Re: [PATCH 7/7] md/raid5: fix handling of degraded stripes in batches.

[Shaohua Li <shli@kernel.org>]

On Wed, May 13, 2015 at 10:56:04AM +1000, NeilBrown wrote:
> On Fri, 8 May 2015 12:12:23 -0700 Shaohua Li <shli@kernel.org> wrote:
> 
> > On Fri, May 08, 2015 at 06:56:12PM +1000, NeilBrown wrote:
> > > There is no need for special handling of stripe-batches when the array
> > > is degraded.
> > > 
> > > There may be if there is a failure in the batch, but STRIPE_DEGRADED
> > > does not imply an error.
> > > 
> > > So don't set STRIPE_BATCH_ERR in ops_run_io just because the array is
> > > degraded.
> > > This actually causes a bug: the STRIPE_DEGRADED flag gets cleared in
> > > check_break_stripe_batch_list() and so the bitmap bit gets cleared
> > > when it shouldn't.
> > > 
> > > So in check_break_stripe_batch_list(), split the batch up completely -
> > > again STRIPE_DEGRADED isn't meaningful.
> > > 
> > > Also don't set STRIPE_BATCH_ERR when there is a write error to a
> > > replacement device.  This simply removes the replacement device and
> > > requires no extra handling.
> > > 
> > > Signed-off-by: NeilBrown <neilb@suse.de>
> > > ---
> > >  drivers/md/raid5.c |   17 +++--------------
> > >  1 file changed, 3 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> > > index 3873eaa6fa2e..1ba97fdc6df1 100644
> > > --- a/drivers/md/raid5.c
> > > +++ b/drivers/md/raid5.c
> > > @@ -1078,9 +1078,6 @@ again:
> > >  			pr_debug("skip op %ld on disc %d for sector %llu\n",
> > >  				bi->bi_rw, i, (unsigned long long)sh->sector);
> > >  			clear_bit(R5_LOCKED, &sh->dev[i].flags);
> > > -			if (sh->batch_head)
> > > -				set_bit(STRIPE_BATCH_ERR,
> > > -					&sh->batch_head->state);
> > >  			set_bit(STRIPE_HANDLE, &sh->state);
> > >  		}
> > 
> > Patches look good to me. I had a question here. Is it possible some stripes in
> > a batch become degraded here but some not? Seems possible, then the batch
> > should be splitted too.
> 
> Why?
> 
> I don't really understand the purpose of splitting up the batch.
> The only possible error handling on a full-stripe write is:
>  - fail a device, or
>  - record a bad-block.
> 
> The first case affects all stripes in a batch equally so there is no need to
> split it up.
> The second case it is probably best to record the bad blocks while iterating
> through the batch in handle_stripe_clean_event().
> 
> What exactly do you expect to happen after the stripes in a batch after they
> have been split up?

My original concern is a device failure can causes some stripes fail but some
not, eg, get rdev returns NULL in ops_run_io for some stripes but not all of a
batch. There is no any locking, so seems possible. But you are right, the
stripes without error in ops_run_io will get an IO error eventually, the whole
batch stripes are still in the same state. So my concern is invalid, but I
forgot to reply the email, sorry.

The batch split is to handle IO error, eg, record bad-block and so on. I'm not
confident to change existing code to handle the error case, so I feel spliting
it and handling the stripe in normal way is the best thing to do. There
certainly might be better way. Again the device failure case should be ignored,
which I didn't realize originally.

BTW, can you apply the fix reported by Maxime, which is introduced by the batch patch.
http://marc.info/?l=linux-raid&m=143153461415534&w=2

Thanks,
Shaohua