Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange two lists of pages.

[Matthew Wilcox]

On Fri, Feb 15, 2019 at 02:08:26PM -0800, Zi Yan wrote:
> +struct page_flags {
> +	unsigned int page_error :1;
> +	unsigned int page_referenced:1;
> +	unsigned int page_uptodate:1;
> +	unsigned int page_active:1;
> +	unsigned int page_unevictable:1;
> +	unsigned int page_checked:1;
> +	unsigned int page_mappedtodisk:1;
> +	unsigned int page_dirty:1;
> +	unsigned int page_is_young:1;
> +	unsigned int page_is_idle:1;
> +	unsigned int page_swapcache:1;
> +	unsigned int page_writeback:1;
> +	unsigned int page_private:1;
> +	unsigned int __pad:3;
> +};

I'm not sure how to feel about this.  It's a bit fragile versus somebody adding
new page flags.  I don't know whether it's needed or whether you can just
copy page->flags directly because you're holding PageLock.

> +static void exchange_page(char *to, char *from)
> +{
> +	u64 tmp;
> +	int i;
> +
> +	for (i = 0; i < PAGE_SIZE; i += sizeof(tmp)) {
> +		tmp = *((u64 *)(from + i));
> +		*((u64 *)(from + i)) = *((u64 *)(to + i));
> +		*((u64 *)(to + i)) = tmp;
> +	}
> +}

I have a suspicion you'd be better off allocating a temporary page and
using copy_page().  Some architectures have put a lot of effort into
making copy_page() run faster.

> +		xa_lock_irq(&to_mapping->i_pages);
> +
> +		to_pslot = radix_tree_lookup_slot(&to_mapping->i_pages,
> +			page_index(to_page));

This needs to be converted to the XArray.  radix_tree_lookup_slot() is
going away soon.  You probably need:

	XA_STATE(to_xas, &to_mapping->i_pages, page_index(to_page));

This is a lot of code and I'm still trying to get my head aroud it all.
Thanks for putting in this work; it's good to see this approach being
explored.