Re: [PATCH 1/2] btrfs: Factor out read portion of btrfs_get_blocks_direct

[Nikolay Borisov <nborisov@suse.com>]

On 10.04.2018 18:49, David Sterba wrote:
> On Thu, Feb 22, 2018 at 06:12:13PM +0200, Nikolay Borisov wrote:
>> Currently this function handles both the READ and WRITE dio cases. This
>> is facilitated by a bunch of 'if' statements, a goto short-circuit
>> statement and a very perverse aliasing of "!created"(READ) case
>> by setting lockstart = lockend and checking for lockstart < lockend for
>> detecting the write. Let's simplify this mess by extracting the
>> READ-only code into a separate __btrfs_get_block_direct_read function.
>> This is only the first step, the next one will be to factor out the
>> write side as well. The end goal will be to have the common locking/
>> unlocking code in btrfs_get_blocks_direct and then it will call either
>> the read|write subvariants. No functional changes.
> 
> Reviewed-by: David Sterba <dsterba@suse.com>
> 
> uh what a convoluted code to untangle. A few style notes below.
>>
>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
>> ---
>>  fs/btrfs/inode.c | 49 ++++++++++++++++++++++++++++++++++++-------------
>>  1 file changed, 36 insertions(+), 13 deletions(-)
>>
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index 491a7397f6fa..fd99347d0c91 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -7677,6 +7677,27 @@ static struct extent_map *create_io_em(struct inode *inode, u64 start, u64 len,
>>  	return em;
>>  }
>>  
>> +
>> +static int __btrfs_get_blocks_direct_read(struct extent_map *em,
> 
> Please drop the "__" the function is static and there's no
> underscore-less version.
> 
>> +					 struct buffer_head *bh_result,
>> +					 struct inode *inode,
>> +					 u64 start, u64 len)
>> +{
>> +	if (em->block_start == EXTENT_MAP_HOLE ||
>> +			test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
>> +		return -ENOENT;
>> +
>> +	len = min(len, em->len - (start - em->start));
>> +
>> +	bh_result->b_blocknr = (em->block_start + (start - em->start)) >>
>> +		inode->i_blkbits;
>> +	bh_result->b_size = len;
>> +	bh_result->b_bdev = em->bdev;
>> +	set_buffer_mapped(bh_result);
>> +
>> +	return 0;
>> +}
>> +
>>  static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
>>  				   struct buffer_head *bh_result, int create)
>>  {
>> @@ -7745,11 +7766,22 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
>>  		goto unlock_err;
>>  	}
>>  
>> -	/* Just a good old fashioned hole, return */
>> -	if (!create && (em->block_start == EXTENT_MAP_HOLE ||
>> -			test_bit(EXTENT_FLAG_PREALLOC, &em->flags))) {
>> +	if (!create) {
>> +		ret = __btrfs_get_blocks_direct_read(em, bh_result, inode,
>> +						   start, len);
>> +		/* Can be negative only if we read from a hole */
>> +		if (ret < 0) {
>> +			ret = 0;
>> +			free_extent_map(em);
>> +			goto unlock_err;
>> +		}
>> +		/*
>> +		 * We need to unlock only the end area that we aren't using
>> +		 * if there is any leftover space
>> +		 */
>> +		free_extent_state(cached_state);
>>  		free_extent_map(em);
>> -		goto unlock_err;
>> +		return 0;
> 
> Please add a separate label for that, the funcion uses the single exit
> block style (labels and one-or-two returns).

So I think this is unnecessary because in 2/2 I factor out common code
i.e. the free_extent_map(em); return 0; outside of the 'if' branch and
so this return disappears. The 3rd hunk in 2/2 begins with:

@@ -7780,106 +7882,8 @@ static int btrfs_get_blocks_direct(struct inode
*inode, sector_t iblock,
 		 * if there is any leftover space
 		 */
 		free_extent_state(cached_state);
-		free_extent_map(em);
-		return 0;
-	}




> 
>>  	}
>>  
>>  	/*
>> @@ -7761,12 +7793,6 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
>>  	 * just use the extent.
>>  	 *
>>  	 */
>> -	if (!create) {
>> -		len = min(len, em->len - (start - em->start));
>> -		lockstart = start + len;
>> -		goto unlock;
>> -	}
>> -
>>  	if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags) ||
>>  	    ((BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW) &&
>>  	     em->block_start != EXTENT_MAP_HOLE)) {
>> @@ -7853,10 +7879,7 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
>>  		clear_extent_bit(&BTRFS_I(inode)->io_tree, lockstart,
>>  				 lockend, unlock_bits, 1, 0,
>>  				 &cached_state);
>> -	} else {
>> -		free_extent_state(cached_state);
>>  	}
>> -
>>  	free_extent_map(em);
>>  
>>  	return 0;