From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx184.postini.com [74.125.245.184]) by kanga.kvack.org (Postfix) with SMTP id 4AF0E6B006C for ; Mon, 19 Nov 2012 18:42:42 -0500 (EST) Date: Mon, 19 Nov 2012 15:42:40 -0800 From: Andrew Morton Subject: Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pages occupied by memmap Message-Id: <20121119154240.91efcc53.akpm@linux-foundation.org> In-Reply-To: <1353254850-27336-5-git-send-email-jiang.liu@huawei.com> References: <20121115112454.e582a033.akpm@linux-foundation.org> <1353254850-27336-1-git-send-email-jiang.liu@huawei.com> <1353254850-27336-5-git-send-email-jiang.liu@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Jiang Liu Cc: Wen Congyang , David Rientjes , Jiang Liu , Maciej Rutecki , Chris Clayton , "Rafael J . Wysocki" , Mel Gorman , Minchan Kim , KAMEZAWA Hiroyuki , Michal Hocko , Jianguo Wu , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Mon, 19 Nov 2012 00:07:29 +0800 Jiang Liu wrote: > If SPARSEMEM is enabled, it won't build page structures for > non-existing pages (holes) within a zone, so provide a more accurate > estimation of pages occupied by memmap if there are big holes within > the zone. > > And pages for highmem zones' memmap will be allocated from lowmem, > so charge nr_kernel_pages for that. > > ... > > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4435,6 +4435,22 @@ void __init set_pageblock_order(void) > > #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */ > > +static unsigned long calc_memmap_size(unsigned long spanned_pages, > + unsigned long present_pages) > +{ > + unsigned long pages = spanned_pages; > + > + /* > + * Provide a more accurate estimation if there are big holes within > + * the zone and SPARSEMEM is in use. > + */ > + if (spanned_pages > present_pages + (present_pages >> 4) && > + IS_ENABLED(CONFIG_SPARSEMEM)) > + pages = present_pages; > + > + return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT; > +} Please explain the ">> 4" heuristc more completely - preferably in both the changelog and code comments. Why can't we calculate this requirement exactly? That might require a second pass, but that's OK for code like this? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org