Re: [PATCH v20 4/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

On 4/15/21 1:40 AM, Muchun Song wrote:

Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4 (__NR_USED_SUBPAGE) struct page structures
to store metadata associated with each HugeTLB.

There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.

When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().

The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.

Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Tested-by: Chen Huang <redacted>
Tested-by: Bodeddula Balasubramaniam <redacted>
Acked-by: Michal Hocko <mhocko@suse.com>

There may need to be some trivial rebasing due to Oscar's changes
when they go in.

Reviewed-by: Mike Kravetz <redacted>
-- 
Mike Kravetz