linux-kernel - Re: [PATCH v1 2/2] zram: remove init_lock in zram_make

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Date:	Mon, 2 Feb 2015 12:41:00 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Ganesh Mahendran <opensource.ganesh@...il.com>
Subject: Re: [PATCH v1 2/2] zram: remove init_lock in zram_make_request

Separate another issue from my patch.

On Mon, Feb 02, 2015 at 11:44:06AM +0900, Minchan Kim wrote:
> On Mon, Feb 02, 2015 at 10:48:00AM +0900, Sergey Senozhatsky wrote:
> > Hello Minchan,
> > 
> > On (02/02/15 10:30), Minchan Kim wrote:
> > > > >  static inline int init_done(struct zram *zram)
> > > > >  {
> > > > > -	return zram->meta != NULL;
> > > > > +	return zram->disksize != 0;
> > > > 
> > > > we don't set ->disksize to 0 when create device. and I think
> > > > it's better to use refcount here, but set it to 0 during device creation.
> > > > (see the patch below)
> > > 
> > > There was a reason I didn't use refcount there.
> > > I should have written down it.
> > > 
> > > We need something to prevent further I/O handling on other CPUs.
> > > Otherwise, it's livelock. For example, new 'A' I/O rw path on CPU 1
> > > can see non-zero refcount if another CPU is going on rw.
> > > Then, another new 'B' I/O rw path on CPU 2 can see non-zero refcount
> > > if A I/O is going on. Then, another new 'C' I/O rw path on CPU 3 can
> > > see non-zero refcount if B I/O is going on. Finally, 'A' IO is done
> > > on CPU 1 and next I/O 'D' on CPU 1 can see non-zero refcount because
> > > 'C' on CPU 3 is going on. Infinite loop.
> > 
> > sure, I did think about this. and I actually didn't find any reason not
> > to use ->refcount there. if user wants to reset the device, he first
> > should umount it to make bdev->bd_holders check happy. and that's where
> > IOs will be failed. so it makes sense to switch to ->refcount there, IMHO.
> 
> If we use zram as block device itself(not a fs or swap) and open the
> block device as !FMODE_EXCL, bd_holders will be void.
> 
> Another topic: As I didn't see enough fs/block_dev.c bd_holders in zram
> would be mess. I guess we need to study hotplug of device and implement
> it for zram reset rather than strange own konb. It should go TODO. :(

Actually, I thought bd_mutex use from custom driver was terrible idea
so we should walk around with device hotplug but as I look through
another drivers, they have used the lock for a long time.
Maybe it's okay to use it in zram?
If so, Ganesh's patch is no problem to me although I didn't' review it in detail.
One thing I want to point out is that it would be better to change bd_holders
with bd_openers to filter out because dd test opens block device as !EXCL
so bd_holders will be void.

What do you think about it?

-- 
Kind regards,
Minchan Kim
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