linux-kernel - Re: BUG_ON(!mapping_empty(&inode->i

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Message-ID: <20210402132708.GM351017@casper.infradead.org>
Date:   Fri, 2 Apr 2021 14:27:08 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: BUG_ON(!mapping_empty(&inode->i_data))

On Fri, Apr 02, 2021 at 04:13:05AM +0100, Matthew Wilcox wrote:
> +	for (;;) {
> +		xas_load(xas);
> +		if (!xas_is_node(xas))
> +			break;
> +		xas_delete_node(xas);
> +		xas->xa_index -= XA_CHUNK_SIZE;
> +		if (xas->xa_index < index)
> +			break;

That's a bug.  index can be 0, so the condition would never be true.
It should be:

		if (xas->xa_index <= (index | XA_CHUNK_MASK))
			break;
		xas->xa_index -= XA_CHUNK_SIZE;

The test doesn't notice this bug because the tree is otherwise empty,
and the !xas_is_node(xas) condition is hit first.  The next test will
notice this.