Re: [PATCH v4] mm: don't cap request size based on read-ahead setting

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Nov 17, 2016 at 02:23:10PM -0700, Jens Axboe wrote:
> We ran into a funky issue, where someone doing 256K buffered reads saw
> 128K requests at the device level. Turns out it is read-ahead capping
> the request size, since we use 128K as the default setting. This doesn't
> make a lot of sense - if someone is issuing 256K reads, they should see
> 256K reads, regardless of the read-ahead setting, if the underlying
> device can support a 256K read in a single command.
> 
> To make matters more confusing, there's an odd interaction with the
> fadvise hint setting. If we tell the kernel we're doing sequential IO on
> this file descriptor, we can get twice the read-ahead size. But if we
> tell the kernel that we are doing random IO, hence disabling read-ahead,
> we do get nice 256K requests at the lower level. This is because
> ondemand and forced read-ahead behave differently, with the latter doing
> the right thing. An application developer will be, rightfully,
> scratching his head at this point, wondering wtf is going on. A good one
> will dive into the kernel source, and silently weep.

With the FADV_RANDOM part of the changelog updated, this looks good to
me. Just a few nitpicks below.

> This patch introduces a bdi hint, io_pages. This is the soft max IO size
> for the lower level, I've hooked it up to the bdev settings here.
> Read-ahead is modified to issue the maximum of the user request size,
> and the read-ahead max size, but capped to the max request size on the
> device side. The latter is done to avoid reading ahead too much, if the
> application asks for a huge read. With this patch, the kernel behaves
> like the application expects.
> 
> Signed-off-by: Jens Axboe <axboe@fb.com>

> @@ -207,12 +207,17 @@ int __do_page_cache_readahead(struct address_space
> *mapping, struct file *filp,
>   * memory at once.
>   */
>  int force_page_cache_readahead(struct address_space *mapping, struct file
> *filp,

Linewrap (but you already knew that ;))

> -		pgoff_t offset, unsigned long nr_to_read)
> +		               pgoff_t offset, unsigned long nr_to_read)
>  {
> +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> +	struct file_ra_state *ra = &filp->f_ra;
> +	unsigned long max_pages;
> +
>  	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
>  		return -EINVAL;
>
> -	nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
> +	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
> +	nr_to_read = min(nr_to_read, max_pages);

It would be useful to have the comment on not capping below optimal IO
size from ondemand_readahead() here as well.

> @@ -369,10 +374,18 @@ ondemand_readahead(struct address_space *mapping,
>  		   bool hit_readahead_marker, pgoff_t offset,
>  		   unsigned long req_size)
>  {
> -	unsigned long max = ra->ra_pages;
> +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> +	unsigned long max_pages = ra->ra_pages;
>  	pgoff_t prev_offset;
> 
>  	/*
> +	 * If the request exceeds the readahead window, allow the read to
> +	 * be up to the optimal hardware IO size
> +	 */
> +	if (req_size > max_pages && bdi->io_pages > max_pages)
> +		max_pages = min(req_size, bdi->io_pages);
> +
> +	/*
>  	 * start of file
>  	 */
>  	if (!offset)

Please feel free to add:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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