Date:	Fri, 8 May 2015 09:15:01 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCHv4 00/10] add on-demand device creation

On (05/08/15 00:11), Minchan Kim wrote:
> It's good timing. When I'm about to dig in, I got your patches
> and tested it.
> 
> Passed my test script!
> 
> BTW, patches you sent landed on mainline?
> 
> Anyway, Thanks!
> 

Hello Minchan,

thanks a ton!

not mainlined yet, to the best of my knowledge. will keep an eye on these patches.


I'll rename zram-contol interface and resubmit zram hot add/remove
patchset today.
thanks.

	-ss

> > From 7056096317825dbe582d005385bc4077ecb62c76 Mon Sep 17 00:00:00 2001
> > From: NeilBrown <neilb@...e.de>
> > Date: Mon, 27 Apr 2015 04:12:22 +1000
> > Subject: [PATCH 2/2] block: destroy bdi before blockdev is unregistered.
> > 
> > Because of the peculiar way that md devices are created (automatically
> > when the device node is opened), a new device can be created and
> > registered immediately after the
> > 	blk_unregister_region(disk_devt(disk), disk->minors);
> > call in del_gendisk().
> > 
> > Therefore it is important that all visible artifacts of the previous
> > device are removed before this call.  In particular, the 'bdi'.
> > 
> > Since:
> > commit c4db59d31e39ea067c32163ac961e9c80198fd37
> > Author: Christoph Hellwig <hch@....de>
> >     fs: don't reassign dirty inodes to default_backing_dev_info
> > 
> > moved the
> >    device_unregister(bdi->dev);
> > call from bdi_unregister() to bdi_destroy() it has been quite easy to
> > lose a race and have a new (e.g.) "md127" be created after the
> > blk_unregister_region() call and before bdi_destroy() is ultimately
> > called by the final 'put_disk', which must come after del_gendisk().
> > 
> > The new device finds that the bdi name is already registered in sysfs
> > and complains
> > 
> > > [ 9627.630029] WARNING: CPU: 18 PID: 3330 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x5a/0x70()
> > > [ 9627.630032] sysfs: cannot create duplicate filename '/devices/virtual/bdi/9:127'
> > 
> > We can fix this by moving the bdi_destroy() call out of
> > blk_release_queue() (which can happen very late when a refcount
> > reaches zero) and into blk_cleanup_queue() - which happens exactly when the md
> > device driver calls it.
> > 
> > Then it is only necessary for md to call blk_cleanup_queue() before
> > del_gendisk().  As loop.c devices are also created on demand by
> > opening the device node, we make the same change there.
> > 
> > Fixes: c4db59d31e39ea067c32163ac961e9c80198fd37
> > Reported-by: Azat Khuzhin <a3at.mail@...il.com>
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: stable@...r.kernel.org (v4.0)
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > Reviewed-by: Christoph Hellwig <hch@....de>
> > Signed-off-by: Jens Axboe <axboe@...com>
> > ---
> >  block/blk-core.c     | 2 ++
> >  block/blk-sysfs.c    | 2 --
> >  drivers/block/loop.c | 2 +-
> >  drivers/md/md.c      | 4 ++--
> >  4 files changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index fd154b9..7871603 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -552,6 +552,8 @@ void blk_cleanup_queue(struct request_queue *q)
> >  		q->queue_lock = &q->__queue_lock;
> >  	spin_unlock_irq(lock);
> >  
> > +	bdi_destroy(&q->backing_dev_info);
> > +
> >  	/* @q is and will stay empty, shutdown and put */
> >  	blk_put_queue(q);
> >  }
> > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > index faaf36a..2b8fd30 100644
> > --- a/block/blk-sysfs.c
> > +++ b/block/blk-sysfs.c
> > @@ -522,8 +522,6 @@ static void blk_release_queue(struct kobject *kobj)
> >  
> >  	blk_trace_shutdown(q);
> >  
> > -	bdi_destroy(&q->backing_dev_info);
> > -
> >  	ida_simple_remove(&blk_queue_ida, q->id);
> >  	call_rcu(&q->rcu_head, blk_free_queue_rcu);
> >  }
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index ae3fcb4..d7173cb 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -1620,8 +1620,8 @@ out:
> >  
> >  static void loop_remove(struct loop_device *lo)
> >  {
> > -	del_gendisk(lo->lo_disk);
> >  	blk_cleanup_queue(lo->lo_queue);
> > +	del_gendisk(lo->lo_disk);
> >  	blk_mq_free_tag_set(&lo->tag_set);
> >  	put_disk(lo->lo_disk);
> >  	kfree(lo);
> > diff --git a/drivers/md/md.c b/drivers/md/md.c
> > index d4f31e1..593a024 100644
> > --- a/drivers/md/md.c
> > +++ b/drivers/md/md.c
> > @@ -4818,12 +4818,12 @@ static void md_free(struct kobject *ko)
> >  	if (mddev->sysfs_state)
> >  		sysfs_put(mddev->sysfs_state);
> >  
> > +	if (mddev->queue)
> > +		blk_cleanup_queue(mddev->queue);
> >  	if (mddev->gendisk) {
> >  		del_gendisk(mddev->gendisk);
> >  		put_disk(mddev->gendisk);
> >  	}
> > -	if (mddev->queue)
> > -		blk_cleanup_queue(mddev->queue);
> >  
> >  	kfree(mddev);
> >  }
> > -- 
> > 2.4.0
> > 
> 
> > From ef8878259ce360ec352b591746761249393cf952 Mon Sep 17 00:00:00 2001
