From nobody Fri Jun 19 20:14:09 2026 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 B2AC2C433EF for ; Tue, 29 Mar 2022 04:57:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232213AbiC2E73 (ORCPT ); Tue, 29 Mar 2022 00:59:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43680 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230226AbiC2E7Z (ORCPT ); Tue, 29 Mar 2022 00:59:25 -0400 Received: from mail-pj1-x102f.google.com (mail-pj1-x102f.google.com [IPv6:2607:f8b0:4864:20::102f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0FF4115E88F for ; Mon, 28 Mar 2022 21:57:43 -0700 (PDT) Received: by mail-pj1-x102f.google.com with SMTP id h23-20020a17090a051700b001c9c1dd3acbso1629151pjh.3 for ; Mon, 28 Mar 2022 21:57:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=n5vkvz8TS8A/TWoh79lH9r9rs5OiOfx+UY7K0dS1wLk=; b=nmgg6DBPyTvI7tSYY3ftXQGAw7pj5EX77afDdie4czwYrHwuk0wemqLC04SZv3KVxK 2Nqso/uWLrG+LZzRWX1Y5j7i/s82vOAhgZIvFehA2k78wOPzs0nqTy+lHQgboDXP2EUs SbYnDIPcuUuuV/Ab1x/3rbV3xuoJfK/tP7YCNXlP6JVLE2pUh2+0JaLHz8HY30VojbGZ hX0tdY8SeYM5cFY7AX9bdXgTkZqTZJss79AI8VU2iYBbESzmmLvbVz7H/SZBUIXV5bo7 gTqrP46DZL4pRQRCJy2gyP8tA6y1ADB+IGcfkGPZSEbodQydRdtuZDe0wBvTxAB1rIoS ng5A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=n5vkvz8TS8A/TWoh79lH9r9rs5OiOfx+UY7K0dS1wLk=; b=PUzhNinJLx6t4+/iB7RIxCcp32spJLayU8DFKxR7UrVosLDRVMC/qfPOXaWlss0jRh oiMqObPMHg9fh2OtvdZ8y7PFc1YszaCi8c84L+p8lPObAexKElqHi0aUJz8hXXy5+RbQ yNKCmiLbcRF4lBeW0B0mklKM3i5VnsPE+7jBQ9MJxQJGVmD/vI5JRJ/WDgxiPRZTOAtf fUR8kV7AzxJZisfE1NPNJtj0fe4aIJJkJaHzSCnv6Bxp+0ZZMjwJoV98UJAbwgZElC0A oALPMWOWDogeowsNDGCl1mDQwV43C8pZkXh+2mjMEYQJbFjajsH50E+MXwf0zoDnf8BR TnUA== X-Gm-Message-State: AOAM532OERvor0vFUAYFO7tBOUqAEysorn1HlfD4f+/AY9WvmuEjuN+0 wEK2V3Bz+P4hGIhyTOggE30z3Q== X-Google-Smtp-Source: ABdhPJx9DzqtPZfaVooKrZXY1FuovRY9AVAwCLfsV5rTw8g7K2Xw9YVRhfamuhpewUfkJ1SkatF8Ow== X-Received: by 2002:a17:902:b202:b0:151:4f64:e516 with SMTP id t2-20020a170902b20200b001514f64e516mr29078442plr.16.1648529862520; Mon, 28 Mar 2022 21:57:42 -0700 (PDT) Received: from localhost.localdomain ([139.177.225.239]) by smtp.gmail.com with ESMTPSA id d18-20020a056a0010d200b004fa2e13ce80sm18333011pfu.76.2022.03.28.21.57.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Mar 2022 21:57:42 -0700 (PDT) From: Muchun Song To: will@kernel.org, akpm@linux-foundation.org, david@redhat.com, bodeddub@amazon.com, osalvador@suse.de, mike.kravetz@oracle.com, rientjes@google.com, mark.rutland@arm.com, catalin.marinas@arm.com, james.morse@arm.com, 21cnbao@gmail.com Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, duanxiongchun@bytedance.com, fam.zheng@bytedance.com, smuchun@gmail.com, Muchun Song Subject: [PATCH v2 RESEND] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB Date: Tue, 29 Mar 2022 12:55:35 +0800 Message-Id: <20220329045535.45641-1-songmuchun@bytedance.com> X-Mailer: git-send-email 2.32.0 (Apple Git-132) MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" The feature of minimizing overhead of struct page associated with each HugeTLB page aims to free its vmemmap pages (used as struct page) to save memory, where is ~14GB/16GB per 1TB HugeTLB pages (2MB/1GB type). In short, when a HugeTLB page is allocated or freed, the vmemmap array representing the range associated with the page will need to be remapped. When a page is allocated, vmemmap pages are freed after remapping. When a page is freed, previously discarded vmemmap pages must be allocated before remapping. More implementations and details can be found here [1]. The preparation of freeing vmemmap pages associated with each HugeTLB page is ready, so we can support this feature for arm64 now. The flush_dcache_page() need to be adapted to operate on the head page's flags since the tail vmemmap pages are mapped with read-only after the feature is enabled (clear operation is not permitted). There was some discussions about this in the thread [2], but there was no conclusion in the end. And I copied the concern proposed by Anshuman to here. 1st concern: ''' But what happens when a hot remove section's vmemmap area (which is being teared down) is nearby another vmemmap area which