Re: [PATCH 1/7] brd: use XArray instead of radix-tree to index backing pages

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 14, 2023 at 01:46:31PM +0200, Hannes Reinecke wrote:
>  static void brd_free_pages(struct brd_device *brd)
>  {
> -	unsigned long pos = 0;
> -	struct page *pages[FREE_BATCH];
> -	int nr_pages;
> -
> -	do {
> -		int i;
> -
> -		nr_pages = radix_tree_gang_lookup(&brd->brd_pages,
> -				(void **)pages, pos, FREE_BATCH);
> -
> -		for (i = 0; i < nr_pages; i++) {
> -			void *ret;
> -
> -			BUG_ON(pages[i]->index < pos);
> -			pos = pages[i]->index;
> -			ret = radix_tree_delete(&brd->brd_pages, pos);
> -			BUG_ON(!ret || ret != pages[i]);
> -			__free_page(pages[i]);
> -		}
> -
> -		pos++;
> +	struct page *page;
> +	pgoff_t idx;
>  
> -		/*
> -		 * It takes 3.4 seconds to remove 80GiB ramdisk.
> -		 * So, we need cond_resched to avoid stalling the CPU.
> -		 */
> -		cond_resched();
> +	xa_for_each(&brd->brd_pages, idx, page) {
> +		__free_page(page);
> +		cond_resched_rcu();

This should be a regular cond_resched().  The body of the loop is run
without the RCU read lock held.  Surprised none of the bots have noticed
an unlock-underflow.  Perhaps they don't test brd ;-)

With that fixed,

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>