Re: [External] Re: [RFC PATCH 00/24] mm/hugetlb: Free some vmemmap pages of hugetlb page

[Muchun Song]

On Wed, Sep 16, 2020 at 1:39 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote:
> > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote:
> > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote:
> > > > > > This patch series will free some vmemmap pages(struct page structures)
> > > > > > associated with each hugetlbpage when preallocated to save memory.
> > > > >
> > > > > It would be lovely to be able to do this.  Unfortunately, it's completely
> > > > > impossible right now.  Consider, for example, get_user_pages() called
> > > > > on the fifth page of a hugetlb page.
> > > >
> > > > Can you elaborate on the problem? Thanks so much.
> > >
> > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page
> > > on a 4kB base page system.  Today, that results in a bio_vec containing
> > > {head+5, 0, 0x800}.  Then we call page_to_phys() on that (head+5) struct
> > > page to get the physical address of the I/O, and we turn it into a struct
> > > scatterlist, which similarly has a reference to the page (head+5).
> >
> > As I know, in this case, the get_user_pages() will get a reference
> > to the head page (head+0) before returning such that the hugetlb
> > page can not be freed. Although get_user_pages() returns the
> > page (head+5) and the scatterlist has a reference to the page
> > (head+5), this patch series can handle this situation. I can not
> > figure out where the problem is. What I missed? Thanks.
>
> You freed pages 4-511 from the vmemmap so they could be used for
> something else.  Page 5 isn't there any more.  So if you return head+5,
> then when we complete the I/O, we'll look for the compound_head() of
> head+5 and we won't find head.
>

We do not free pages 4-511 from the vmemmap. Actually, we only
free pages 128-511 from the vmemmap.

The 512 struct pages occupy 8 pages of physical memory. We only
free 6 physical page frames to the buddy. But we will create a new
mapping just like below. The virtual address of the freed pages will
remap to the second page frame. So the second page frame is
reused.

When a hugetlbpage is preallocated, we can change the mapping to
bellow.

   hugetlbpage                   struct page(8 pages)          page
frame(8 pages)
  +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
  |           |                     |     0     | -------------> |     0     |
  |           |                     |     1     | -------------> |     1     |
  |           |                     |     2     | -------------> +-----------+
  |           |                     |     3     | -----------------^ ^ ^ ^ ^
  |           |                     |     4     | -------------------+ | | |
  |     2M    |                     |     5     | ---------------------+ | |
  |           |                     |     6     | -----------------------+ |
  |           |                     |     7     | -------------------------+
  |           |                     +-----------+
  |           |
  |           |
  +-----------+

As you can see, we reuse the first tail page.

-- 
Yours,
Muchun