From: Michal Hocko <mhocko@suse.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
Mike Kravetz <mike.kravetz@oracle.com>,
Thomas Gleixner <tglx@linutronix.de>,
mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
dave.hansen@linux.intel.com, luto@kernel.org,
Peter Zijlstra <peterz@infradead.org>,
viro@zeniv.linux.org.uk,
Andrew Morton <akpm@linux-foundation.org>,
paulmck@kernel.org, mchehab+huawei@kernel.org,
pawan.kumar.gupta@linux.intel.com,
Randy Dunlap <rdunlap@infradead.org>,
oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
Mina Almasry <almasrymina@google.com>,
David Rientjes <rientjes@google.com>,
Matthew Wilcox <willy@infradead.org>,
Oscar Salvador <osalvador@suse.de>,
"Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
Xiongchun duan <duanxiongchun@bytedance.com>,
linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page
Date: Mon, 23 Nov 2020 13:48:13 +0100 [thread overview]
Message-ID: <20201123124813.GN27488@dhcp22.suse.cz> (raw)
In-Reply-To: <CAMZfGtVboaBuP_jYHeaQHwQ4gJoXuJC47g1UwTa+aUL4bqo=zw@mail.gmail.com>
On Mon 23-11-20 20:40:40, Muchun Song wrote:
> On Mon, Nov 23, 2020 at 8:18 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Mon 23-11-20 20:07:23, Muchun Song wrote:
> > > On Mon, Nov 23, 2020 at 7:32 PM Michal Hocko <mhocko@suse.com> wrote:
> > [...]
> > > > > > > > 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
> > > > >
> > > > > Yeah, you are right. But it may not be a problem for HugeTLB pages.
> > > > > Because in most cases, we only read the tail struct page and get the
> > > > > head struct page through compound_head() when the pfn is within
> > > > > a HugeTLB range. Right?
> > > >
> > > > Many pfn walkers would encounter the head page first and then skip over
> > > > the rest. Those should be reasonably safe. But there is no guarantee and
> > > > the fact that you need a valid page->compound_head which might get
> > > > scribbled over once you have the struct page makes this extremely
> > > > subtle.
> > >
> > > In this patch series, we can guarantee that the page->compound_head
> > > is always valid. Because we reuse the first tail page. Maybe you need to
> > > look closer at this series. Thanks.
> >
> > I must be really terrible exaplaining my concern. Let me try one last
> > time. It is really _irrelevant_ what you do with tail pages. The
> > underlying problem is that you are changing struct pages under users
> > without any synchronization. What used to be a valid struct page will
> > turn into garbage as soon as you remap vmemmap page tables.
>
> Thank you very much for your patient explanation. So if the pfn walkers
> always try get the head struct page through compound_head() when it
> encounter a tail struct page. There will be no concerns. Do you agree?
No, I do not agree. Please read again. The content of the struct page
might be a complete garbage at any time after pfn_to_page returns a
struct page. So there is no valid compound_head anywamore.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-11-23 12:48 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-20 6:43 [PATCH v5 00/21] Free some vmemmap pages of hugetlb page Muchun Song
2020-11-20 6:43 ` [PATCH v5 01/21] mm/memory_hotplug: Move bootmem info registration API to bootmem_info.c Muchun Song
2020-11-20 6:43 ` [PATCH v5 02/21] mm/memory_hotplug: Move {get,put}_page_bootmem() " Muchun Song
2020-11-20 6:43 ` [PATCH v5 03/21] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP Muchun Song
2020-11-20 7:49 ` Michal Hocko
2020-11-20 8:35 ` [External] " Muchun Song
2020-11-20 8:47 ` Michal Hocko
2020-11-20 8:53 ` Muchun Song
