linux-kernel - Re: [PATCH v4 09/31] dio: create a dio

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Date:	Fri, 23 Nov 2012 11:45:22 -0600
From:	Dave Kleikamp <dave.kleikamp@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Zach Brown <zab@...bo.net>,
	"Maxim V. Patlasov" <mpatlasov@...allels.com>
Subject: Re: [PATCH v4 09/31] dio: create a dio_aligned() helper function



On 11/23/2012 02:19 AM, Christoph Hellwig wrote:
> On Wed, Nov 21, 2012 at 04:40:49PM -0600, Dave Kleikamp wrote:
>> From: Zach Brown <zab@...bo.net>
>>
>> __blockdev_direct_IO() had two instances of the same code to determine
>> if a given offset wasn't aligned first to the inode's blkbits and then
>> to the underlying device's blkbits.   This was confusing enough but
>> we're about to add code that performs the same check on offsets in bvec
>> arrays.  Rather than add yet more copies of this code let's have
>> everyone call a helper.
>>
>> Signed-off-by: Dave Kleikamp <dave.kleikamp@...cle.com>
>> Cc: Zach Brown <zab@...bo.net>
>> ---
>>  fs/direct-io.c | 59 ++++++++++++++++++++++++++++++++++++----------------------
>>  1 file changed, 37 insertions(+), 22 deletions(-)
>>
>> diff --git a/fs/direct-io.c b/fs/direct-io.c
>> index f86c720..035c0a3 100644
>> --- a/fs/direct-io.c
>> +++ b/fs/direct-io.c
>> @@ -1020,6 +1020,39 @@ static inline int drop_refcount(struct dio *dio)
>>  }
>>  
>>  /*
>> + * Returns true if the given offset is aligned to either the IO size
>> + * specified by the given blkbits or by the logical block size of the
>> + * given block device.
>> + *
>> + * If the given offset isn't aligned to the blkbits arguments as this is
>> + * called then blkbits is set to the block size of the specified block
>> + * device.  The call can then return either true or false.
>> + *
>> + * This bizarre calling convention matches the code paths that
>> + * duplicated the functionality that this helper was built from.  We
>> + * reproduce the behaviour to avoid introducing subtle bugs.
>> + */
>> +static int dio_aligned(unsigned long offset, unsigned *blkbits,
>> +		       struct block_device *bdev)
>> +{
>> +	unsigned mask = (1 << *blkbits) - 1;
>> +
>> +	/*
>> +	 * Avoid references to bdev if not absolutely needed to give
>> +	 * the early prefetch in the caller enough time.
>> +	 */
>> +
>> +	if (offset & mask) {
>> +		if (bdev)
>> +			*blkbits = blksize_bits(bdev_logical_block_size(bdev));
> 
> I don't like having the blkbits assignment hidden in the helper,
> shouldn't we have a:
> 
> 	if (bdev)
> 		blkbits = blksize_bits(bdev_logical_block_size(bdev));
> 	else
> 		blkbits = inode->i_blkbits;
> 
> in the caller instead?

Yeah, that absolutely makes more sense.

Shaggy
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