM1gCjewPP/s60/XBQnjUD4s1RYWwFcL xrHL7Au0NxGstZSg8PskKYlxAtfTqNpHytl0FmtOZZSZIqSGvb2O6n+4VbKJneS+2060 hCgX+NzTZZH9YrOVywym3u+FKnC/VSeQ4xjJ28pSBpim+Kqj96pwDDTBRDEMSWSYh6al EdKO+dHyP65OUPgB+lk5nAKi2HTnekBqsCmayZwI89H+0Bn7ujlyHFL6BEDMP/Gr3QpA xXzg== X-Forwarded-Encrypted: i=1; AKwUvBy3rYpSlG7pyBCeXxvxPex/9bXNUysOboj+PzKXGTwsB5mbCpKr2118w4j+RHE1FSJNveqw96M6@vger.kernel.org X-Gm-Message-State: AFq9FYL3s//WEf+5rafm4Jh1KxrgE6wW6e+n7owY1giT0E+k0VGhLl7Z ANHkOUdt3cFakpntXzPhxnB+2KAbIWXUXAPbXPmdGJtDAsxc3SUiX3GqeOuMTnOprA== X-Gm-Gg: AYBFou1FZ6wZjKADijvF/FiRyMC/uPVkZ0UfjbuDENe+poe41XXBBLpozQz7Khqw0nm klkzyPJBAERItRb51j3yKeQVlKUwShHcV99vuoFPrpl12z5wYJWCoY3G0BzHaXiveUaFDN2kmeq af2IQTe5jvzezUapfqy3P8qP6e7FMcmzOOMnE3qUGVrsaGe2lD1Wy/wAe2GmBEYhZxZfcVwm3mn ZoAiZJF8cboa4zOzzKmqCAQGG0j0NGBXD4SLGgt2KBA5FIWk+W2KUp2T5wSHhNMk/li5pbsbX/3 rGCd8Q+mUwCwwZHCot6hscXSha1LXse8CAkdGOpBcjJMWeHKWOAeISNPwyMFSC7jQtHj1ylBSYI dwdeSrKlBiegg3EuHDOCk/82k2F90d5Lo2aBlbdFqmUh1iSNo2mO7mi2fPpM4z/otqtgstekBAb Y+8D74gjx0HB/ZpjLIYnaUi4gGoS1wL91bjP0VktmdpAIStgtCJym7+CuZz+Yy13KSXptMWJsit CgCwO/vgpSKiGBE7RuSSSv08+hDvDe7JnYVjPXZbI5ryGiO2rFqpYKBZuonRvgws6hyfi4sfu7h NcuuSFGGYbivpBQhguWWyJr+ X-Received: by 2002:a05:600c:524b:b0:4a1:f720:74cd with SMTP id 5b1f17b1804b1-4a211ed3f5emr23048395e9.24.1792743494996; Fri, 23 Oct 2026 01:18:14 -0700 (PDT) Received: from google.com (197.183.140.34.bc.googleusercontent.com. [34.140.183.197]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a21045df9dsm68556225e9.8.2026.10.23.01.18.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2026 01:18:14 -0700 (PDT) Date: Fri, 23 Oct 2026 09:18:10 +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-ppc@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: <20261022181817.49717-1-amachhiw@linux.ibm.com> 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; > > user_thread = current->mm != NULL; > } > > base-commit: 7be4bc52fd9e0e55dc9a2e0232b89427d2c4b614 > -- > 2.54.0 (Apple Git-157) > -- Vincent