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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no 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 C2F4AC43461 for ; Tue, 15 Sep 2020 18:04:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5E69F206B2 for ; Tue, 15 Sep 2020 18:04:19 +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="hRt4BX3Q" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727936AbgIOSER (ORCPT ); Tue, 15 Sep 2020 14:04:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35906 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727910AbgIORkS (ORCPT ); Tue, 15 Sep 2020 13:40:18 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 583FAC06174A; Tue, 15 Sep 2020 10:40:14 -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=R6wtnhjzOsOiJ+klikAlQCnUOZdsHt22W63kKyTGmA4=; b=hRt4BX3Qa/pHh5niIZM8vSwTDW azS00Kn1oHngezR/Un12ACAP87cSOwa/z52k7SuaEGVNOMqCnfDTGPJISGtzjFogOcIMPGFaXL+nv Rm3SQRcS2XV2jY6eHefJgtKRIlB8uN+GWZ8tmlf2IhiqF+Ne8MrFYG4eX0T6SwNCtnkHksRJ0aVky a549EeQTQIfe1ToTtT6bJovd6V8Q36MkUGC8NC4qJAWUcfb3K6Z6qdxmhFfkSgk86VHPftms2wKur 2i7VoJNs8IDwmNPqhS5WdsQyjo15s11Qw2lG6850wQMptmNv9PVt1Iu4uMVVHcRrE3X0mbilre/r3 u95WBVqw==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kIEvk-0001oD-H5; Tue, 15 Sep 2020 17:39:48 +0000 Date: Tue, 15 Sep 2020 18:39:48 +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: <20200915173948.GK5449@casper.infradead.org> References: <20200915125947.26204-1-songmuchun@bytedance.com> <20200915143241.GH5449@casper.infradead.org> <20200915154213.GI5449@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote: > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox wrote: > > > > 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). > > As I know, in this case, the get_user_pages() will get a reference > to the head page (head+0) before returning such that the hugetlb > page can not be freed. Although get_user_pages() returns the > page (head+5) and the scatterlist has a reference to the page > (head+5), this patch series can handle this situation. I can not > figure out where the problem is. What I missed? Thanks. You freed pages 4-511 from the vmemmap so they could be used for something else. Page 5 isn't there any more. So if you return head+5, then when we complete the I/O, we'll look for the compound_head() of head+5 and we won't find head.