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

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Date:   Tue, 18 Feb 2020 05:57:36 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        ocfs2-devel@....oracle.com, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v6 04/19] mm: Rearrange readahead loop

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@...radead.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".