Re: [PATCH v6 05/19] mm: Remove 'page_offset' from readahead loop

From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>, <linux-fsdevel@vger.kernel.org>
Cc: <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
	<linux-btrfs@vger.kernel.org>, <linux-erofs@lists.ozlabs.org>,
	<linux-ext4@vger.kernel.org>,
	<linux-f2fs-devel@lists.sourceforge.net>,
	<cluster-devel@redhat.com>, <ocfs2-devel@oss.oracle.com>,
	<linux-xfs@vger.kernel.org>
Subject: Re: [PATCH v6 05/19] mm: Remove 'page_offset' from readahead loop
Date: Tue, 18 Feb 2020 15:08:49 -0800	[thread overview]
Message-ID: <af3a8179-ea65-7a47-3b96-70aeceac0352@nvidia.com> (raw)
In-Reply-To: <20200217184613.19668-8-willy@infradead.org>

On 2/17/20 10:45 AM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Eliminate the page_offset variable which was confusing with the
> 'offset' parameter and record the start of each consecutive run of
> pages in the readahead_control.

...presumably for the benefit of a subsequent patch, since it's not
consumed in this patch.

Thanks for breaking these up, btw, it really helps.

> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/readahead.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 3eca59c43a45..74791b96013f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -162,6 +162,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	struct readahead_control rac = {
>  		.mapping = mapping,
>  		.file = filp,
> +		._start = offset,
>  		._nr_pages = 0,
>  	};
>  
> @@ -175,12 +176,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	 */
>  	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
>  		struct page *page;
> -		pgoff_t page_offset = offset + page_idx;

OK, this is still something I want to mention (I wrote the same thing when reviewing 
the wrong version of this patch, a moment ago).

You know...this ends up incrementing offset each time through the
loop, so yes, the behavior is the same as when using "offset + page_idx".
However, now it's a little harder to see that.

IMHO the page_offset variable is not actually a bad thing, here. I'd rather
keep it, all other things being equal (and I don't see any other benefits
here: line count is about the same, for example).

What do you think? (I don't feel strongly about this fine point.)

thanks,
-- 
John Hubbard
NVIDIA

>  
> -		if (page_offset > end_index)
> +		if (offset > end_index)
>  			break;
>  
> -		page = xa_load(&mapping->i_pages, page_offset);
> +		page = xa_load(&mapping->i_pages, offset);
>  		if (page && !xa_is_value(page)) {
>  			/*
>  			 * Page already present?  Kick off the current batch
> @@ -196,16 +196,18 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  		page = __page_cache_alloc(gfp_mask);
>  		if (!page)
>  			break;
> -		page->index = page_offset;
> +		page->index = offset;
>  		list_add(&page->lru, &page_pool);
>  		if (page_idx == nr_to_read - lookahead_size)
>  			SetPageReadahead(page);
>  		rac._nr_pages++;
> +		offset++;
>  		continue;
>  read:
>  		if (readahead_count(&rac))
>  			read_pages(&rac, &page_pool, gfp_mask);
>  		rac._nr_pages = 0;
> +		rac._start = ++offset;
>  	}
>  
>  	/*
> 