[md PATCH 09/15] md/raid10: stop using bi_phys_segments

NeilBrown
2017-03-15T03:05:13Z

raid10 currently repurposes bi_phys_segments on each
incoming bio to count how many r10bio was used to encode the
request.

We need to know when the number of attached r10bio reaches
zero to:
1/ call bio_endio() when all IO on the bio is finished
2/ decrement ->nr_pending so that resync IO can proceed.

Now that the bio has its own __bi_remaining counter, that
can be used instead. We can call bio_inc_remaining to
increment the counter and call bio_endio() every time an
r10bio completes, rather than only when bi_phys_segments
reaches zero.

This addresses point 1, but not point 2. bio_endio()
doesn't (and cannot) report when the last r10bio has
finished, so a different approach is needed.

So: instead of counting bios in ->nr_pending, count r10bios.
i.e. every time we attach a bio, increment nr_pending.
Every time an r10bio completes, decrement nr_pending.

Normally we only increment nr_pending after first checking
that ->barrier is zero, or some other non-trivial tests and
possible waiting. When attaching multiple r10bios to a bio,
we only need the tests and the waiting once. After the
first increment, subsequent increments can happen
unconditionally as they are really all part of the one
request.

So introduce inc_pending() which can be used when we know
that nr_pending is already elevated.

Note that this fixes a bug. freeze_array() contains the line
atomic_read(&conf->nr_pending) == conf->nr_queued+extra,
which implies that the units for ->nr_pending, ->nr_queued and extra
are the same.
->nr_queue and extra count r10_bios, but prior to this patch,
->nr_pending counted bios. If a bio ever resulted in multiple
r10_bios (due to bad blocks), freeze_array() would not work correctly.
Now it does.

Signed-off-by: NeilBrown <neilb@suse.com>
---
drivers/md/raid10.c | 76 +++++++++++++++++----------------------------------
1 file changed, 25 insertions(+), 51 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 20029345f8b6..288e39abae11 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -301,27 +301,18 @@ static void reschedule_retry(struct r10bio *r10_bio)
static void raid_end_bio_io(struct r10bio *r10_bio)
{
struct bio *bio = r10_bio->master_bio;
- int done;
struct r10conf *conf = r10_bio->mddev->private;

- if (bio->bi_phys_segments) {
- unsigned long flags;
- spin_lock_irqsave(&conf->device_lock, flags);
- bio->bi_phys_segments--;
- done = (bio->bi_phys_segments == 0);
- spin_unlock_irqrestore(&conf->device_lock, flags);
- } else
- done = 1;
if (!test_bit(R10BIO_Uptodate, &r10_bio->state))
bio->bi_error = -EIO;
- if (done) {
- bio_endio(bio);
- /*
- * Wake up any possible resync thread that waits for the device
- * to go idle.
- */
- allow_barrier(conf);
- }
+
+ bio_endio(bio);
+ /*
+ * Wake up any possible resync thread that waits for the device
+ * to go idle.
+ */
+ allow_barrier(conf);
+
free_r10bio(r10_bio);
}

@@ -984,6 +975,15 @@ static void wait_barrier(struct r10conf *conf)
spin_unlock_irq(&conf->resync_lock);
}

+static void inc_pending(struct r10conf *conf)
+{
+ /* The current request requires multiple r10_bio, so
+ * we need to increment the pending count.
+ */
+ WARN_ON(!atomic_read(&conf->nr_pending));
+ atomic_inc(&conf->nr_pending);
+}
+
static void allow_barrier(struct r10conf *conf)
{
if ((atomic_dec_and_test(&conf->nr_pending)) ||
@@ -1161,12 +1161,8 @@ static void raid10_read_request(struct mddev *mddev, struct bio *bio,
sectors_handled = (r10_bio->sector + max_sectors
- bio->bi_iter.bi_sector);
r10_bio->sectors = max_sectors;
- spin_lock_irq(&conf->device_lock);
- if (bio->bi_phys_segments == 0)
- bio->bi_phys_segments = 2;
- else
- bio->bi_phys_segments++;
- spin_unlock_irq(&conf->device_lock);
+ inc_pending(conf);
+ bio_inc_remaining(bio);
/*
* Cannot call generic_make_request directly as that will be
* queued in __generic_make_request and subsequent
@@ -1261,9 +1257,7 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,
* on which we have seen a write error, we want to avoid
* writing to those blocks. This potentially requires several
* writes to write around the bad blocks. Each set of writes
- * gets its own r10_bio with a set of bios attached. The number
- * of r10_bios is recored in bio->bi_phys_segments just as with
- * the read case.
+ * gets its own r10_bio with a set of bios attached.
*/

r10_bio->read_slot = -1; /* make sure repl_bio gets freed */
@@ -1494,15 +1488,9 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,
*/

if (sectors_handled < bio_sectors(bio)) {
- /* We need another r10_bio and it needs to be counted
- * in bio->bi_phys_segments.
- */
- spin_lock_irq(&conf->device_lock);
- if (bio->bi_phys_segments == 0)
- bio->bi_phys_segments = 2;
- else
- bio->bi_phys_segments++;
- spin_unlock_irq(&conf->device_lock);
+ /* We need another r10_bio and it needs to be counted */
+ inc_pending(conf);
+ bio_inc_remaining(bio);
one_write_done(r10_bio);
r10_bio = mempool_alloc(conf->r10bio_pool, GFP_NOIO);

@@ -1531,16 +1519,6 @@ static void __make_request(struct mddev *mddev, struct bio *bio)
r10_bio->sector = bio->bi_iter.bi_sector;
r10_bio->state = 0;

- /*
- * We might need to issue multiple reads to different devices if there
- * are bad blocks around, so we keep track of the number of reads in
- * bio->bi_phys_segments. If this is 0, there is only one r10_bio and
- * no locking will be needed when the request completes. If it is
- * non-zero, then it is the number of not-completed requests.
- */
- bio->bi_phys_segments = 0;
- bio_clear_flag(bio, BIO_SEG_VALID);
-
if (bio_data_dir(bio) == READ)
raid10_read_request(mddev, bio, r10_bio);
else
@@ -2692,12 +2670,8 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
r10_bio->sector + max_sectors
- mbio->bi_iter.bi_sector;
r10_bio->sectors = max_sectors;
- spin_lock_irq(&conf->device_lock);
- if (mbio->bi_phys_segments == 0)
- mbio->bi_phys_segments = 2;
- else
- mbio->bi_phys_segments++;
- spin_unlock_irq(&conf->device_lock);
+ bio_inc_remaining(mbio);
+ inc_pending(conf);
generic_make_request(bio);

r10_bio = mempool_alloc(conf->r10bio_pool,