From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=209.85.128.43 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793033518; b=m0WWirPOwU18O9du3tema0+lQSd9VtZlREK283MU9AG7uelmom3NvQk0V0JPXXTUKrSv QVwGf2W3/6Dy2BjD5yAkEXeQsQr7XGZ3Pt3zxOIMXUKm5e38cbScuhAwqeOskZfdf2wIe VF9jgu+77fxs4vUlacjdAzWD+tF2F+Mx5miJqmNftDQUpaAPTeQJZxEioSk4H+1xTGhM+ /flcgjMbGeHz7H1PQDEO7HH9x3KBQ+RGpWVfxrQ24HYZ9f5gWa++5x1s+guZtfbAhHCK9 CNT9FCJa2jxFnVFo18SeA/4yY16iZj0xNtagyPIrrkSGaSIBcWPdzFvUqbyK2+WyHKQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793033518; 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=2/2qQgHjg7B4L90wJY7NcYQb/sw77pDt+0YufHxfkwg=; b=KpM4k39o5E9LSOLz5WvcKZmg7x9AuWmt7N9PaKmPWl9yWLeTWLZps8zNEk23cLq/wxnf 7ysJhMD3Z80I45PVU1FOQ1Aea2OGQ9OY7337NRRwdoSL08D4hbk7tYCx/BR5NYca9laY/ 79TBYh2XeiymciaXQ/WeFlnbvluoLPnczpCbBhAcp9tUS4mod576r0xzFxA6ckJ4/I2LC 2u1b68JSTbk2D25puT5/dR/WUfGVCltCvGpWeLIujXfAdqodh7zGA2uP3XCo/U+vKt8cp 4L99iVew//j4fy8yn7MZuOiYQ+PvefyB0MR9AWNzSLu6mIeCEY8N6Q7F216sPGQ3jbw== 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=uMdD4p6K; arc=none smtp.remote-ip=209.85.128.43 Received: from mail-wm1-f43.google.com (mail-wm1-f43.google.com [209.85.128.43]) (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 291CD606A32D for ; Mon, 26 Oct 2026 16:51:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 291CD606A32D 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=uMdD4p6K Received: by mail-wm1-f43.google.com with SMTP id 5b1f17b1804b1-4a5b2be99a8so11886525e9.2 for ; Mon, 26 Oct 2026 09:51:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1793033516; x=1793638316; 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=2/2qQgHjg7B4L90wJY7NcYQb/sw77pDt+0YufHxfkwg=; b=uMdD4p6KVakawh179kk8FHBLF5CXO9wel4SSzout3Cv109M7RXUmUmrC9sJPIgWsSH 9ABB0JV8Z5NUOTGY7GHDP30lVe2PF06Z5Rb3sGZu9AHhb2etfX/cHvQCyiGlPRRn3Cbe CFLeOIxpliD0sRukb/bWQTt0J7k1R+JKXgZVJ1KMgCeuSSmAA57l+zaNLFl6HnQ/Ahaw zQi33bNvGhiw52l+kM3vvg4nLkAUFO3pt8PlK45P8ZDXb1WCt8rTiBonnjcILrrKElFt fjlgVO/cXdwsEDjIa5uw8AyiN4qBUQXEww/1M11No4eGietgwgSL41okjBF4DwT+0EjC VbRQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1793033516; x=1793638316; 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=2/2qQgHjg7B4L90wJY7NcYQb/sw77pDt+0YufHxfkwg=; b=ZjwHY9183ko+ez/C9Lw5FbaSQVF5nZgE28PCmc8ehC8uMjuk2IINk/3CIhgxVc5VMi NylFfuwif0gdfn+csMGrPXa7I/kZpnAOOo71SGsqiVzA9kid8B6Zyu2RRPTspa4sD+Pt UtI2KDRgKcPRCURyu6jJUOSksCu75J3Zqa38VbtlKRwEnfP/obXEKatN0F+QawuXNX3Z zjHSJ5k+Bjsai4QxwBeq5vzTFU03bInomO5ZRfSHBQHaTe06sKE/wCBofOwsnIWuYNL1 BFdeqpYkr8CvnwaSp6TpsUcX0JEOyacFyeh4q3CLFuvi/bWAZuU7iEFNoPtPQ1WLi/YF uftQ== X-Forwarded-Encrypted: i=1; AKwUvBxuCE8aLh15CskOcCPo1oWo5BhbjJhoCvkX+uqbFda9kKBXzKt1dffYGpLVnOCPDALRfeo=@vger.kernel.org X-Gm-Message-State: AFq9FYJggmesUuIjMq3qAg8OWoYzIAphXzJtXVw0/u5OBw5dEJccSbYt y94Nl2GtS94C/CaH7bnicMfvpI0Ae2vknzAHT2jbhT2+z19bjLU3C/IdWQl6okHyTg== X-Gm-Gg: