Re: [PATCH 12/12] iomap: Convert from readpages to readahead

[Dave Chinner <david@fromorbit.com>]

On Fri, Jan 24, 2020 at 05:35:53PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Use the new readahead operation in XFS and iomap.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: linux-xfs@vger.kernel.org

....
> +unsigned
> +iomap_readahead(struct address_space *mapping, pgoff_t start,
>  		unsigned nr_pages, const struct iomap_ops *ops)
>  {
>  	struct iomap_readpage_ctx ctx = {
> -		.pages		= pages,
>  		.is_readahead	= true,
>  	};
> -	loff_t pos = page_offset(list_entry(pages->prev, struct page, lru));
> -	loff_t last = page_offset(list_entry(pages->next, struct page, lru));
> -	loff_t length = last - pos + PAGE_SIZE, ret = 0;
> +	loff_t pos = start * PAGE_SIZE;
> +	loff_t length = nr_pages * PAGE_SIZE;
>  
> -	trace_iomap_readpages(mapping->host, nr_pages);
> +	trace_iomap_readahead(mapping->host, nr_pages);
>  
>  	while (length > 0) {
> -		ret = iomap_apply(mapping->host, pos, length, 0, ops,
> -				&ctx, iomap_readpages_actor);
> +		loff_t ret = iomap_apply(mapping->host, pos, length, 0, ops,
> +				&ctx, iomap_readahead_actor);
>  		if (ret <= 0) {
>  			WARN_ON_ONCE(ret == 0);
> -			goto done;
> +			break;
>  		}
>  		pos += ret;
>  		length -= ret;
>  	}
> -	ret = 0;
> -done:
> +
>  	if (ctx.bio)
>  		submit_bio(ctx.bio);
> -	if (ctx.cur_page) {
> -		if (!ctx.cur_page_in_bio)
> -			unlock_page(ctx.cur_page);
> +	if (ctx.cur_page && ctx.cur_page_in_bio)
>  		put_page(ctx.cur_page);
> -	}
>  
> -	/*
> -	 * Check that we didn't lose a page due to the arcance calling
> -	 * conventions..
> -	 */
> -	WARN_ON_ONCE(!ret && !list_empty(ctx.pages));
> -	return ret;
> +	return length / PAGE_SIZE;

Took me quite some time to get my head around whether this was
correct or not.

I'm still not certain in the cases where block size != page size and
we've got an extent boundary in the middle of the page and had a
read error on the second extent in the page. In this case,
ctx.cur_page_in_bio is true so we drop the readahead reference to
the page. Also, length is not a multiple of page size, and so the
nr_pages value returned includes the partial page that we have IO
underway on.

That, I think, leads to both a double unlock and a double put_page()
of the partial page in question.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com