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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 04161C433EF for ; Wed, 30 Mar 2022 09:09:35 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 7F8FF8D0002; Wed, 30 Mar 2022 05:09:35 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7AA358D0001; Wed, 30 Mar 2022 05:09:35 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 670C78D0002; Wed, 30 Mar 2022 05:09:35 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.25]) by kanga.kvack.org (Postfix) with ESMTP id 536CD8D0001 for ; Wed, 30 Mar 2022 05:09:35 -0400 (EDT) Received: from smtpin08.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 1861CC3F for ; Wed, 30 Mar 2022 09:09:35 +0000 (UTC) X-FDA: 79300479510.08.FDDF90D Received: from mail-ej1-f54.google.com (mail-ej1-f54.google.com [209.85.218.54]) by imf22.hostedemail.com (Postfix) with ESMTP id 91161C0003 for ; Wed, 30 Mar 2022 09:09:34 +0000 (UTC) Received: by mail-ej1-f54.google.com with SMTP id pv16so40205112ejb.0 for ; Wed, 30 Mar 2022 02:09:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=2B7F41ws4olLWAaIXnAdGV8fIlHulrUokTuiMGzfmnw=; b=FzR/KbCmNwWMlmFy0d61cIFF+xES/VqpAoijU7PVL0MLqCr1pxhjRNBHkzDXqLFPmg +1QNSshW5SfY2XhlByHFG0ssVgSe6NrL6oTVRg+bPuyio9MRaa2zvcRXmWWJCP0O/UUQ tdsEbVr2VVNavvm+lFcBGmAP5N0ZJo6v35B/Rb9rjHvZ428TE55cxXdysaG6B9t3dzqc RlBCsc3o/ermfPND5HrwBtKVCX6eu7/nWynDLaMAaLmTT3JboNjgENtMxlsPYNxIcfpz jO3q5VKHK+dFj/7MJnys7XgwnxgGU/qaBGjjP2L9FVWFPHIGCLqrVttfx+AbckPIbeKX u/jQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=2B7F41ws4olLWAaIXnAdGV8fIlHulrUokTuiMGzfmnw=; b=HOL15xfm7SPGb/DilLJa3YXz13rf5OFe6vcUAN5qZ60/d1CtK3lvuQe3/ZU0iXXeoU 3xL6RGWXHV85nyGvXukVxfFYpVTYqNjHnRCjiBpWIW6wzltK2Va42LvssEj0OrokFvNt dwy16pE7ekS4e9R5CdrbLbyKRECmSrVC3ibCJTsbGGjTLziFnWRsuEF6UifqDj3Vm1V/ ooDK/1KpZGanhqxUJugxQsGr0g+ruTz/tVsPcqOmQ7Fcrw5cfpY+FEQN7L3hvKdMHSKQ R5MrneiM+YYJrjOFNifYKkJMkBvKRV8kX8P/MpOFPDHK24quutVQZVJRqpHxq8gwE2DQ kEfQ== X-Gm-Message-State: AOAM533eWIVQp7jBJxtU9o+3HshBDrl6sCFU75urbn0xRmgAdoFgHqkp 8T8yc5DBxfLCkoPPXnKe9zmRNJJH4zLTR5rNhP0= X-Google-Smtp-Source: ABdhPJzOqkMoif51zT1EA5s03BklIZ8II21tK0zwJ6ibs0HOvImhSlTORDyrO6i0FZkvJUebj2vqYqjnZf2rBRTbh50= X-Received: by 2002:a17:907:94d5:b0:6e0:2924:bd54 with SMTP id dn21-20020a17090794d500b006e02924bd54mr37982877ejc.170.1648631373078; Wed, 30 Mar 2022 02:09:33 -0700 (PDT) MIME-Version: 1.0 References: <20220329045535.45641-1-songmuchun@bytedance.com> In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Wed, 30 Mar 2022 22:09:21 +1300 Message-ID: Subject: Re: [PATCH v2 RESEND] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB To: Muchun Song Cc: Will Deacon , Andrew Morton , David Hildenbrand , "Bodeddula, Balasubramaniam" , Oscar Salvador , Mike Kravetz , David Rientjes , Mark Rutland , Catalin Marinas , james.morse@arm.com, LAK , LKML , Linux-MM , Xiongchun duan , Fam Zheng , Muchun Song Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b="FzR/KbCm"; spf=pass (imf22.hostedemail.com: domain of 21cnbao@gmail.com designates 209.85.218.54 as permitted sender) smtp.mailfrom=21cnbao@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Stat-Signature: omzho6aospqypg7sqa9uiiqkjm7ozpsa X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 91161C0003 X-HE-Tag: 1648631374-727705 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Mar 30, 2022 at 7:53 PM Muchun Song wrote: > > On Tue, Mar 29, 2022 at 7:44 PM Barry Song <21cnbao@gmail.com> wrote: > > > > On Tue, Mar 29, 2022 at 5:57 PM Muchun Song wrote: > > > > > > 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 >= the section size > > > 3) The HugeTLB size >= 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 memory > > > 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()). > > > > Is this still true if hugetlb_free_vmemmap_enabled() is false? > > No. Do you think it is better to add hugetlb_free_vmemmap_enabled() > into the if block? Something like the following? yep, with if (hugetlb_free_vmemmap_enabled() && PageHuge(page)), i guess we won't need the "ifdef" any more? > > + if (hugetlb_free_vmemmap_enabled() && PageHuge(page)) > + page = compound_head(page); > > > > > btw, the subject is a bit confusing as it seems it is not bringing up > > HUGETLB_PAGE_FREE_VMEMMAP and it seems the feature > > has been already there, but we are lacking some fixes for some > > functions to make it work. > > Right. > > > could we explain this clear in commit > > log? maybe we need a better subject for the commit as well. > > Will do. Thanks Barry