From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx150.postini.com [74.125.245.150]) by kanga.kvack.org (Postfix) with SMTP id 231A06B005D for ; Tue, 20 Nov 2012 10:18:49 -0500 (EST) Received: by mail-pb0-f41.google.com with SMTP id xa7so4675150pbc.14 for ; Tue, 20 Nov 2012 07:18:48 -0800 (PST) Message-ID: <50AB9F4A.5050500@gmail.com> Date: Tue, 20 Nov 2012 23:18:34 +0800 From: Jiang Liu MIME-Version: 1.0 Subject: Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pages occupied by memmap 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> <20121119154240.91efcc53.akpm@linux-foundation.org> In-Reply-To: <20121119154240.91efcc53.akpm@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton 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 11/20/2012 07:42 AM, Andrew Morton wrote: > 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? Hi Andrew, A normal x86 platform always have some holes within the DMA ZONE, so the ">> 4" heuristic is to avoid applying this adjustment to the DMA ZONE on x86 platforms. Because the memmap_size is just an estimation, I feel it's OK to remove the ">> 4" heuristic, that shouldn't affect much. Thanks Gerry -- 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