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Date:   Mon, 15 Feb 2021 11:08:31 -0500
From:   Mike Snitzer <snitzer@redhat.com>
To:     Sergei Shtepa <sergei.shtepa@veeam.com>
Cc:     "Damien.LeMoal@wdc.com" <Damien.LeMoal@wdc.com>,
        "hare@suse.de" <hare@suse.de>,
        "ming.lei@redhat.com" <ming.lei@redhat.com>,
        "agk@redhat.com" <agk@redhat.com>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "axboe@kernel.dk" <axboe@kernel.dk>, "jack@suse.cz" <jack@suse.cz>,
        "johannes.thumshirn@wdc.com" <johannes.thumshirn@wdc.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "koct9i@gmail.com" <koct9i@gmail.com>,
        "steve@sk2.org" <steve@sk2.org>,
        "dm-devel@redhat.com" <dm-devel@redhat.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Pavel Tide <Pavel.TIde@veeam.com>
Subject: Re: [PATCH v5 5/6] dm: add 'noexcl' option for dm-linear
Message-ID: <20210215160831.GA5371@redhat.com>
References: <1612881028-7878-1-git-send-email-sergei.shtepa@veeam.com>
 <1612881028-7878-6-git-send-email-sergei.shtepa@veeam.com>
 <20210211175151.GA13839@redhat.com>
 <20210212113438.GA9877@veeam.com>
 <20210212160631.GA19424@redhat.com>
 <20210215103444.GA11820@veeam.com>
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On Mon, Feb 15 2021 at  5:34am -0500,
Sergei Shtepa <sergei.shtepa@veeam.com> wrote:

> The 02/12/2021 19:06, Mike Snitzer wrote:
> > On Fri, Feb 12 2021 at  6:34am -0500,
> > Sergei Shtepa <sergei.shtepa@veeam.com> wrote:
> > 
> > > The 02/11/2021 20:51, Mike Snitzer wrote:
> > > > On Tue, Feb 09 2021 at  9:30am -0500,
> > > > Sergei Shtepa <sergei.shtepa@veeam.com> wrote:
> > > > 
> > > > > The 'noexcl' option allow to open underlying block-device
> > > > > without FMODE_EXCL.
> > > > > 
> > > > > Signed-off-by: Sergei Shtepa <sergei.shtepa@veeam.com>
> > > > > ---
> > > > >  drivers/md/dm-linear.c        | 14 +++++++++++++-
> > > > >  drivers/md/dm-table.c         | 14 ++++++++------
> > > > >  drivers/md/dm.c               | 26 +++++++++++++++++++-------
> > > > >  drivers/md/dm.h               |  2 +-
> > > > >  include/linux/device-mapper.h |  7 +++++++
> > > > >  5 files changed, 48 insertions(+), 15 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/md/dm-linear.c b/drivers/md/dm-linear.c
> > > > > index 00774b5d7668..b16d89802b9d 100644
> > > > > --- a/drivers/md/dm-linear.c
> > > > > +++ b/drivers/md/dm-linear.c
> > > > > @@ -33,7 +33,7 @@ static int linear_ctr(struct dm_target *ti, unsigned int argc, char **argv)
> > > > >  	char dummy;
> > > > >  	int ret;
> > > > >  
> > > > > -	if (argc != 2) {
> > > > > +	if ((argc < 2) || (argc > 3)) {
> > > > >  		ti->error = "Invalid argument count";
> > > > >  		return -EINVAL;
> > > > >  	}
> > > > > @@ -51,6 +51,18 @@ static int linear_ctr(struct dm_target *ti, unsigned int argc, char **argv)
> > > > >  	}
> > > > >  	lc->start = tmp;
> > > > >  
> > > > > +	ti->non_exclusive = false;
> > > > > +	if (argc > 2) {
> > > > > +		if (strcmp("noexcl", argv[2]) == 0)
> > > > > +			ti->non_exclusive = true;
> > > > > +		else if (strcmp("excl", argv[2]) == 0)
> > > > > +			ti->non_exclusive = false;
> > > > > +		else {
> > > > > +			ti->error = "Invalid exclusive option";
> > > > > +			return -EINVAL;
> > > > > +		}
> > > > > +	}
> > > > > +
> > > > >  	ret = dm_get_device(ti, argv[0], dm_table_get_mode(ti->table), &lc->dev);
> > > > >  	if (ret) {
> > > > >  		ti->error = "Device lookup failed";
> > > > > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > > > > index 4acf2342f7ad..f020459465bd 100644
> > > > > --- a/drivers/md/dm-table.c
> > > > > +++ b/drivers/md/dm-table.c
> > > > > @@ -322,7 +322,7 @@ static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
> > > > >   * device and not to touch the existing bdev field in case
> > > > >   * it is accessed concurrently.
