Re: [md PATCH 01/14] md/raid5: use md_write_start to count stripes, not bios

NeilBrown
2017-02-17T02:04:44Z

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On Thu, Feb 16 2017, Shaohua Li wrote:

> On Thu, Feb 16, 2017 at 03:39:01PM +1100, Neil Brown wrote:
>> We use md_write_start() to increase the count of pending writes, and
>> md_write_end() to decrement the count. We currently count bios
>> submitted to md/raid5. Change it count stripe_heads that a WRITE bio
>> has been attached to.
>>
>> So now, raid5_make_request() calls md_write_start() and then
>> md_write_end() to keep the count elevated during the setup of the
>> request.
>>
>> add_stripe_bio() calls md_write_start() for each stripe_head, and the
>> completion routines always call md_write_end(), instead of only
>> calling it when raid5_dec_bi_active_stripes() returns 0.
>> make_discard_request also calls md_write_start/end().
>>
>> The parallel between md_write_{start,end} and use of bi_phys_segments
>> can be seen in that:
>> Whenever we set bi_phys_segments to 1, we now call md_write_start.
>> Whenever we increment it on non-read requests with
>> raid5_inc_bi_active_stripes(), we now call md_write_start().
>> Whenever we decrement bi_phys_segments on non-read requsts with
>> raid5_dec_bi_active_stripes(), we now call md_write_end().
>>
>> This reduces our dependence on keeping a per-bio count of active
>> stripes in bi_phys_segments.
>>
>> Signed-off-by: NeilBrown <neilb@suse.com>
>> ---
>> drivers/md/raid5-cache.c | 2 +-
>> drivers/md/raid5.c | 27 +++++++++++++--------------
>> 2 files changed, 14 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
>> index 302dea3296ba..4b211f914d17 100644
>> --- a/drivers/md/raid5-cache.c
>> +++ b/drivers/md/raid5-cache.c
>> @@ -265,8 +265,8 @@ r5c_return_dev_pending_writes(struct r5conf *conf, struct r5dev *dev,
>> while (wbi && wbi->bi_iter.bi_sector <
>> dev->sector + STRIPE_SECTORS) {
>> wbi2 = r5_next_bio(wbi, dev->sector);
>> + md_write_end(conf->mddev);
>> if (!raid5_dec_bi_active_stripes(wbi)) {
>> - md_write_end(conf->mddev);
>> bio_list_add(return_bi, wbi);
>> }
>> wbi = wbi2;
>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>> index 3c7e106c12a2..760b726943c9 100644
>> --- a/drivers/md/raid5.c
>> +++ b/drivers/md/raid5.c
>> @@ -3075,6 +3075,7 @@ static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx,
>> bi->bi_next = *bip;
>> *bip = bi;
>> raid5_inc_bi_active_stripes(bi);
>> + md_write_start(conf->mddev, bi);
>>
>> if (forwrite) {
>> /* check if page is covered */
>> @@ -3198,10 +3199,9 @@ handle_failed_stripe(struct r5conf *conf, struct stripe_head *sh,
>> struct bio *nextbi = r5_next_bio(bi, sh->dev[i].sector);
>>
>> bi->bi_error = -EIO;
>> - if (!raid5_dec_bi_active_stripes(bi)) {
>> - md_write_end(conf->mddev);
>> + md_write_end(conf->mddev);
>> + if (!raid5_dec_bi_active_stripes(bi))
>> bio_list_add(return_bi, bi);
>> - }
>> bi = nextbi;
>> }
>> if (bitmap_end)
>> @@ -3222,10 +3222,9 @@ handle_failed_stripe(struct r5conf *conf, struct stripe_head *sh,
>> struct bio *bi2 = r5_next_bio(bi, sh->dev[i].sector);
>>
>> bi->bi_error = -EIO;
>> - if (!raid5_dec_bi_active_stripes(bi)) {
>> - md_write_end(conf->mddev);
>> + md_write_end(conf->mddev);
>> + if (!raid5_dec_bi_active_stripes(bi))
>> bio_list_add(return_bi, bi);
>> - }
>> bi = bi2;
>> }
>>
>> @@ -3582,10 +3581,9 @@ static void handle_stripe_clean_event(struct r5conf *conf,
>> while (wbi && wbi->bi_iter.bi_sector <
>> dev->sector + STRIPE_SECTORS) {
>> wbi2 = r5_next_bio(wbi, dev->sector);
>> - if (!raid5_dec_bi_active_stripes(wbi)) {
>> - md_write_end(conf->mddev);
>> + md_write_end(conf->mddev);
>> + if (!raid5_dec_bi_active_stripes(wbi))
>> bio_list_add(return_bi, wbi);
>> - }
>> wbi = wbi2;
>> }
>> bitmap_endwrite(conf->mddev->bitmap, sh->sector,
>> @@ -5268,6 +5266,7 @@ static void make_discard_request(struct mddev *mddev, struct bio *bi)
>>
>> bi->bi_next = NULL;
>> bi->bi_phys_segments = 1; /* over-loaded to count active stripes */
>> + md_write_start(mddev, bi);
>
> md_write_start calls wait_event, so it can't be called with lock hold. Maybe
> add a special __md_write_start which only increases the count, we already wait
> in make_request.

I cannot see what lock is held here, but I can see a that a lock is held
below while md_write_start() is called.

An "md_write_incr()" or similar would certainly make sense. I'll add
that in.

Thanks,
NeilBrown

>
>> stripe_sectors = conf->chunk_sectors *
>> (conf->raid_disks - conf->max_degraded);
>> @@ -5314,6 +5313,7 @@ static void make_discard_request(struct mddev *mddev, struct bio *bi)
>> sh->dev[d].towrite = bi;
>> set_bit(R5_OVERWRITE, &sh->dev[d].flags);
>> raid5_inc_bi_active_stripes(bi);
>> + md_write_start(mddev, bi);
>
> Ditto.
>
> Thanks,
> Shaohua
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