From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta1.migadu.com (out-174.mta1.migadu.com [95.215.58.174]) (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 78F3C42376D; Wed, 15 Jul 2026 09:20:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784107209; cv=none; b=btbBLw7Ntk8uB1rUhH53p8zqZZSQOvxo5tgcq95Xkpu8FH/0eAdg/C5O+klfRH6wwCCmwws8W0LvVz9K/UGSe5xNEw9XS6eh5teaawdiFwba08KKEVPIzZimVhRKIcfIIW+orATkwyKH0IVVm3SucnMMthCo2t00vtImHBOqrhg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784107209; c=relaxed/simple; bh=7bqX575miDuDXxtkUM5rIRiOrm7H9ZQm+Q8G5UwWC5E=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=f5sBWNMYOJhcOVkASaxCEdl4cDtwDw9cW6iFMh3Cgr9qSd/4im0ZdHSDeNSq40hLZ07b/KdyqYFaNWTMqRvRU1ymNZSZ5K1t0nFbSxcFWGdUEM1QnB6JT5YbyqKdrOj7gvlP2UcTR/uinmgKl2qeA2hzCH5nUivJO7HPe5Uhqm0= 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=SKTE+a+J; arc=none smtp.client-ip=95.215.58.174 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="SKTE+a+J" Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784107205; 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=bO9YvcI54ue/gwDNdgB1p3Ix0P3xCAMWlTg6ILi3wPw=; b=SKTE+a+Jqj1MwRmJW7eGwo0/jz6WimdogmPp1HPKMXNzQl/nGGV6NKOLoVkLUBQJajM1gD 41oVl93srSOA3OKg6v6h0WJTIMxAQTIaZYM+m3tR9qN8L5LKnE+4Or3zdDA0nkQ0ASEcJm a39ToRz0q7nGV7XgjMxLSoh3WZaQIcg= Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.600.51.1.1\)) Subject: Re: [PATCH v6 4/8] mm: add a template-based fast path for zone-device page init X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: Date: Wed, 15 Jul 2026 17:19:21 +0800 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 Content-Transfer-Encoding: 7bit Message-Id: References: <20260709112520.24857-1-lizhe.67@bytedance.com> <20260709112520.24857-5-lizhe.67@bytedance.com> <08d40cad-8f61-4aba-9556-5a84067c6494@linux.dev> To: Li Zhe X-Migadu-Flow: FLOW_OUT > On Jul 15, 2026, at 14:11, Li Zhe wrote: > > On 7/14/26 4:38 PM, Muchun Song wrote: >> >> >> 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%. >>> >>> 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); >> >> I'm not sure why we need to make this. I think we can go ahead and >> perform this optimization unconditionally. >> >> If we look at why the tracepoint was added in the first place, it's >> clear that its primary purpose is for debugging, particularly regarding >> why CMA memory pages cannot be allocated (see commit 95813b8faa0). But >> at this stage, the struct page hasn't even been initialized yet; we >> are currently in the middle of initialization. From the perspective of >> the page being initialized, the user doesn't need to care at all about >> how the reference count is changing. Therefore, I don't think we need to >> include these checks. It would only increase the complexity of the code. >> >> I suggest deleting this section of code to keep things simple. >> >> Thanks. > > I would prefer to keep this check. > > When page_ref_set debugging is enabled, preserving the existing per-page > set_page_count() behavior is still useful, including during > initialization, because it keeps the debug trace complete and unchanged. When you say 'useful,' what aspects are you referring to? > > In the normal case, page_ref_set is off by default, so this only affects > an explicit debugging configuration and leaves the production fast path > unchanged. The extra logic here is also fairly small and stays local to > this optimization path. I know there's no real overhead here. The point I'm actually trying to make is that since deleting the code has no impact, why not go with the simpler approach? Thanks. > > Thanks, > Zhe > >>> +} >>> + >>> +/* >>> + * '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