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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 9E210C2D0E4 for ; Mon, 23 Nov 2020 10:43:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2B4A6208C3 for ; Mon, 23 Nov 2020 10:43:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="DtL35ygU" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728724AbgKWKnC (ORCPT ); Mon, 23 Nov 2020 05:43:02 -0500 Received: from mx2.suse.de ([195.135.220.15]:39626 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725907AbgKWKnB (ORCPT ); Mon, 23 Nov 2020 05:43:01 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1606128180; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=pFsAgqEF6rCorJPEzwduvsXSgM44bZXUvbIYui99MCA=; b=DtL35ygUL3Uzf5GavQeNA+QTIh5N0c7udmZkQriWUhO5FMnjuGAXKRDfZd3V+VKKdLLp5l TayBlp1wsYFV+BPvDH1ToIhuqojQSqFMlPKZl48S7JJwJe4hyTi4+vMxSTeOe3Zzgmjzsd 8AXQLYnutgm23bZAGnyR9o3Jt7Nv2SA= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CF697ABCE; Mon, 23 Nov 2020 10:42:59 +0000 (UTC) Date: Mon, 23 Nov 2020 11:42:58 +0100 From: Michal Hocko 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, Mina Almasry , David Rientjes , Matthew Wilcox , Oscar Salvador , "Song Bao Hua (Barry Song)" , Xiongchun duan , linux-doc@vger.kernel.org, LKML , Linux Memory Management List , linux-fsdevel Subject: Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page Message-ID: <20201123104258.GJ27488@dhcp22.suse.cz> References: <20201120064325.34492-1-songmuchun@bytedance.com> <20201120084202.GJ3200@dhcp22.suse.cz> <20201120131129.GO3200@dhcp22.suse.cz> <20201123074046.GB27488@dhcp22.suse.cz> <20201123094344.GG27488@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 23-11-20 18:36:33, Muchun Song wrote: > On Mon, Nov 23, 2020 at 5:43 PM Michal Hocko wrote: > > > > On Mon 23-11-20 16:53:53, Muchun Song wrote: > > > On Mon, Nov 23, 2020 at 3:40 PM Michal Hocko wrote: > > > > > > > > On Fri 20-11-20 23:44:26, Muchun Song wrote: > > > > > On Fri, Nov 20, 2020 at 9:11 PM Michal Hocko wrote: > > > > > > > > > > > > On Fri 20-11-20 20:40:46, Muchun Song wrote: > > > > > > > On Fri, Nov 20, 2020 at 4:42 PM Michal Hocko wrote: > > > > > > > > > > > > > > > > On Fri 20-11-20 14:43:04, Muchun Song wrote: > > > > > > > > [...] > > > > > > > > > > > > > > > > Thanks for improving the cover letter and providing some numbers. I have > > > > > > > > only glanced through the patchset because I didn't really have more time > > > > > > > > to dive depply into them. > > > > > > > > > > > > > > > > Overall it looks promissing. To summarize. I would prefer to not have > > > > > > > > the feature enablement controlled by compile time option and the kernel > > > > > > > > command line option should be opt-in. I also do not like that freeing > > > > > > > > the pool can trigger the oom killer or even shut the system down if no > > > > > > > > oom victim is eligible. > > > > > > > > > > > > > > Hi Michal, > > > > > > > > > > > > > > I have replied to you about those questions on the other mail thread. > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > One thing that I didn't really get to think hard about is what is the > > > > > > > > effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be > > > > > > > > invalid when racing with the split. How do we enforce that this won't > > > > > > > > blow up? > > > > > > > > > > > > > > This feature depends on the CONFIG_SPARSEMEM_VMEMMAP, > > > > > > > in this case, the pfn_to_page can work. The return value of the > > > > > > > pfn_to_page is actually the address of it's struct page struct. > > > > > > > I can not figure out where the problem is. Can you describe the > > > > > > > problem in detail please? Thanks. > > > > > > > > > > > > struct page returned by pfn_to_page might get invalid right when it is > > > > > > returned because vmemmap could get freed up and the respective memory > > > > > > released to the page allocator and reused for something else. See? > > > > > > > > > > If the HugeTLB page is already allocated from the buddy allocator, > > > > > the struct page of the HugeTLB can be freed? Does this exist? > > > > > > > > Nope, struct pages only ever get deallocated when the respective memory > > > > (they describe) is hotremoved via hotplug. > > > > > > > > > If yes, how to free the HugeTLB page to the buddy allocator > > > > > (cannot access the struct page)? > > > > > > > > But I do not follow how that relates to my concern above. > > > > > > Sorry. I shouldn't understand your concerns. > > > > > > vmemmap pages page frame > > > +-----------+ mapping to +-----------+ > > > | | -------------> | 0 | > > > +-----------+ +-----------+ > > > | | -------------> | 1 | > > > +-----------+ +-----------+ > > > | | -------------> | 2 | > > > +-----------+ +-----------+ > > > | | -------------> | 3 | > > > +-----------+ +-----------+ > > > | | -------------> | 4 | > > > +-----------+ +-----------+ > > > | | -------------> | 5 | > > > +-----------+ +-----------+ > > > | | -------------> | 6 | > > > +-----------+ +-----------+ > > > | | -------------> | 7 | > > > +-----------+ +-----------+ > > > > > > In this patch series, we will free the page frame 2-7 to the > > > buddy allocator. You mean that pfn_to_page can return invalid > > > value when the pfn is the page frame 2-7? Thanks. > > > > No I really mean that pfn_to_page will give you a struct page pointer > > from pages which you release from the vmemmap page tables. Those pages > > might get reused as soon sa they are freed to the page allocator. > > We will remap vmemmap pages 2-7 (virtual addresses) to page > frame 1. And then we free page frame 2-7 to the buddy allocator. And this doesn't really happen in an atomic fashion from the pfn walker POV, right? So it is very well possible that struct page *page = pfn_to_page(); // remapping happens here // page content is no longer valid because its backing memory can be // reused for whatever purpose. -- Michal Hocko SUSE Labs