Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page

[Singh, Balbir]

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