Re: [PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Matthew Wilcox]

On Wed, Jul 14, 2021 at 05:17:58PM +0800, Muchun Song wrote:
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +extern bool hugetlb_free_vmemmap_enabled;
> +
> +/*
> + * If the feature of freeing some vmemmap pages associated with each HugeTLB
> + * page is enabled, the head vmemmap page frame is reused and all of the tail
> + * vmemmap addresses map to the head vmemmap page frame (furture details can
> + * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
> + * word, there are more than one page struct with PG_head associated with each
> + * HugeTLB page.  We __know__ that there is only one head page struct, the tail
> + * page structs with PG_head are fake head page structs.  We need an approach
> + * to distinguish between those two different types of page structs so that
> + * compound_head() can return the real head page struct when the parameter is
> + * the tail page struct but with PG_head. This is what page_head_if_fake()
> + * does.
> + *
> + * The page_head_if_fake() returns the real head page struct iff the @page may
> + * be fake, otherwise, returns NULL if the @page cannot be a fake page struct.
> + * The following figure describes how to distinguish between real and fake head
> + * page struct.
> + *
> + *	if (test_bit(PG_head, &page->flags)) {
> + *		unsigned long head = READ_ONCE(page[1].compound_head);
> + *
> + *		if (head & 1) {
> + *			if (head == (unsigned long)page + 1)
> + *				==> head page struct
> + *			else
> + *				==> tail page struct
> + *		} else
> + *			==> head page struct
> + *	} else
> + *		==> cannot be fake head page struct

I'm not sure we need the pseudocode when the code is right there ...

> + * We can safely access the field of the @page[1] with PG_head because it means
> + * that the @page is a compound page composed with at least two contiguous
> + * pages.
> + */
> +static __always_inline struct page *page_head_if_fake(const struct page *page)
> +{
> +	if (!hugetlb_free_vmemmap_enabled)
> +		return NULL;
> +
> +	/*
> +	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> +	 * struct page. The alignment check aims to avoid access the fields (
> +	 * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> +	 * cold cacheline in some cases.
> +	 */
> +	if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> +	    test_bit(PG_head, &page->flags)) {
> +		unsigned long head = READ_ONCE(page[1].compound_head);
> +
> +		if (likely(head & 1))
> +			return (struct page *)(head - 1);
> +	}
> +
> +	return NULL;
> +}

Why return 'NULL' instead of 'page'?

This is going to significantly increase the cost of calling
compound_page() (by whichever spelling it has).  That will make
the folio patchset more compelling ;-)