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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 26977C83F10 for ; Thu, 31 Aug 2023 06:22:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240014AbjHaGWy (ORCPT ); Thu, 31 Aug 2023 02:22:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41160 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346086AbjHaGWe (ORCPT ); Thu, 31 Aug 2023 02:22:34 -0400 Received: from out-249.mta0.migadu.com (out-249.mta0.migadu.com [91.218.175.249]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C8927170C for ; Wed, 30 Aug 2023 23:21:55 -0700 (PDT) Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1693462911; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=XpQNp0A8i0S7SQfKTwp86WbNGnfzkLE8YkfFQsWuvUM=; b=X140LA5XVSse65boP6zP2hcTtma2Vup+iEfNBHXkmt0K3YzHrytzoMCTuIRmymld+BWC9O BSseNNTlDRlB4rJ90jaxCNTS7a6as5yKaFlXaFcbmlQ/tFnvOv8nfND+MM8RcvqHyUzesV jh57Am7jhW9FRzWn8Vw7VVDzAAJ/1KA= Mime-Version: 1.0 Subject: Re: [External] [v3 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: Date: Thu, 31 Aug 2023 14:21:06 +0800 Cc: Linux-MM , Mike Kravetz , Mike Rapoport , LKML , Muchun Song , fam.zheng@bytedance.com, liangma@liangbit.com, punit.agrawal@bytedance.com, Andrew Morton , Usama Arif Content-Transfer-Encoding: quoted-printable Message-Id: References: <20230825111836.1715308-1-usama.arif@bytedance.com> <20230825111836.1715308-5-usama.arif@bytedance.com> <486CFF93-3BB1-44CD-B0A0-A47F560F2CAE@linux.dev> To: Mel Gorman X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Aug 30, 2023, at 18:27, Usama Arif = wrote: > On 28/08/2023 12:33, Muchun Song wrote: >>> On Aug 25, 2023, at 19:18, Usama Arif = wrote: >>>=20 >>> The new boot flow when it comes to initialization of gigantic pages >>> is as follows: >>> - At boot time, for a gigantic page during __alloc_bootmem_hugepage, >>> the region after the first struct page is marked as noinit. >>> - This results in only the first struct page to be >>> initialized in reserve_bootmem_region. As the tail struct pages are >>> not initialized at this point, there can be a significant saving >>> in boot time if HVO succeeds later on. >>> - Later on in the boot, HVO is attempted. If its successful, only = the first >>> HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct = pages >>> after the head struct page are initialized. If it is not successful, >>> then all of the tail struct pages are initialized. >>>=20 >>> Signed-off-by: Usama Arif >> This edition is simpler than before ever, thanks for your work. >> There is premise that other subsystems do not access vmemmap pages >> before the initialization of vmemmap pages associated withe HugeTLB >> pages allocated from bootmem for your optimization. However, IIUC, = the >> compacting path could access arbitrary struct page when memory fails >> to be allocated via buddy allocator. So we should make sure that >> those struct pages are not referenced in this routine. And I know >> if CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, it will encounter >> the same issue, but I don't find any code to prevent this from >> happening. I need more time to confirm this, if someone already = knows, >> please let me know, thanks. So I think HugeTLB should adopt the = similar >> way to prevent this. >> Thanks. >=20 > Thanks for the reviews. >=20 > So if I understand it correctly, the uninitialized pages due to the = optimization in this patch and due to DEFERRED_STRUCT_PAGE_INIT should = be treated in the same way during compaction. I see that in = isolate_freepages during compaction there is a check to see if PageBuddy = flag is set and also there are calls like __pageblock_pfn_to_page to = check if the pageblock is valid. >=20 > But if the struct page is uninitialized then they would contain random = data and these checks could pass if certain bits were set? >=20 > Compaction is done on free list. I think the uninitialized struct = pages atleast from DEFERRED_STRUCT_PAGE_INIT would be part of freelist, = so I think their pfn would be considered for compaction. >=20 > Could someone more familiar with DEFERRED_STRUCT_PAGE_INIT and = compaction confirm how the uninitialized struct pages are handled when = compaction happens? Thanks! Hi Mel, Could you help us answer this question? I think you must be the expert = of CONFIG_DEFERRED_STRUCT_PAGE_INIT. I summarize the context here. As we = all know, some struct pages are uninnitialized when = CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, if someone allocates a larger memory (e.g. order is 4) via = buddy allocator and fails to allocate the memory, then we will go into the = compacting routine, which will traverse all pfns and use pfn_to_page to access its = struct page, however, those struct pages may be uninnitialized (so it's = arbitrary data). Our question is how to prevent the compacting routine from accessing = those uninitialized struct pages? We'll be appreciated if you know the answer. Thanks.