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SubjectRe: [PATCH v3 07/11] md/raid5: split bio for chunk_aligned_read
On 05/07/2015 09:14 PM, NeilBrown wrote:
> On Wed, 6 May 2015 23:34:17 -0700 Ming Lin <mlin@kernel.org> 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@suse.de>
>> Cc: linux-raid@vger.kernel.org
>> Signed-off-by: Ming Lin <mlin@kernel.org>
>> ---
>> 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.

From d40e9dfaae261cc86170193305e2022d2e1cda1a Mon Sep 17 00:00:00 2001
From: Ming Lin <mlin@kernel.org>
Date: Wed, 6 May 2015 22:51:24 -0700
Subject: [PATCH 07/11] md/raid5: split bio for chunk_aligned_read

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@suse.de>
Cc: linux-raid@vger.kernel.org
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Ming Lin <mlin@kernel.org>
---
drivers/md/raid5.c | 37 ++++++++++++++++++++++++++++++++-----
1 file changed, 32 insertions(+), 5 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 7f4a717..1978aa9 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 +4822,31 @@ 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 = chunk_sects - (sector & (chunk_sects-1));
+
+ if (sectors < bio_sectors(raid_bio)) {
+ split = bio_split(raid_bio, sectors, GFP_NOIO, fs_bio_set);
+ bio_chain(split, raid_bio);
+ } else
+ split = raid_bio;
+
+ if (!raid5_read_one_chunk(mddev, split)) {
+ if (split != raid_bio)
+ generic_make_request(raid_bio);
+ return split;
+ }
+ } while (split != raid_bio);
+
+ return NULL;
+}
+
/* __get_priority_stripe - get the next stripe to process
*
* Full stripe writes are allowed to pass preread active stripes up until
@@ -5099,9 +5124,11 @@ static void make_request(struct mddev *mddev, struct bio * bi)
* data on failed drives.
*/
if (rw == READ && mddev->degraded == 0 &&
- mddev->reshape_position == MaxSector &&
- chunk_aligned_read(mddev,bi))
- return;
+ mddev->reshape_position == MaxSector) {
+ bi = chunk_aligned_read(mddev, bi);
+ if (!bi)
+ return;
+ }

if (unlikely(bi->bi_rw & REQ_DISCARD)) {
make_discard_request(mddev, bi);
--
1.9.1

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