is either created or being destroyed for HugeTLB alloc/free purpose. As you mentioned HugeTLB pages inside the hot remove section might be safe. But what about other HugeTLB areas whose vmemmap area shares page table entries with vmemmap entries for a section being hot removed ? Massive HugeTLB alloc /use/free test cycle using memory just adjacent to a memory hotplug area, which is always added and removed periodically, should be able to expose this problem. ''' Answer: At the time memory is removed, all HugeTLB pages either have been migrated away or dissolved. So there is no race between memory hot remove and free_huge_page_vmemmap(). Therefore, HugeTLB pages inside the hot remove section is safe. Let's talk your question "what about other HugeTLB areas whose vmemmap area shares page table entries with vmemmap entries for a section being hot removed ?", the question is not established. The minimal granularity size of hotplug memory 128MB (on arm64, 4k base page), any HugeTLB smaller than 128MB is within a section, then, there is no share PTE page tables between HugeTLB in this section and ones in other sections and a HugeTLB page could not cross two sections. In this case, the section cannot be freed. Any HugeTLB bigger than 128MB (section size) whose vmemmap pages is an integer multiple of 2MB (PMD-mapped). As long as: 1) HugeTLBs are naturally aligned, power-of-two sizes 2) The HugeTLB size >=3D the section size 3) The HugeTLB size >=3D the vmemmap leaf mapping size Then a HugeTLB will not share any leaf page table entries with *anything else*, but will share intermediate entries. In this case, at the time memo= ry is removed, all HugeTLB pages either have been migrated away or dissolved. So there is also no race between memory hot remove and free_huge_page_vmemmap(). 2nd concern: ''' differently, not sure if ptdump would require any synchronization. Dumping an wrong value is probably okay but crashing because a page table entry is being freed after ptdump acquired the pointer is bad. On arm64, ptdump() is protected against hotremove via [get|put]_online_mems(). ''' Answer: The ptdump should be fine since vmemmap_remap_free() only exchanges PTEs or split the PMD entry (which means allocating a PTE page table). Both operations do not free any page tables (PTE), so ptdump cannot run into a UAF on any page tables. The wrost case is just dumping an wrong value. [1] https://lore.kernel.org/all/20210510030027.56044-1-songmuchun@bytedance= .com/ [2] https://lore.kernel.org/all/20210518091826.36937-1-songmuchun@bytedance= .com/ Signed-off-by: Muchun Song --- Changes in v2: - Update commit message (Mark Rutland). - Fix flush_dcache_page(). arch/arm64/mm/flush.c | 14 ++++++++++++++ fs/Kconfig | 2 +- 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c index a06c6ac770d4..705484a9b9df 100644 --- a/arch/arm64/mm/flush.c +++ b/arch/arm64/mm/flush.c @@ -75,6 +75,20 @@ EXPORT_SYMBOL_GPL(__sync_icache_dcache); */ void flush_dcache_page(struct page *page) { +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP + /* + * Only the head page's flags of HugeTLB can be cleared since the tail + * vmemmap pages associated with each HugeTLB page are mapped with + * read-only when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is enabled (more + * details can refer to vmemmap_remap_pte()). Although + * __sync_icache_dcache() only set PG_dcache_clean flag on the head + * page struct, some tail page structs still can see the flag since + * the head vmemmap page frame is reused (more details can refer to + * the comments above page_fixed_fake_head()). + */ + if (PageHuge(page)) + page =3D compound_head(page); +#endif if (test_bit(PG_dcache_clean, &page->flags)) clear_bit(PG_dcache_clean, &page->flags); } diff --git a/fs/Kconfig b/fs/Kconfig index 7a2b11c0b803..04cfd5bf5ec9 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -247,7 +247,7 @@ config HUGETLB_PAGE =20 config HUGETLB_PAGE_FREE_VMEMMAP def_bool HUGETLB_PAGE - depends on X86_64 + depends on X86_64 || ARM64 depends on SPARSEMEM_VMEMMAP =20 config HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON --=20 2.11.0