Re: [PATCH v3 07/11] md/raid5: split bio for chunk_aligned_read

[Ming Lin]

On 05/07/2015 09:14 PM, NeilBrown wrote:
> On Wed,  6 May 2015 23:34:17 -0700 Ming Lin <mlin@xxxxxxxxxx> wrote:
> 
>> If a read request fits entirely in a chunk, it will be passed directly to the
>> underlying device (providing it hasn't failed of course).  If it doesn't fit,
>> the slightly less efficient path that uses the stripe_cache is used.
>> Requests that get to the stripe cache are always completely split up as
>> necessary.
>>
>> So with RAID5, ripping out the merge_bvec_fn doesn't cause it to stop work,
>> but could cause it to take the less efficient path more often.
>>
>> All that is needed to manage this is for 'chunk_aligned_read' do some bio
>> splitting, much like the RAID0 code does.
>>
>> Cc: Neil Brown <neilb@xxxxxxx>
>> Cc: linux-raid@xxxxxxxxxxxxxxx
>> Signed-off-by: Ming Lin <mlin@xxxxxxxxxx>
>> ---
>>  drivers/md/raid5.c | 42 +++++++++++++++++++++++++++++++++++++-----
>>  1 file changed, 37 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>> index 7f4a717..b18f548 100644
>> --- a/drivers/md/raid5.c
>> +++ b/drivers/md/raid5.c
>> @@ -4738,7 +4738,7 @@ static void raid5_align_endio(struct bio *bi, int error)
>>  	add_bio_to_retry(raid_bi, conf);
>>  }
>>  
>> -static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
>> +static int raid5_read_one_chunk(struct mddev *mddev, struct bio *raid_bio)
>>  {
>>  	struct r5conf *conf = mddev->private;
>>  	int dd_idx;
>> @@ -4747,7 +4747,7 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
>>  	sector_t end_sector;
>>  
>>  	if (!in_chunk_boundary(mddev, raid_bio)) {
>> -		pr_debug("chunk_aligned_read : non aligned\n");
>> +		pr_debug("%s: non aligned\n", __func__);
>>  		return 0;
>>  	}
>>  	/*
>> @@ -4822,6 +4822,36 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
>>  	}
>>  }
>>  
>> +static struct bio *chunk_aligned_read(struct mddev *mddev, struct bio *raid_bio)
>> +{
>> +	struct bio *split;
>> +
>> +	do {
>> +		sector_t sector = raid_bio->bi_iter.bi_sector;
>> +		unsigned chunk_sects = mddev->chunk_sectors;
>> +		unsigned sectors;
>> +
>> +		if (likely(is_power_of_2(chunk_sects)))
>> +			sectors = chunk_sects - (sector & (chunk_sects-1));
>> +		else
>> +			sectors = chunk_sects - sector_div(sector, chunk_sects);
> 
> RAID5 doesn't currently allow non-power-of-2 chunks.  So this test is
> pointless, but not really harmful.  Maybe someday we will.
> 
> I'm equally happy for it to stay or go.

Then it's better for it to go.
Thanks.