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Date: Thu, 4 Mar 2021 11:36:44 +0800
From: Muchun Song <songmuchun@...edance.com>
To: "Singh, Balbir" <bsingharora@...il.com>
Cc: Jonathan Corbet <corbet@....net>,
Mike Kravetz <mike.kravetz@...cle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, bp@...en8.de, x86@...nel.org,
hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
Peter Zijlstra <peterz@...radead.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
anshuman.khandual@....com, jroedel@...e.de,
Mina Almasry <almasrymina@...gle.com>,
David Rientjes <rientjes@...gle.com>,
Matthew Wilcox <willy@...radead.org>,
Oscar Salvador <osalvador@...e.de>,
Michal Hocko <mhocko@...e.com>,
"Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
David Hildenbrand <david@...hat.com>,
HORIGUCHI NAOYA(堀口 直也)
<naoya.horiguchi@....com>,
Joao Martins <joao.m.martins@...cle.com>,
Xiongchun duan <duanxiongchun@...edance.com>,
linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
Linux Memory Management List <linux-mm@...ck.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page
On Thu, Mar 4, 2021 at 11:14 AM Singh, Balbir <bsingharora@...il.com> wrote:
>
> On 26/2/21 12:21 am, Muchun Song wrote:
> > Hi all,
> >
> > This patch series will free some vmemmap pages(struct page structures)
> > associated with each hugetlbpage when preallocated to save memory.
> >
> > In order to reduce the difficulty of the first version of code review.
> > From this version, we disable PMD/huge page mapping of vmemmap if this
> > feature was enabled. This accutualy eliminate a bunch of the complex code
> > doing page table manipulation. When this patch series is solid, we cam add
> > the code of vmemmap page table manipulation in the future.
> >
> > The struct page structures (page structs) are used to describe a physical
> > page frame. By default, there is a one-to-one mapping from a page frame to
> > it's corresponding page struct.
> >
> > The HugeTLB pages consist of multiple base page size pages and is supported
> > by many architectures. See hugetlbpage.rst in the Documentation directory
> > for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
> > are currently supported. Since the base page size on x86 is 4KB, a 2MB
> > HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> > 4096 base pages. For each base page, there is a corresponding page struct.
> >
> > Within the HugeTLB subsystem, only the first 4 page structs are used to
> > contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> > provides this upper limit. The only 'useful' information in the remaining
> > page structs is the compound_head field, and this field is the same for all
> > tail pages.
>
> The HUGETLB_CGROUP_MIN_ORDER is only when CGROUP_HUGETLB is enabled, but I guess
> that does not matter
Agree.
>
> >
> > By removing redundant page structs for HugeTLB pages, memory can returned to
> > the buddy allocator for other uses.
> >
> > When the system boot up, every 2M HugeTLB has 512 struct page structs which
> > size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
> >
> > HugeTLB struct pages(8 pages) page frame(8 pages)
> > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > | | | 0 | -------------> | 0 |
> > | | +-----------+ +-----------+
> > | | | 1 | -------------> | 1 |
> > | | +-----------+ +-----------+
> > | | | 2 | -------------> | 2 |
> > | | +-----------+ +-----------+
> > | | | 3 | -------------> | 3 |
> > | | +-----------+ +-----------+
> > | | | 4 | -------------> | 4 |
> > | 2MB | +-----------+ +-----------+
> > | | | 5 | -------------> | 5 |
> > | | +-----------+ +-----------+
> > | | | 6 | -------------> | 6 |
> > | | +-----------+ +-----------+
> > | | | 7 | -------------> | 7 |
> > | | +-----------+ +-----------+
> > | |
> > | |
> > | |
> > +-----------+
> >
> > The value of page->compound_head is the same for all tail pages. The first
> > page of page structs (page 0) associated with the HugeTLB page contains the 4
> > page structs necessary to describe the HugeTLB. The only use of the remaining
> > pages of page structs (page 1 to page 7) is to point to page->compound_head.
> > Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> > will be used for each HugeTLB page. This will allow us to free the remaining
> > 6 pages to the buddy allocator.
>
> What is page 1 used for? page 0 carries the 4 struct pages needed, does compound_head
> need a full page? IOW, why do we need two full pages -- may be the patches have the
> answer to something I am missing?
Yeah. It really can free 7 pages. But we need some work to support this. Why?
Now for the 2MB HugeTLB page, we only free 6 vmemmap pages. we really can
free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
structures have been set PG_head flag. If we can adjust compound_head()
slightly and make compound_head() return the real head struct page when
the parameter is the tail struct page but with PG_head flag set.
In order to make the code evolution route clearer. This feature can be
a separate patch (and send it out) after this patchset is solid and applied.
>
> >
> > Here is how things look after remapping.
> >
> > HugeTLB struct pages(8 pages) page frame(8 pages)
> > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > | | | 0 | -------------> | 0 |
> > | | +-----------+ +-----------+
> > | | | 1 | -------------> | 1 |
> > | | +-----------+ +-----------+
> > | | | 2 | ----------------^ ^ ^ ^ ^ ^
> > | | +-----------+ | | | | |
> > | | | 3 | ------------------+ | | | |
> > | | +-----------+ | | | |
> > | | | 4 | --------------------+ | | |
> > | 2MB | +-----------+ | | |
> > | | | 5 | ----------------------+ | |
> > | | +-----------+ | |
> > | | | 6 | ------------------------+ |
> > | | +-----------+ |
> > | | | 7 | --------------------------+
> > | | +-----------+
> > | |
> > | |
> > | |
> > +-----------+
> >
> > When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> > vmemmap pages and restore the previous mapping relationship.
> >
>
> Can these 6 pages come from the hugeTLB page itself? When you say 6 pages,
> I presume you mean 6 pages of PAGE_SIZE
There was a decent discussion about this in a previous version of the
series starting here:
https://lore.kernel.org/linux-mm/20210126092942.GA10602@linux/
In this thread various other options were suggested and discussed.
Thanks.
>
> > Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
> > to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
> > pages.
> >
> > In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
> > very substantial gain. On our server, run some SPDK/QEMU applications which
> > will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
> > (1G hugepage)/~12GB (2MB hugepage) memory.
>
> Thanks,
> Balbir Singh
>
>
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