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

[Balbir Singh]

On Thu, Mar 04, 2021 at 11:36:44AM +0800, Muchun Song wrote:
> On Thu, Mar 4, 2021 at 11:14 AM Singh, Balbir <bsingharora@xxxxxxxxx> 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.
>

Makes sense!
 
> >
> > >
> > > 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,
Balbir Singh