linux-kernel - Re: [PATCH 06/12] btrfs: Convert from readpages to readahead

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Message-ID: <20200129004609.GI18610@dread.disaster.area>
Date:   Wed, 29 Jan 2020 11:46:09 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 06/12] btrfs: Convert from readpages to readahead

On Fri, Jan 24, 2020 at 05:35:47PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> 
> Use the new readahead operation in btrfs
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: linux-btrfs@...r.kernel.org
> ---
>  fs/btrfs/extent_io.c | 15 ++++-----------
>  fs/btrfs/extent_io.h |  2 +-
>  fs/btrfs/inode.c     | 18 +++++++++---------
>  3 files changed, 14 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 2f4802f405a2..b1e2acbec165 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4283,7 +4283,7 @@ int extent_writepages(struct address_space *mapping,
>  	return ret;
>  }
>  
> -int extent_readpages(struct address_space *mapping, struct list_head *pages,
> +unsigned extent_readahead(struct address_space *mapping, pgoff_t start,
>  		     unsigned nr_pages)
>  {
>  	struct bio *bio = NULL;
> @@ -4294,20 +4294,13 @@ int extent_readpages(struct address_space *mapping, struct list_head *pages,
>  	int nr = 0;
>  	u64 prev_em_start = (u64)-1;
>  
> -	while (!list_empty(pages)) {
> +	while (nr_pages) {
>  		u64 contig_end = 0;
>  
> -		for (nr = 0; nr < ARRAY_SIZE(pagepool) && !list_empty(pages);) {
> -			struct page *page = lru_to_page(pages);
> +		for (nr = 0; nr < ARRAY_SIZE(pagepool) && nr_pages--;) {

What is stopping nr_pages from going negative here, and then looping
forever on the outer nr_pages loop? Perhaps "while(nr_pages > 0) {"
would be better there?

-Dave.
-- 
Dave Chinner
david@...morbit.com