> > > > >   */
> > > > > -static int upgrade_mode(struct dm_dev_internal *dd, fmode_t new_mode,
> > > > > +static int upgrade_mode(struct dm_dev_internal *dd, fmode_t new_mode, bool non_exclusive,
> > > > >  			struct mapped_device *md)
> > > > >  {
> > > > >  	int r;
> > > > > @@ -330,8 +330,8 @@ static int upgrade_mode(struct dm_dev_internal *dd, fmode_t new_mode,
> > > > >  
> > > > >  	old_dev = dd->dm_dev;
> > > > >  
> > > > > -	r = dm_get_table_device(md, dd->dm_dev->bdev->bd_dev,
> > > > > -				dd->dm_dev->mode | new_mode, &new_dev);
> > > > > +	r = dm_get_table_device(md, dd->dm_dev->bdev->bd_dev, dd->dm_dev->mode | new_mode,
> > > > > +				non_exclusive, &new_dev);
> > > > >  	if (r)
> > > > >  		return r;
> > > > >  
> > > > > @@ -387,7 +387,8 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > > > >  		if (!dd)
> > > > >  			return -ENOMEM;
> > > > >  
> > > > > -		if ((r = dm_get_table_device(t->md, dev, mode, &dd->dm_dev))) {
> > > > > +		r = dm_get_table_device(t->md, dev, mode, ti->non_exclusive, &dd->dm_dev);
> > > > > +		if (r) {
> > > > >  			kfree(dd);
> > > > >  			return r;
> > > > >  		}
> > > > > @@ -396,8 +397,9 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > > > >  		list_add(&dd->list, &t->devices);
> > > > >  		goto out;
> > > > >  
> > > > > -	} else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
> > > > > -		r = upgrade_mode(dd, mode, t->md);
> > > > > +	} else if ((dd->dm_dev->mode != (mode | dd->dm_dev->mode)) &&
> > > > > +		   (dd->dm_dev->non_exclusive != ti->non_exclusive)) {
> > > > > +		r = upgrade_mode(dd, mode, ti->non_exclusive, t->md);
> > > > >  		if (r)
> > > > >  			return r;
> > > > >  	}
> > > > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > > > > index 00c41aa6d092..c25dcc2fdb89 100644
> > > > > --- a/drivers/md/dm.c
> > > > > +++ b/drivers/md/dm.c
> > > > > @@ -1117,33 +1117,44 @@ static void close_table_device(struct table_device *td, struct mapped_device *md
> > > > >  	if (!td->dm_dev.bdev)
> > > > >  		return;
> > > > >  
> > > > > -	bd_unlink_disk_holder(td->dm_dev.bdev, dm_disk(md));
> > > > > -	blkdev_put(td->dm_dev.bdev, td->dm_dev.mode | FMODE_EXCL);
> > > > > +	if (td->dm_dev.non_exclusive)
> > > > > +		blkdev_put(td->dm_dev.bdev, td->dm_dev.mode);
> > > > > +	else {
> > > > > +		bd_unlink_disk_holder(td->dm_dev.bdev, dm_disk(md));
> > > > > +		blkdev_put(td->dm_dev.bdev, td->dm_dev.mode | FMODE_EXCL);
> > > > > +	}
> > > > > +
> > > > > +
> > > > > +	blkdev_put(td->dm_dev.bdev, td->dm_dev.mode);
> > > > > +
> > > > >  	put_dax(td->dm_dev.dax_dev);
> > > > >  	td->dm_dev.bdev = NULL;
> > > > >  	td->dm_dev.dax_dev = NULL;
> > > > > +	td->dm_dev.non_exclusive = false;
> > > > >  }
> > > > >  
> > > > >  static struct table_device *find_table_device(struct list_head *l, dev_t dev,
> > > > > -					      fmode_t mode)
> > > > > +					      fmode_t mode, bool non_exclusive)
> > > > >  {
> > > > >  	struct table_device *td;
> > > > >  
> > > > >  	list_for_each_entry(td, l, list)
> > > > > -		if (td->dm_dev.bdev->bd_dev == dev && td->dm_dev.mode == mode)
> > > > > +		if (td->dm_dev.bdev->bd_dev == dev &&
> > > > > +		    td->dm_dev.mode == mode &&
> > > > > +		    td->dm_dev.non_exclusive == non_exclusive)
