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

[Jens Axboe]

On 11/16/2016 08:12 AM, Jens Axboe wrote:
> 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.

Johannes pointed out that I'm an idiot - a last minute edit introduced
this typo, and I was too blind to spot it when you sent that email this
morning. So yes, it should of course be:

	max_pages = min(io_pages, max_pages);

like the first version I posted. I'll post a v3...

--
Jens Axboe