AYBFou3F6E3TwIo5YasFsI1yD28+/qIyfT9D09HVxYfI0t/ETZ0XyLw4BPVEtX1DC57 wcCvyLe18Tzmzvl+SoLhkELE5ntJttvfg4r/AS6mi5ojZooVEOaCgdPg5g9fG3qJHDi+t0C4Gv5 y5iT8qiVkAHwH1Khq5duIR2eN1nHuMVHKHZdh2JnLhrehlDR6mcsbr1j49OdMRepCktSToBtneK 2juM+Ze+v5uygGAzeOfJ0NPqDPIXhMZgdl1WsNeGqq8hDwTh06f8oFXl13L8OrAZHNrJLZKI1uf FTw0BtUpYdkDM1lC5kElxU0qvdDUzeWI1COOsF3Ov2K/Dz5rIN4se8+rZB/13hlB1che0bdTN8O Fc0qZU8eTccvZ+ii4FhjxsoEbVk68L+uZVzNMpGNV/8VUiR503R/RoboavU/X1VO+C7VZkV3N54 Go/dAnvJNOsxN1qOQ9nGzbgxQiYisen7Q/6SJsJyNkxU3rdJSCQEvm0Jvwde4gqYOQ2b8yUSzGJ 9o2ULwlygdbr4h8uHObq62tyMCjFFy5f+RgkLGhS3NIFBth1b8nd7kVUzCsz88M3wbrH9HZlwrx tomhQtwh9VZ609QpXZXw4UFr X-Received: by 2002:a05:600c:46c5:b0:4a2:1364:252b with SMTP id 5b1f17b1804b1-4a35acba20dmr143926245e9.35.1793033515974; Mon, 26 Oct 2026 09:51:55 -0700 (PDT) Received: from google.com (197.183.140.34.bc.googleusercontent.com. [34.140.183.197]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a35a58a293sm204540985e9.12.2026.10.26.09.51.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Oct 2026 09:51:55 -0700 (PDT) Date: Mon, 26 Oct 2026 16:51:52 +0000 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 v4] ring-buffer: Do not check si_mem_available() before deferred init completes Message-ID: References: <20261023145459.47516-1-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: <20261023145459.47516-1-amachhiw@linux.ibm.com> On Fri, Oct 23, 2026 at 08:24:59PM +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. > > Use IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) to set i = LONG_MAX during > this window, unconditionally bypassing the threshold check. On kernels > without deferred init IS_ENABLED() compiles the branch away and the original > check is preserved unchanged. Commit messages are not LLMs' strong suit. Otherwise. Reviewed-by: Vincent Donnefort > > 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 > --- > Changes since v3: > - Replace #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT / #else / #endif with > IS_ENABLED() + i = LONG_MAX to avoid duplicating the threshold check. > (Vincent Donnefort) > - Trim the in-code comment to one sentence. (Vincent Donnefort) > > 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 | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 9c4e40847e43..09c02e838c4d 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -2463,8 +2463,16 @@ 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, si_mem_available() > + * will reflect the actual available memory only after > + * SYSTEM_FREEING_INITMEM. > */ > i = si_mem_available(); > + if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) && > + system_state < SYSTEM_FREEING_INITMEM) > + i = LONG_MAX; > + > if (i < (nr_pages << subbuf_order)) > return -ENOMEM; > > > base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b > -- > 2.54.0 (Apple Git-157) >