From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 37C2E7A for ; Wed, 20 Apr 2022 21:40:38 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 84B26C385A0; Wed, 20 Apr 2022 21:40:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1650490837; bh=vmOOYXgIqVmJv6Ynfv4pVSdE8+/S6hF5D1Kl1qwaVvU=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=thNo9wgIAXrJWqhwyyf3EdE8u9+lQc1YnozYsvYbJp4fyJqyGr0tPvqiLnmBgdkjv Ld7BFCZqCj8XKMBcUiMUYrkRvdEPr4/8AUHYDQQxFgECzXlSxUNS+UmKIvlDxJTKPX /UD26xfNslVtpTXROF0h+5R80+RqKP37IJOsKZOM= Date: Wed, 20 Apr 2022 14:40:36 -0700 From: Andrew Morton To: Joao Martins Cc: linux-mm@kvack.org, Dan Williams , Vishal Verma , Matthew Wilcox , Jason Gunthorpe , Jane Chu , Muchun Song , Mike Kravetz , Jonathan Corbet , Christoph Hellwig , nvdimm@lists.linux.dev, linux-doc@vger.kernel.org Subject: Re: [PATCH v9 4/5] mm/sparse-vmemmap: improve memory savings for compound devmaps Message-Id: <20220420144036.0bd0955e5d8478ca64f2df19@linux-foundation.org> In-Reply-To: <20220420155310.9712-5-joao.m.martins@oracle.com> References: <20220420155310.9712-1-joao.m.martins@oracle.com> <20220420155310.9712-5-joao.m.martins@oracle.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: nvdimm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 20 Apr 2022 16:53:09 +0100 Joao Martins wrote: > A compound devmap is a dev_pagemap with @vmemmap_shift > 0 and it > means that pages are mapped at a given huge page alignment and utilize > uses compound pages as opposed to order-0 pages. > > Take advantage of the fact that most tail pages look the same (except > the first two) to minimize struct page overhead. Allocate a separate > page for the vmemmap area which contains the head page and separate for > the next 64 pages. The rest of the subsections then reuse this tail > vmemmap page to initialize the rest of the tail pages. > > Sections are arch-dependent (e.g. on x86 it's 64M, 128M or 512M) and > when initializing compound devmap with big enough @vmemmap_shift (e.g. > 1G PUD) it may cross multiple sections. The vmemmap code needs to > consult @pgmap so that multiple sections that all map the same tail > data can refer back to the first copy of that data for a given > gigantic page. > > On compound devmaps with 2M align, this mechanism lets 6 pages be > saved out of the 8 necessary PFNs necessary to set the subsection's > 512 struct pages being mapped. On a 1G compound devmap it saves > 4094 pages. > > Altmap isn't supported yet, given various restrictions in altmap pfn > allocator, thus fallback to the already in use vmemmap_populate(). It > is worth noting that altmap for devmap mappings was there to relieve the > pressure of inordinate amounts of memmap space to map terabytes of pmem. > With compound pages the motivation for altmaps for pmem gets reduced. > > ... > > @@ -665,12 +770,19 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn, > { > unsigned long start = (unsigned long) pfn_to_page(pfn); > unsigned long end = start + nr_pages * sizeof(struct page); > + int r; > > if (WARN_ON_ONCE(!IS_ALIGNED(pfn, PAGES_PER_SUBSECTION) || > !IS_ALIGNED(nr_pages, PAGES_PER_SUBSECTION))) > return NULL; > > - if (vmemmap_populate(start, end, nid, altmap)) > + if (is_power_of_2(sizeof(struct page)) && Note that Muchun is working on a compile-time STRUCT_PAGE_SIZE_IS_POWER_OF_2 which this site should be able to utilize. https://lkml.kernel.org/r/20220413144748.84106-2-songmuchun@bytedance.com > + pgmap && pgmap_vmemmap_nr(pgmap) > 1 && !altmap) > + r = vmemmap_populate_compound_pages(pfn, start, end, nid, pgmap); > + else > + r = vmemmap_populate(start, end, nid, altmap); > + > + if (r < 0) > return NULL; > > return pfn_to_page(pfn);