Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page

[Matthew Wilcox]

On Wed, Nov 18, 2020 at 12:29:07AM +0800, Muchun Song wrote:
> > ideally, we should be able to free PageTail if we change struct page in some way.
> > Then we will save much more for 2MB hugetlb. but it seems it is not easy.
> 
> Now for the 2MB HugrTLB page, we only free 6 vmemmap pages.
> But your words woke me up. Maybe we really can free 7 vmemmap
> pages. In this case, we can see 8 of the 512 struct page structures
> has beed 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. I will start an investigation and a test.

What are you thinking?

static inline struct page *compound_head(struct page *page)
{
        unsigned long head = READ_ONCE(page->compound_head);

        if (unlikely(head & 1))
                return (struct page *) (head - 1);
+	if (unlikely(page->flags & PG_head))
+		return (struct page *)(page[1]->compound_head - 1)
        return page;
}

... because if it's that, there are code paths which also just test
PageHead, and so we'd actually need to change PageHead to be something
like:

static inline bool PageHead(struct page *page)
{
	return (page->flags & PG_head) &&
		(page[1]->compound_head == (unsigned long)page + 1);
}

I'm not sure if that's worth doing -- there may be other things I
haven't thought of.