linux-kernel - Re: [PATCH] page_alloc: Fix freeing non-compound pages

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Message-ID: <20200922143559.GF32101@casper.infradead.org>
Date:   Tue, 22 Sep 2020 15:35:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nick Piggin <npiggin@...e.de>, Hugh Dickins <hughd@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] page_alloc: Fix freeing non-compound pages

On Tue, Sep 22, 2020 at 03:00:17PM +0100, Matthew Wilcox (Oracle) wrote:
>  void __free_pages(struct page *page, unsigned int order)
>  {
>  	if (put_page_testzero(page))
>  		free_the_page(page, order);
> +	else
> +		while (order-- > 0)
> +			free_the_page(page + (1 << order), order);
>  }
>  EXPORT_SYMBOL(__free_pages);

... a three line patch and one of them is wrong.

-       else
+       else if (!PageHead(page))

Anyone got a smart idea about how to _test_ this code path?  I'm
wondering about loading one kernel module which wanders through memmap
calling
	if (page_cache_get_speculative(page)) put_page(page);
and another kernel module that calls
	__free_pages(alloc_page(GFP_KERNEL, 1), 1);

and putting in a printk to let me know when we hit it.