> > From: NeilBrown <neilb@...e.de>
> > Date: Thu, 7 May 2015 21:49:41 +0900
> > Subject: [PATCH 1/2] block: bdi_unregister() now contains very little
> >  functionality.
> > 
> > It contains a "WARN_ON" if bdi->dev is NULL.  This warning is of no
> > real consequence as bdi->dev isn't needed by anything else in the function,
> > and it triggers if
> >    blk_cleanup_queue() -> bdi_destroy()
> > is called before bdi_unregister, which a subsequent patch will make happen.
> > So this isn't wanted.
> > 
> > It also calls bdi_set_min_ratio().  This needs to be called after
> > writes through the bdi have all been flushed, and before the bdi is destroyed.
> > Calling it early is better than calling it late as it frees up a global
> > resource.
> > 
> > Calling it immediately after bdi_wb_shutdown() in bdi_destroy()
> > perfectly fits these requirements.
> > 
> > So bdi_unregister can be discarded with the important content moved to
> > bdi_destroy, as can the
> >   writeback_bdi_unregister
> > event which is already not used.
> > 
> > This is tagged for 'stable' as it is a pre-requisite for a subsequent
> > patch which moves calls to blk_cleanup_queue() before calls to
> > del_gendisk().  The commit identified as 'Fixes' removed a lot of
> > other functionality from bdi_unregister(), and made a change which
> > necessitated moving the blk_cleanup_queue() calls.
> > 
> > Reported-by: Mike Snitzer <snitzer@...hat.com>
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: stable@...r.kernel.org (v4.0)
> > Fixes: c4db59d31e39ea067c32163ac961e9c80198fd37
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > ---
> >  block/genhd.c                    |  1 -
> >  include/linux/backing-dev.h      |  1 -
> >  include/trace/events/writeback.h |  1 -
> >  mm/backing-dev.c                 | 18 +-----------------
> >  4 files changed, 1 insertion(+), 20 deletions(-)
> > 
> > diff --git a/block/genhd.c b/block/genhd.c
> > index 64600e9..2fba82c 100644
> > --- a/block/genhd.c
> > +++ b/block/genhd.c
> > @@ -653,7 +653,6 @@ void del_gendisk(struct gendisk *disk)
> >  	disk->flags &= ~GENHD_FL_UP;
> >  
> >  	sysfs_remove_link(&disk_to_dev(disk)->kobj, "bdi");
> > -	bdi_unregister(&disk->queue->backing_dev_info);
> >  	blk_unregister_queue(disk);
> >  	blk_unregister_region(disk_devt(disk), disk->minors);
> >  
> > diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> > index aff923a..d87d8ec 100644
> > --- a/include/linux/backing-dev.h
> > +++ b/include/linux/backing-dev.h
> > @@ -116,7 +116,6 @@ __printf(3, 4)
> >  int bdi_register(struct backing_dev_info *bdi, struct device *parent,
> >  		const char *fmt, ...);
> >  int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev);
> > -void bdi_unregister(struct backing_dev_info *bdi);
> >  int __must_check bdi_setup_and_register(struct backing_dev_info *, char *);
> >  void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
> >  			enum wb_reason reason);
> > diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> > index 880dd74..c178d13 100644
> > --- a/include/trace/events/writeback.h
> > +++ b/include/trace/events/writeback.h
> > @@ -250,7 +250,6 @@ DEFINE_EVENT(writeback_class, name, \
> >  DEFINE_WRITEBACK_EVENT(writeback_nowork);
> >  DEFINE_WRITEBACK_EVENT(writeback_wake_background);
> >  DEFINE_WRITEBACK_EVENT(writeback_bdi_register);
> > -DEFINE_WRITEBACK_EVENT(writeback_bdi_unregister);
> >  
> >  DECLARE_EVENT_CLASS(wbc_class,
> >  	TP_PROTO(struct writeback_control *wbc, struct backing_dev_info *bdi),
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index 6dc4580..000e7b3 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -359,23 +359,6 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
> >  	flush_delayed_work(&bdi->wb.dwork);
> >  }
> >  
> > -/*
> > - * Called when the device behind @bdi has been removed or ejected.
> > - *
> > - * We can't really do much here except for reducing the dirty ratio at
> > - * the moment.  In the future we should be able to set a flag so that
> > - * the filesystem can handle errors at mark_inode_dirty time instead
> > - * of only at writeback time.
> > - */
> > -void bdi_unregister(struct backing_dev_info *bdi)
> > -{
> > -	if (WARN_ON_ONCE(!bdi->dev))
> > -		return;
> > -
> > -	bdi_set_min_ratio(bdi, 0);
> > -}
> > -EXPORT_SYMBOL(bdi_unregister);
> > -
> >  static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
> >  {
> >  	memset(wb, 0, sizeof(*wb));
> > @@ -443,6 +426,7 @@ void bdi_destroy(struct backing_dev_info *bdi)
> >  	int i;
> >  
> >  	bdi_wb_shutdown(bdi);
> > +	bdi_set_min_ratio(bdi, 0);
> >  
> >  	WARN_ON(!list_empty(&bdi->work_list));
> >  	WARN_ON(delayed_work_pending(&bdi->wb.dwork));
> > -- 
> > 2.4.0
> > 
> 
> 
> -- 
> Kind regards,
> Minchan Kim
> 
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