Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page

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