Re: [PATCH 27/33] readahead: laptop mode

[Andrew Morton]

Wu Fengguang <wfg@xxxxxxxxxxxxxxxx> wrote:
>
>   /*
>  + * Set a new look-ahead mark at @new_index.
>  + * Return 0 if the new mark is successfully set.
>  + */
>  +static inline int renew_lookahead(struct address_space *mapping,
>  +				struct file_ra_state *ra,
>  +				pgoff_t index, pgoff_t new_index)
>  +{
>  +	struct page *page;
>  +
>  +	if (index == ra->lookahead_index &&
>  +			new_index >= ra->readahead_index)
>  +		return 1;
>  +
>  +	page = find_page(mapping, new_index);
>  +	if (!page)
>  +		return 1;
>  +
>  +	__SetPageReadahead(page);
>  +	if (ra->lookahead_index == index)
>  +		ra->lookahead_index = new_index;
>  +
>  +	return 0;
>  +}
>  +

This is a pagecache page and other CPUs can look it up and play with it. 
The __SetPageReadahead() is quite wrong here.

And we don't have a reference on this page, so this code appears to be racy.

You could fix that by taking and dropping a ref on the page, but it'd be
quicker to take tree_lock and do the SetPageReadahead() while holding it.

This function is too large to inline.
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