linux-ext4 - Re: [RFC PATCH V2 05/11] ext4: Decrypt all boundary blocks when doing buffered write

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Date:   Tue, 20 Feb 2018 17:01:55 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     Chandan Rajendra <chandan@...ux.vnet.ibm.com>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, tytso@....edu
Subject: Re: [RFC PATCH V2 05/11] ext4: Decrypt all boundary blocks when
 doing buffered write

Hi Chandan,

On Mon, Feb 12, 2018 at 03:13:41PM +0530, Chandan Rajendra wrote:
> With block size < page size, ext4_block_write_begin() can have up to two
> blocks to decrypt. Hence this commit invokes fscrypt_decrypt_block() for
> each of those blocks.
> 
> Signed-off-by: Chandan Rajendra <chandan@...ux.vnet.ibm.com>
> ---
>  fs/ext4/inode.c | 33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 69a4fd6..180dd2d 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1158,12 +1158,13 @@ static int ext4_block_write_begin(struct page *page, loff_t pos, unsigned len,
>  	unsigned to = from + len;
>  	struct inode *inode = page->mapping->host;
>  	unsigned block_start, block_end;
> -	sector_t block;
> +	sector_t block, page_blk_nr;
>  	int err = 0;
>  	unsigned blocksize = inode->i_sb->s_blocksize;
>  	unsigned bbits;
>  	struct buffer_head *bh, *head, *wait[2], **wait_bh = wait;
>  	bool decrypt = false;
> +	int i;
>  
>  	BUG_ON(!PageLocked(page));
>  	BUG_ON(from > PAGE_SIZE);
> @@ -1224,18 +1225,30 @@ static int ext4_block_write_begin(struct page *page, loff_t pos, unsigned len,
>  	/*
>  	 * If we issued read requests, let them complete.
>  	 */
> -	while (wait_bh > wait) {
> -		wait_on_buffer(*--wait_bh);
> -		if (!buffer_uptodate(*wait_bh))
> +	for (i = 0; &wait[i] < wait_bh; i++) {
> +		wait_on_buffer(wait[i]);
> +		if (!buffer_uptodate(wait[i]))
>  			err = -EIO;
> 	}

[...]

> +		for (i = 0; &wait[i] < wait_bh; i++) {
> +			int err2;
> +
> +			--wait_bh;
> +			block = page_blk_nr + (bh_offset(wait[i]) >> bbits);
> +			err2 = fscrypt_decrypt_block(page->mapping->host, page,
> +						wait[i]->b_size,
> +						bh_offset(wait[i]),
> +						block);
> +			if (err2) {
> +				clear_buffer_uptodate(wait[i]);
> +				err = err2;
> +			}
> +		}

These are very confusing ways to iterate through an array, especially the second
loop which is actually going in reverse order (why?).  Why not just use a
variable like 'nr_wait' for the number of valid buffer_head's like I had
suggested?  Then you can just do 'for (i = 0; i < nr_wait; i++)'.

- Eric