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Subject: Re: [PATCH v6 04/16] mm: Tweak readahead loop slightly
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>
References: <20200217184613.19668-1-willy@infradead.org>
 <20200217184613.19668-6-willy@infradead.org>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <7691abe7-d0e9-e091-b158-764fb624c2d7@nvidia.com>
Date:   Tue, 18 Feb 2020 14:57:43 -0800
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On 2/17/20 10:45 AM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Eliminate the page_offset variable which was just confusing;
> record the start of each consecutive run of pages in the


OK...presumably for the benefit of a following patch, since it is not 
actually consumed in this patch.

> readahead_control, and move the 'kick off a fresh batch' code to
> the end of the function for easier use in the next patch.


That last bit was actually done in the previous patch, rather than this
one, right?

> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/readahead.c | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 15329309231f..74791b96013f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -154,7 +154,6 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  		unsigned long lookahead_size)
>  {
>  	struct inode *inode = mapping->host;
> -	struct page *page;
>  	unsigned long end_index;	/* The last page we want to read */
>  	LIST_HEAD(page_pool);
>  	int page_idx;
> @@ -163,6 +162,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	struct readahead_control rac = {
>  		.mapping = mapping,
>  		.file = filp,
> +		._start = offset,
>  		._nr_pages = 0,
>  	};
>  
> @@ -175,32 +175,39 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	 * Preallocate as many pages as we will need.
>  	 */
>  	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
> -		pgoff_t page_offset = offset + page_idx;


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 the same, for example).

What do you think?


thanks,
-- 
John Hubbard
NVIDIA

> +		struct page *page;
>  
> -		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 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);
>  		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;
>  	}
>  
>  	/*
>