Re: [PATCH v6 04/19] mm: Rearrange readahead loop

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 18, 2020 at 05:57:36AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 04:08:24PM +1100, Dave Chinner wrote:
> > On Mon, Feb 17, 2020 at 10:45:45AM -0800, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > > 
> > > Move the declaration of 'page' to inside the loop and move the 'kick
> > > off a fresh batch' code to the end of the function for easier use in
> > > subsequent patches.
> > 
> > Stale? the "kick off" code is moved to the tail of the loop, not the
> > end of the function.
> 
> Braino; I meant to write end of the loop.
> 
> > > @@ -183,14 +183,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
> > >  		page = xa_load(&mapping->i_pages, page_offset);
> > >  		if (page && !xa_is_value(page)) {
> > >  			/*
> > > -			 * Page already present?  Kick off the current batch of
> > > -			 * contiguous pages before continuing with the next
> > > -			 * batch.
> > > +			 * Page already present?  Kick off the current batch
> > > +			 * of contiguous pages before continuing with the
> > > +			 * next batch.  This page may be the one we would
> > > +			 * have intended to mark as Readahead, but we don't
> > > +			 * have a stable reference to this page, and it's
> > > +			 * not worth getting one just for that.
> > >  			 */
> > > -			if (readahead_count(&rac))
> > > -				read_pages(&rac, &page_pool, gfp_mask);
> > > -			rac._nr_pages = 0;
> > > -			continue;
> > > +			goto read;
> > >  		}
> > >  
> > >  		page = __page_cache_alloc(gfp_mask);
> > > @@ -201,6 +201,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
> > >  		if (page_idx == nr_to_read - lookahead_size)
> > >  			SetPageReadahead(page);
> > >  		rac._nr_pages++;
> > > +		continue;
> > > +read:
> > > +		if (readahead_count(&rac))
> > > +			read_pages(&rac, &page_pool, gfp_mask);
> > > +		rac._nr_pages = 0;
> > >  	}
> > 
> > Also, why? This adds a goto from branched code that continues, then
> > adds a continue so the unbranched code doesn't execute the code the
> > goto jumps to. In absence of any explanation, this isn't an
> > improvement and doesn't make any sense...
> 
> I thought I was explaining it ... "for easier use in subsequent patches".

Sorry, my braino there. :) I commented on the problem with the first
part of the sentence, then the rest of the sentence completely
failed to sink in.

-Dave.
-- 
Dave Chinner
david@fromorbit.com