Re: [PATCH RFC] mm: Let readahead submit larger batches of pages in case of ra->ra_pages == 0

[Bean Huo]

On Fri, 2020-09-04 at 11:09 -0700, Andrew Morton wrote:
> On Fri,  4 Sep 2020 16:48:07 +0200 Bean Huo <huobean@xxxxxxxxx>
> wrote:
> 
> > From: Bean Huo <beanhuo@xxxxxxxxxx>
> > 
> > Current generic_file_buffered_read() will break up the larger
> > batches of pages
> > and read data in single page length in case of ra->ra_pages == 0.
> > This patch is
> > to allow it to pass the batches of pages down to the device if the
> > supported
> > maximum IO size >= the requested size.
> > 
> > ...
> > 
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -2062,6 +2062,7 @@ ssize_t generic_file_buffered_read(struct
> > kiocb *iocb,
> >  	struct file *filp = iocb->ki_filp;
> >  	struct address_space *mapping = filp->f_mapping;
> >  	struct inode *inode = mapping->host;
> > +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> >  	struct file_ra_state *ra = &filp->f_ra;
> >  	loff_t *ppos = &iocb->ki_pos;
> >  	pgoff_t index;
> > @@ -2098,9 +2099,14 @@ ssize_t generic_file_buffered_read(struct
> > kiocb *iocb,
> >  		if (!page) {
> >  			if (iocb->ki_flags & IOCB_NOIO)
> >  				goto would_block;
> > -			page_cache_sync_readahead(mapping,
> > -					ra, filp,
> > -					index, last_index - index);
> > +
> > +			if (!ra->ra_pages && bdi->io_pages >=
> > last_index - index)
> > +				__do_page_cache_readahead(mapping,
> > filp, index,
> > +							  last_index -
> > index, 0);
> > +			else
> > +				page_cache_sync_readahead(mapping, ra,
> > filp,
> > +							  index,
> > +							  last_index -
> > index);
> >  			page = find_get_page(mapping, index);
> >  			if (unlikely(page == NULL))
> >  				goto no_cached_page;
> 
> I assume this is a performance patch.  What are the observed changes
> in behaviour?
> 
> What is special about ->ra_pages==0?  Wouldn't this optimization
> still
> be valid if ->ra_pages==2?
> 
> Doesn't this defeat the purpose of having ->ra_pages==0?


Hi Andrew
Sorry, I am still not quite understanding your above three questions. 

Based on my shallow understanding, ra_pages is associated with
read_ahead_kb. Seems ra_pages controls the maximum read-ahead window
size, but it doesn't work when the requested size exceeds ra_pages. 

If I set the read_ahead_kb to 0, also, as Christoph mentioned, MTD
forcibly sets ra_pages to 0.  I think the intention is that only wants
to disable read-ahead, however, doesn't want
generic_file_buffered_read() to split the request and read data with
4KB chunk size separately.

Thanks,
Bean