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Date:   Mon, 26 Jul 2021 18:19:37 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc:     linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org,
        linux-fscrypt@vger.kernel.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH] fs: reduce pointers while using file_ra_state_init()
Message-ID: <YP7uqRrXsbCqTpfx@casper.infradead.org>
References: <20210726164647.brx3l2ykwv3zz7vr@fiona>
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In-Reply-To: <20210726164647.brx3l2ykwv3zz7vr@fiona>
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On Mon, Jul 26, 2021 at 11:46:47AM -0500, Goldwyn Rodrigues wrote:
> Simplification.
> 
> file_ra_state_init() take struct address_space *, just to use inode
> pointer by dereferencing from mapping->host.
> 
> The callers also derive mapping either by file->f_mapping, or
> even file->f_mapping->host->i_mapping.
> 
> Change file_ra_state_init() to accept struct inode * to reduce pointer
> dereferencing, both in the callee and the caller.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

(some adjacent comments)

> diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
> index 4806295116d8..c43bf9915cda 100644
> --- a/fs/btrfs/free-space-cache.c
> +++ b/fs/btrfs/free-space-cache.c
> @@ -351,7 +351,7 @@ static void readahead_cache(struct inode *inode)
>  	if (!ra)
>  		return;
>  
> -	file_ra_state_init(ra, inode->i_mapping);
> +	file_ra_state_init(ra, inode);
>  	last_index = (i_size_read(inode) - 1) >> PAGE_SHIFT;
>  
>  	page_cache_sync_readahead(inode->i_mapping, ra, NULL, 0, last_index);

Why does btrfs allocate a file_ra_state using kmalloc instead of
on the stack?

> +++ b/include/linux/fs.h
> @@ -3260,7 +3260,7 @@ extern long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
>  
>  
>  extern void
> -file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping);
> +file_ra_state_init(struct file_ra_state *ra, struct inode *inode);

This should move to pagemap.h (and lose the extern).
I'd put it near the definition of VM_READAHEAD_PAGES.

> diff --git a/mm/readahead.c b/mm/readahead.c
> index d589f147f4c2..3541941df5e7 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -31,9 +31,9 @@
>   * memset *ra to zero.
>   */
>  void
> -file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
> +file_ra_state_init(struct file_ra_state *ra, struct inode *inode)
>  {
> -	ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
> +	ra->ra_pages = inode_to_bdi(inode)->ra_pages;
>  	ra->prev_pos = -1;
>  }
>  EXPORT_SYMBOL_GPL(file_ra_state_init);

I'm not entirely sure why this function is out-of-line, tbh.
Would it make more sense for it to be static inline in a header?