From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7334E371878 for ; Mon, 13 Jul 2026 13:28:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783949304; cv=none; b=MKF8ja4I0GHhTQkTkRdiVWNzP156AYWazaS66S4lvUkBJuJOGJiV4HzynWG3LWJP4NmuB45N/AGmTdmL7D+JS+9Kbe20blspHFJl6aACzWzon2Ncva6EVA96kZHLjI8JPSuEbP+di4cWiHlOnhfZdZxmUPa4W/RGprBP9X7Bdcg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783949304; c=relaxed/simple; bh=ioNx/vZiww8QXaRnyf9DxKh6eU5D8RXGfIcVFBFbMwE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=NuuqI9xOifhgqOvF+qeFisuBkjNbrjQLu7zzRxfv1waGIlI96S5Tx3/TEa4wj7OHlBS1aY3R6ym1kju7fqh3orvpsza5ypskTpQSViKAhGt4+WCMf9oQ34QH3pLXSKXHu/oWKMasArVGhU6nCWeQmR002h+n1vI0wko6LQzuPg8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=IbpaFlKp; arc=none smtp.client-ip=91.218.175.189 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="IbpaFlKp" Message-ID: <03158ab2-94e0-4a48-94d1-df2eedbc5545@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783949300; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hCZwaMJokj8MvB23i+KgfL9s2fF07ZtloHz25nGxCcw=; b=IbpaFlKpeA4JFZRTs7k4EM9meHM8rqNlkc7853eC6eHUK3DbIhaVDMxQ+fem+c4f+dtDuE y6OhZo3KMkn3mClYnfAZ9QXDmpNSTEDhd/MA7LJ4NycFEh2iQTAefbclmy+AYROQyxRBxf ornndC/2/V47wWEUo6ZTo1CKDrkwRGA= Date: Mon, 13 Jul 2026 21:28:10 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v6 4/8] mm: add a template-based fast path for zone-device page init To: Li Zhe Cc: linux-arch@vger.kernel.org, linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, apopple@nvidia.com, arnd@arndb.de, balbirs@nvidia.com, bp@alien8.de, dave.hansen@linux.intel.com, david@kernel.org, kees@kernel.org, mingo@redhat.com, rppt@kernel.org, tglx@kernel.org References: <20260709112520.24857-1-lizhe.67@bytedance.com> <20260709112520.24857-5-lizhe.67@bytedance.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: <20260709112520.24857-5-lizhe.67@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2026/7/9 19:25, Li Zhe wrote: > memmap_init_zone_device() repeats nearly identical head-page > initialization for each PFN. Prepare one reusable ZONE_DEVICE head-page > template through the existing slow path, refresh the PFN-dependent > fields in that template before each copy, and memcpy it into each > destination page. > > The optimized path assigns _refcount through the copied template, so > keep it disabled when the page_ref_set tracepoint is enabled. > > This patch accelerates head-page initialization. The pfns_per_compound > == 1 case gets the full benefit here, compound tails are handled in the > next patch. > > Tested in a VM with a 100 GB fsdax namespace device configured with > map=dev on Intel Ice Lake server. This test exercises the nd_pmem rebind > path (pfns_per_compound == 1). > > Test procedure: > Rebind the nd_pmem driver 30 times and collect the memmap initialization > time from the pr_debug() output of memmap_init_zone_device(). > > Base(v7.2-rc1): > First binding: 1456 ms > Average of subsequent rebinds: 244.28 ms > > With this patch and its prerequisites applied: > First binding: 1440 ms > Average of subsequent rebinds: 217.19 ms > > This reduces the average rebind time from 244.28 ms to 217.19 ms, or > about 11%. An 11% reduction is definitely a solid achievement. While the absolute gain is in the range of a few dozen milliseconds, I’d love to better understand the compounding impact. In which specific latency-sensitive scenarios or workloads does saving these few dozen milliseconds translate into a major breakthrough for the user experience? Muchun, Thanks. > > Signed-off-by: Li Zhe > --- > mm/mm_init.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 58 insertions(+), 1 deletion(-) > > diff --git a/mm/mm_init.c b/mm/mm_init.c > index 5fccfbacf855..170021e182e0 100644 > --- a/mm/mm_init.c > +++ b/mm/mm_init.c > @@ -1065,6 +1065,44 @@ static void __ref zone_device_page_init_slow(struct page *page, > set_page_count(page, 0); > } > > +static inline bool zone_device_page_init_optimization_enabled(void) > +{ > + /* > + * The template fast path copies a preinitialized struct page image. > + * Skip it when the page_ref_set tracepoint is enabled. > + */ > + return !page_ref_tracepoint_active(page_ref_set); > +} > + > +/* > + * 'template' is a reusable page prototype rather than a strictly immutable > + * object. Most ZONE_DEVICE fields stay constant across the pages covered by > + * the current template, but section bits and page->virtual may still depend > + * on the PFN. Refresh those PFN-dependent fields in the template before > + * copying it into @page. > + */ > +static inline void zone_device_page_update_template(struct page *template, > + unsigned long pfn) > +{ > + set_page_section_from_pfn(template, pfn); > +#ifdef WANT_PAGE_VIRTUAL > + if (!is_highmem_idx(ZONE_DEVICE)) > + set_page_address(template, __va(pfn << PAGE_SHIFT)); > +#endif > +} > + > +static void zone_device_page_init_from_template(struct page *page, > + unsigned long pfn, struct page *template) > +{ > + /* > + * 'template' carries the invariant portion of a ZONE_DEVICE struct > + * page. Update the PFN-dependent fields in place before copying it > + * to the destination page. > + */ > + zone_device_page_update_template(template, pfn); > + memcpy(page, template, sizeof(*page)); > +} > + > /* > * With compound page geometry and when struct pages are stored in ram most > * tail pages are reused. Consequently, the amount of unique struct pages to > @@ -1120,6 +1158,7 @@ void __ref memmap_init_zone_device(struct zone *zone, > unsigned long nr_pages, > struct dev_pagemap *pgmap) > { > + bool use_template = zone_device_page_init_optimization_enabled(); > unsigned long pfn, end_pfn = start_pfn + nr_pages; > struct pglist_data *pgdat = zone->zone_pgdat; > struct vmem_altmap *altmap = pgmap_altmap(pgmap); > @@ -1127,6 +1166,7 @@ void __ref memmap_init_zone_device(struct zone *zone, > unsigned long zone_idx = zone_idx(zone); > unsigned long start = jiffies; > int nid = pgdat->node_id; > + struct page template; > > if (WARN_ON_ONCE(!pgmap || zone_idx != ZONE_DEVICE)) > return; > @@ -1144,7 +1184,24 @@ void __ref memmap_init_zone_device(struct zone *zone, > for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) { > struct page *page = pfn_to_page(pfn); > > - zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap); > + if (!use_template) { > + zone_device_page_init_slow(page, pfn, zone_idx, > + nid, pgmap); > + } else if (pfn == start_pfn) { > + /* > + * Seed the reusable head-page template from the > + * first real struct page, because the existing > + * page-init and pageblock helpers expect a real > + * memmap entry rather than a stack object. > + */ > + zone_device_page_init_slow(page, pfn, zone_idx, > + nid, pgmap); > + /* init template page */ > + memcpy(&template, page, sizeof(*page)); > + } else { > + zone_device_page_init_from_template(page, pfn, > + &template); > + } > > if (IS_ALIGNED(pfn, PAGES_PER_SECTION)) > cond_resched(); > -- > 2.20.1