2020-11-20 6:43 ` [PATCH v5 04/21] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate Muchun Song
2020-11-20 6:43 ` [PATCH v5 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers Muchun Song
2020-11-20 6:43 ` [PATCH v5 06/21] mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() Muchun Song
2020-11-20 6:43 ` [PATCH v5 07/21] mm/bootmem_info: Combine bootmem info and type into page->freelist Muchun Song
2020-11-20 6:43 ` [PATCH v5 08/21] mm/hugetlb: Initialize page table lock for vmemmap Muchun Song
2020-11-20 6:43 ` [PATCH v5 09/21] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page Muchun Song
2020-11-20 6:43 ` [PATCH v5 10/21] mm/hugetlb: Defer freeing of hugetlb pages Muchun Song
2020-11-20 6:43 ` [PATCH v5 11/21] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page Muchun Song
2020-11-20 8:11 ` Michal Hocko
2020-11-20 8:51 ` [External] " Muchun Song
2020-11-20 9:28 ` Michal Hocko
2020-11-20 9:37 ` Muchun Song
2020-11-20 11:10 ` Michal Hocko
2020-11-20 11:56 ` Muchun Song
2020-11-20 6:43 ` [PATCH v5 12/21] mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper Muchun Song
2020-11-20 6:43 ` [PATCH v5 13/21] mm/hugetlb: Use PG_slab to indicate split pmd Muchun Song
2020-11-20 8:16 ` Michal Hocko
2020-11-20 9:30 ` [External] " Muchun Song
2020-11-23 7:48 ` Michal Hocko
2020-11-23 8:01 ` Muchun Song
2020-11-20 6:43 ` [PATCH v5 14/21] mm/hugetlb: Support freeing vmemmap pages of gigantic page Muchun Song
2020-11-20 6:43 ` [PATCH v5 15/21] mm/hugetlb: Set the PageHWPoison to the raw error page Muchun Song
2020-11-20 8:19 ` Michal Hocko
2020-11-20 10:32 ` [External] " Muchun Song
2020-11-20 6:43 ` [PATCH v5 16/21] mm/hugetlb: Flush work when dissolving hugetlb page Muchun Song
2020-11-20 8:20 ` Michal Hocko
2020-11-20 6:43 ` [PATCH v5 17/21] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap Muchun Song
2020-11-20 8:22 ` Michal Hocko
2020-11-20 10:39 ` [External] " Muchun Song
2020-11-20 6:43 ` [PATCH v5 18/21] mm/hugetlb: Merge pte to huge pmd only for gigantic page Muchun Song
2020-11-20 8:23 ` Michal Hocko
2020-11-20 10:41 ` [External] " Muchun Song
2020-11-20 6:43 ` [PATCH v5 19/21] mm/hugetlb: Gather discrete indexes of tail page Muchun Song
2020-11-20 6:43 ` [PATCH v5 20/21] mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct page Muchun Song
2020-11-20 6:43 ` [PATCH v5 21/21] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two Muchun Song
2020-11-20 8:25 ` Michal Hocko
2020-11-20 9:15 ` David Hildenbrand
2020-11-22 13:30 ` Mike Rapoport
2020-11-22 19:00 ` Matthew Wilcox
2020-11-23 3:14 ` [External] " Muchun Song
2020-11-20 9:16 ` David Hildenbrand
2020-11-20 10:42 ` [External] " Muchun Song
2020-11-20 8:42 ` [PATCH v5 00/21] Free some vmemmap pages of hugetlb page Michal Hocko
2020-11-20 9:27 ` David Hildenbrand
2020-11-20 9:39 ` Michal Hocko
2020-11-20 9:43 ` David Hildenbrand
2020-11-20 17:45 ` Mike Kravetz
2020-11-20 18:00 ` David Hildenbrand
2020-11-22 7:29 ` [External] " Muchun Song
2020-11-23 7:38 ` Michal Hocko
2020-11-23 21:52 ` Mike Kravetz
2020-11-23 22:01 ` Matthew Wilcox
2020-11-20 12:40 ` [External] " Muchun Song
2020-11-20 13:11 ` Michal Hocko
2020-11-20 15:44 ` Muchun Song
2020-11-23 7:40 ` Michal Hocko
2020-11-23 8:53 ` Muchun Song
2020-11-23 9:43 ` Michal Hocko
2020-11-23 10:36 ` Muchun Song
2020-11-23 10:42 ` Michal Hocko
2020-11-23 11:16 ` Muchun Song
2020-11-23 11:32 ` Michal Hocko
2020-11-23 12:07 ` Muchun Song
2020-11-23 12:18 ` Michal Hocko
2020-11-23 12:40 ` Muchun Song
2020-11-23 12:48 ` Michal Hocko [this message]
2020-11-23 12:45 ` Matthew Wilcox
2020-11-23 13:05 ` Muchun Song
2020-11-23 13:13 ` Michal Hocko
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