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

[Jens Axboe]

On 11/16/2016 12:17 AM, Hillf Danton wrote:
> On Wednesday, November 16, 2016 12:31 PM Jens Axboe wrote:
> > @@ -369,10 +369,25 @@ ondemand_readahead(struct address_space *mapping,
> >   		   bool hit_readahead_marker, pgoff_t offset,
> >   		   unsigned long req_size)
> >   {
> > -	unsigned long max = ra->ra_pages;
> > +	unsigned long io_pages, max_pages;
> >   	pgoff_t prev_offset;
> >
> >   	/*
> > +	 * If bdi->io_pages is set, that indicates the (soft) max IO size
> > +	 * per command for that device. If we have that available, use
> > +	 * that as the max suitable read-ahead size for this IO. Instead of
> > +	 * capping read-ahead at ra_pages if req_size is larger, we can go
> > +	 * up to io_pages. If io_pages isn't set, fall back to using
> > +	 * ra_pages as a safe max.
> > +	 */
> > +	io_pages = inode_to_bdi(mapping->host)->io_pages;
> > +	if (io_pages) {
> > +		max_pages = max_t(unsigned long, ra->ra_pages, req_size);
> > +		io_pages = min(io_pages, max_pages);
>
> Doubt if you mean
> 		max_pages = min(io_pages, max_pages);

No, that is what I mean. We want the maximum of the RA setting and the
user IO size, but the minimum of that and the device max command size.

--
Jens Axboe