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

Matthew Wilcox
2020-02-18T13:57:42Z

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".