linux-kernel - Re: [PATCHv2 09/11] dmapool: simplify freeing

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Date:   Fri, 23 Dec 2022 08:38:55 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Keith Busch <kbusch@...a.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Tony Battersby <tonyb@...ernetics.com>,
        Kernel Team <kernel-team@...a.com>,
        Keith Busch <kbusch@...nel.org>
Subject: Re: [PATCHv2 09/11] dmapool: simplify freeing

> @@ -280,14 +268,14 @@ void dma_pool_destroy(struct dma_pool *pool)
>  	mutex_unlock(&pools_reg_lock);
>  
>  	list_for_each_entry_safe(page, tmp, &pool->page_list, page_list) {
> +		if (!is_page_busy(page))
> +			dma_free_coherent(pool->dev, pool->allocation,
> +					  page->vaddr, page->dma);
> +		else
>  			dev_err(pool->dev, "%s %s, %p busy\n", __func__,
>  				pool->name, page->vaddr);
> +		list_del(&page->page_list);
> +		kfree(page);

Hmm.  The is_page_busy case is really a should not happen case.
What is the benefit of skipping the dma_free_coherent and leaking
memory here, vs letting KASAN and friends see the free and possibly
help with debugging?  In other words, why is this not:

		WARN_ON_ONCE(is_page_busy(page));
		dma_free_coherent(pool->dev, pool->allocation, page->vaddr,
				  page->dma);
		...

>  	page->in_use--;
>  	*(int *)vaddr = page->offset;
>  	page->offset = offset;
> -	/*
> -	 * Resist a temptation to do
> -	 *    if (!is_page_busy(page)) pool_free_page(pool, page);
> -	 * Better have a few empty pages hang around.
> -	 */

This doesn't look related to the rest, or am I missing something?