From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EFA09C433E2 for ; Tue, 15 Sep 2020 22:51:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B819920770 for ; Tue, 15 Sep 2020 22:51:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="VfYtjtx1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727564AbgIOWvu (ORCPT ); Tue, 15 Sep 2020 18:51:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45644 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727438AbgIOPmg (ORCPT ); Tue, 15 Sep 2020 11:42:36 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A512AC06174A; Tue, 15 Sep 2020 08:42:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=WN7EmustK264smc9o/ZcxMc3lY45Oy2Z1WSCl7q0HYI=; b=VfYtjtx1jnJJKJuezTF8Wv7kTN zlCLTChtVQU0w+WsT7rLfh1k2wodLScX6Ty6wgSNfOYM41VlJUAn5E0wT6w3EYNXslThrNba9rThI ZZYlnb/ssedTdlgEOS4zIFENEYo3MPuTBEYzuitMiaUUNlQvI8PPP3i71Jr9swVeUInovnUY1A3+D i9YvQoh3MHM1UQUAFk+8eqrpZNsibhZ1xrESnazNUrJOa5x5raC9ClX8jPIN5jBQoeOsGb2mozEQU UGb6nFqz8/ImJNG8M0JlrYbwuPmWsDtAe9YSvx7PZ4qGn6+rOiJaBJQMTzlI57oPsbsnca2pQWGfN gOlQ+fhg==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kID5x-0002Vn-NM; Tue, 15 Sep 2020 15:42:13 +0000 Date: Tue, 15 Sep 2020 16:42:13 +0100 From: Matthew Wilcox To: Muchun Song Cc: Jonathan Corbet , Mike Kravetz , Thomas Gleixner , mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, Peter Zijlstra , viro@zeniv.linux.org.uk, Andrew Morton , paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, Randy Dunlap , oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, almasrymina@google.com, David Rientjes , linux-doc@vger.kernel.org, LKML , Linux Memory Management List , linux-fsdevel@vger.kernel.org Subject: Re: [External] Re: [RFC PATCH 00/24] mm/hugetlb: Free some vmemmap pages of hugetlb page Message-ID: <20200915154213.GI5449@casper.infradead.org> References: <20200915125947.26204-1-songmuchun@bytedance.com> <20200915143241.GH5449@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox wrote: > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > This patch series will free some vmemmap pages(struct page structures) > > > associated with each hugetlbpage when preallocated to save memory. > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > impossible right now. Consider, for example, get_user_pages() called > > on the fifth page of a hugetlb page. > > Can you elaborate on the problem? Thanks so much. OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page on a 4kB base page system. Today, that results in a bio_vec containing {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct page to get the physical address of the I/O, and we turn it into a struct scatterlist, which similarly has a reference to the page (head+5). If you're returning page (head+1) from get_user_pages(), all the calculations will be wrong and you'll do DMA to the wrong address. What needs to happen is a conversion to get_user_bvecs(). That way we can return a bvec which is {head, 0x5000, 0x800} and the calculations will work. So that's going to involve touching a lot of device drivers. Christoph has a start on it here: http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec Overlapping with that is a desire to change the biovec so that it only stores {phys_addr, length} rather than {page, offset, length}. We're also going to have to rework the scatterlist so that it doesn't carry a struct page either.