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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Thu, 4 Mar 2021 11:36:44 +0800
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Subject: Re: [External] Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page
To:     "Singh, Balbir" <bsingharora@gmail.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Matthew Wilcox <willy@infradead.org>,
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On Thu, Mar 4, 2021 at 11:14 AM Singh, Balbir <bsingharora@gmail.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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