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Message-ID: <e9ef3479-24f1-9304-ee0e-6f06fb457d50@gmail.com>
Date: Thu, 4 Mar 2021 14:14:23 +1100
From: "Singh, Balbir" <bsingharora@...il.com>
To: Muchun Song <songmuchun@...edance.com>, corbet@....net,
mike.kravetz@...cle.com, tglx@...utronix.de, mingo@...hat.com,
bp@...en8.de, x86@...nel.org, hpa@...or.com,
dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
paulmck@...nel.org, mchehab+huawei@...nel.org,
pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
osalvador@...e.de, mhocko@...e.com, song.bao.hua@...ilicon.com,
david@...hat.com, naoya.horiguchi@....com,
joao.m.martins@...cle.com
Cc: duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page
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
>
> 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?
>
> 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
> 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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