Re: [Chapter Two] THP shattering: the reverse of collapsing

[Barry Song <21cnbao@gmail.com>]

> In contrast to split, shatter migrates occupied pages in a partially
> mapped THP to a bunch of base folios. IOW, unlike split done in place,
> shatter is the exact opposite of collapse.
> 
> +static void copy_page_owner(struct folio *src)
> +{
> +	int i;
> +	int nr_pages = folio_nr_pages(src);
> +
> +	if (folio_can_split(src))
> +		return;
> +
> +	if (WARN_ON_ONCE(!src->_dst_pp))
> +		return;
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		struct page *dst = folio_dst_page(src, i);
> +
> +		if (dst)
> +			folio_copy_owner(src, page_folio(dst));

folio_copy_owner(page_folio(dst), src) ?

or can we drop this? then dst will have shatter as allocation
stack. this seems more helpful for debugging.
or it is even better we can reflect both the src pageowner and
shatter stack by enhancing pageowner mechinism?

> +	}
> +}

Thanks
Barry