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 A74BDC83F32 for ; Thu, 31 Aug 2023 10:01:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243680AbjHaKB7 (ORCPT ); Thu, 31 Aug 2023 06:01:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40794 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233881AbjHaKB4 (ORCPT ); Thu, 31 Aug 2023 06:01:56 -0400 Received: from out-243.mta0.migadu.com (out-243.mta0.migadu.com [91.218.175.243]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4209BCEE for ; Thu, 31 Aug 2023 03:01:53 -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=1693476111; 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=WKc1BtceIQGmsRun6QDSzaR2M4uTT4/ClHb8w8QgRJs=; b=MIeAL94hzGLpBkuqvJopyawFDuReZXKLmlTgY6nxwsBpwp1nfnT1q1Op78u96Kfi/j63Jt JYYAsAEDps3s8X8P+MrVz29CWb5sDbVWvl9zQiSOAuIh4WVVJwTWP0ksd6Npdv0jZgrpT0 2hNjfFi/8RlzuxCKZFy5KzBvkTcoeEk= 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: <20230831095801.76rtpgdsvdijbw5t@techsingularity.net> Date: Thu, 31 Aug 2023 18:01:08 +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: <07E9202B-CA8B-4E1E-93FC-7BF84CB8E988@linux.dev> References: <20230825111836.1715308-1-usama.arif@bytedance.com> <20230825111836.1715308-5-usama.arif@bytedance.com> <486CFF93-3BB1-44CD-B0A0-A47F560F2CAE@linux.dev> <20230831095801.76rtpgdsvdijbw5t@techsingularity.net> To: Mel Gorman X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Aug 31, 2023, at 17:58, Mel Gorman = wrote: >=20 > On Thu, Aug 31, 2023 at 02:21:06PM +0800, Muchun Song wrote: >>=20 >>=20 >>> 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! >>=20 >> Hi Mel, >>=20 >> 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. >>=20 >=20 > I didn't check the code but IIRC, the struct pages should be at least > valid and not contain arbitrary data once page_alloc_init_late = finishes. However, the buddy allocator is ready before page_alloc_init_late(), so = it may access arbitrary data in compacting routine, right? >=20 > --=20 > Mel Gorman > SUSE Labs