linux-kernel - Re: [PATCH] direct-io: Minor cleanups in do_blockdev_direct

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Date:   Fri, 2 Mar 2018 16:24:00 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH] direct-io: Minor cleanups in do_blockdev_direct_IO

On Mon, 26 Feb 2018 11:54:30 +0200 Nikolay Borisov <nborisov@...e.com> wrote:

> We already get the block counts and the calculate the end block at the
> beginning of the function. Let's use the local variables for consistency and
> readability. No functional changes
> 

Fair enough.

> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -1200,7 +1200,7 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
>  	}
>  
>  	/* watch out for a 0 len io from a tricksy fs */
> -	if (iov_iter_rw(iter) == READ && !iov_iter_count(iter))
> +	if (iov_iter_rw(iter) == READ && !count)
>  		return 0;
>  
>  	dio = kmem_cache_alloc(dio_cache, GFP_KERNEL);
> @@ -1316,8 +1316,7 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
>  
>  	dio->should_dirty = (iter->type == ITER_IOVEC);
>  	sdio.iter = iter;
> -	sdio.final_block_in_request =
> -		(offset + iov_iter_count(iter)) >> blkbits;
> +	sdio.final_block_in_request = end >> blkbits;
>  
>  	/*
>  	 * In case of non-aligned buffers, we may need 2 more

It's always scary to see a local being used in this fashion so far away
from where it was initialized.  However, we can get our warm fuzzy
feelings by doing this:

--- a/fs/direct-io.c~direct-io-minor-cleanups-in-do_blockdev_direct_io-fix
+++ a/fs/direct-io.c
@@ -1178,9 +1178,9 @@ do_blockdev_direct_IO(struct kiocb *iocb
 	unsigned blkbits = i_blkbits;
 	unsigned blocksize_mask = (1 << blkbits) - 1;
 	ssize_t retval = -EINVAL;
-	size_t count = iov_iter_count(iter);
+	const size_t count = iov_iter_count(iter);
 	loff_t offset = iocb->ki_pos;
-	loff_t end = offset + count;
+	const loff_t end = offset + count;
 	struct dio *dio;
 	struct dio_submit sdio = { 0, };
 	struct buffer_head map_bh = { 0, };
_