Messages in this thread | | | Date | Tue, 17 Nov 2020 19:22:23 +0000 | From | Matthew Wilcox <> | Subject | Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page |
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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.
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