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Date: Tue, 18 Feb 2020 07:42:22 -0800
From: Matthew Wilcox <willy@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH v6 07/19] mm: Put readahead pages in cache earlier
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On Tue, Feb 18, 2020 at 05:14:59PM +1100, Dave Chinner wrote:
> On Mon, Feb 17, 2020 at 10:45:52AM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > 
> > At allocation time, put the pages in the cache unless we're using
> > ->readpages.  Add the readahead_for_each() iterator for the benefit of
> > the ->readpage fallback.  This iterator supports huge pages, even though
> > none of the filesystems to be converted do yet.
> 
> This could be better written - took me some time to get my head
> around it and the code.
> 
> "When populating the page cache for readahead, mappings that don't
> use ->readpages need to have their pages added to the page cache
> before ->readpage is called. Do this insertion earlier so that the
> pages can be looked up immediately prior to ->readpage calls rather
> than passing them on a linked list. This early insert functionality
> is also required by the upcoming ->readahead method that will
> replace ->readpages.
> 
> Optimise and simplify the readpage loop by adding a
> readahead_for_each() iterator to provide the pages we need to read.
> This iterator also supports huge pages, even though none of the
> filesystems have been converted to use them yet."

Thanks, I'll use that.

> > +static inline struct page *readahead_page(struct readahead_control *rac)
> > +{
> > +	struct page *page;
> > +
> > +	if (!rac->_nr_pages)
> > +		return NULL;
> 
> Hmmmm.
> 
> > +
> > +	page = xa_load(&rac->mapping->i_pages, rac->_start);
> > +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > +	rac->_batch_count = hpage_nr_pages(page);
> 
> So we could have rac->_nr_pages = 2, and then we get an order 2
> large page returned, and so rac->_batch_count = 4.

Well, no, we couldn't.  rac->_nr_pages is incremented by 4 when we add
an order-2 page to the readahead.  I can put a
	BUG_ON(rac->_batch_count > rac->_nr_pages)
in here to be sure to catch any logic error like that.

> > @@ -159,6 +152,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> >  	unsigned long i;
> >  	loff_t isize = i_size_read(inode);
> >  	gfp_t gfp_mask = readahead_gfp_mask(mapping);
> > +	bool use_list = mapping->a_ops->readpages;
> >  	struct readahead_control rac = {
> >  		.mapping = mapping,
> >  		.file = filp,
> 
> [ I do find these unstructured mixes of declarations and
> initialisations dense and difficult to read.... ]

Fair ... although I didn't create this mess, I can tidy it up a bit.

> > -		page->index = offset;
> > -		list_add(&page->lru, &page_pool);
> > +		if (use_list) {
> > +			page->index = offset;
> > +			list_add(&page->lru, &page_pool);
> > +		} else if (add_to_page_cache_lru(page, mapping, offset,
> > +					gfp_mask) < 0) {
> > +			put_page(page);
> > +			goto read;
> > +		}
> 
> Ok, so that's why you put read code at the end of the loop. To turn
> the code into spaghetti :/
> 
> How much does this simplify down when we get rid of ->readpages and
> can restructure the loop? This really seems like you're trying to
> flatten two nested loops into one by the use of goto....

I see it as having two failure cases in this loop.  One for "page is
already present" (which already existed) and one for "allocated a page,
but failed to add it to the page cache" (which used to be done later).
I didn't want to duplicate the "call read_pages()" code.  So I reshuffled
the code rather than add a nested loop.  I don't think the nested loop
is easier to read (we'll be at 5 levels of indentation for some statements).
Could do it this way ...

@@ -218,18 +218,17 @@ void page_cache_readahead_limit(struct address_space *mapping,
                } else if (add_to_page_cache_lru(page, mapping, offset,
                                        gfp_mask) < 0) {
                        put_page(page);
-                       goto read;
+read:
+                       if (readahead_count(&rac))
+                               read_pages(&rac, &page_pool);
+                       rac._nr_pages = 0;
+                       rac._start = ++offset;
+                       continue;
                }
                if (i == nr_to_read - lookahead_size)
                        SetPageReadahead(page);
                rac._nr_pages++;
                offset++;
-               continue;
-read:
-               if (readahead_count(&rac))
-                       read_pages(&rac, &page_pool);
-               rac._nr_pages = 0;
-               rac._start = ++offset;
        }
 
        /*

but I'm not sure that's any better.