Re: [PATCH v2 04/19] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Oscar Salvador]

On Mon, Oct 26, 2020 at 10:50:59PM +0800, Muchun Song wrote:
> If the size of hugetlb page is 2MB, we need 512 struct page structures
> (8 pages) to be associated with it. As far as I know, we only use the
> first 4 struct page structures.

As Mike pointed out, better describe what those "4" mean.
 
> For tail pages, the value of compound_dtor is the same. So we can reuse

I might be missing something, but HUGETLB_PAGE_DTOR is only set on the
first tail, right?

> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +#define RESERVE_VMEMMAP_NR	2U

Although you can get that from the changelog, maybe a brief comment explaining
why RESERVE_VMEMMAP_NR == 2.
> +
> +static inline unsigned int nr_free_vmemmap(struct hstate *h)
> +{
> +	return h->nr_free_vmemmap_pages;
> +}

Better add this in the patch that is used?

> +	if (vmemmap_pages > RESERVE_VMEMMAP_NR)
> +		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> +	else
> +		h->nr_free_vmemmap_pages = 0;

Can we really have an scenario where we end up with vmemmap_pages < RESERVE_VMEMMAP_NR?

> +
> +	pr_info("HugeTLB: can free %d vmemmap pages for %s\n",
> +		h->nr_free_vmemmap_pages, h->name);

I do not think this is useful unless debugging situations, so I would either
scratch that or make it pr_debug.


-- 
Oscar Salvador
SUSE L3