> > > > >  			return td;
> > > > >  
> > > > >  	return NULL;
> > > > >  }
> > > > >  
> > > > > -int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode,
> > > > > +int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode, bool non_exclusive,
> > > > >  			struct dm_dev **result)
> > > > >  {
> > > > >  	int r;
> > > > >  	struct table_device *td;
> > > > >  
> > > > >  	mutex_lock(&md->table_devices_lock);
> > > > > -	td = find_table_device(&md->table_devices, dev, mode);
> > > > > +	td = find_table_device(&md->table_devices, dev, mode, non_exclusive);
> > > > >  	if (!td) {
> > > > >  		td = kmalloc_node(sizeof(*td), GFP_KERNEL, md->numa_node_id);
> > > > >  		if (!td) {
> > > > > @@ -1154,7 +1165,8 @@ int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode,
> > > > >  		td->dm_dev.mode = mode;
> > > > >  		td->dm_dev.bdev = NULL;
> > > > >  
> > > > > -		if ((r = open_table_device(td, dev, md))) {
> > > > > +		r = open_table_device(td, dev, md, non_exclusive);
> > > > > +		if (r) {
> > > > >  			mutex_unlock(&md->table_devices_lock);
> > > > >  			kfree(td);
> > > > >  			return r;
> > > > > diff --git a/drivers/md/dm.h b/drivers/md/dm.h
> > > > > index fffe1e289c53..7bf20fb2de74 100644
> > > > > --- a/drivers/md/dm.h
> > > > > +++ b/drivers/md/dm.h
> > > > > @@ -179,7 +179,7 @@ int dm_open_count(struct mapped_device *md);
> > > > >  int dm_lock_for_deletion(struct mapped_device *md, bool mark_deferred, bool only_deferred);
> > > > >  int dm_cancel_deferred_remove(struct mapped_device *md);
> > > > >  int dm_request_based(struct mapped_device *md);
> > > > > -int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode,
> > > > > +int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode, bool non_exclusive,
> > > > >  			struct dm_dev **result);
> > > > >  void dm_put_table_device(struct mapped_device *md, struct dm_dev *d);
> > > > >  
> > > > > diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
> > > > > index 61a66fb8ebb3..70002363bfc0 100644
> > > > > --- a/include/linux/device-mapper.h
> > > > > +++ b/include/linux/device-mapper.h
> > > > > @@ -150,6 +150,7 @@ struct dm_dev {
> > > > >  	struct block_device *bdev;
> > > > >  	struct dax_device *dax_dev;
> > > > >  	fmode_t mode;
> > > > > +	bool non_exclusive;
> > > > >  	char name[16];
> > > > >  };
> > > > >  
> > > > > @@ -325,6 +326,12 @@ struct dm_target {
> > > > >  	 * whether or not its underlying devices have support.
> > > > >  	 */
> > > > >  	bool discards_supported:1;
> > > > > +
> > > > > +	/*
> > > > > +	 * Set if this target needs to open device without FMODE_EXCL
> > > > > +	 * mode.
> > > > > +	 */
> > > > > +	bool non_exclusive:1;
> > > > >  };
> > > > >  
> > > > >  void *dm_per_bio_data(struct bio *bio, size_t data_size);
> > > > > -- 
> > > > > 2.20.1
> > > > > 
> > > > 
> > > > I'm really not liking this tug-of-war about FMODE_EXCL vs not.
> > > > Especially dislike the prospect of needing to change _every_ DM target
> > > > that would be made to support blk_interposer.
> > > > 
> > > > I've said this before, private or otherwise, but: Hannes' approach that
> > > > fell back to opening without FMODE_EXCL if FMODE_EXCL open failed.  Have
> > > > you explored that kind of approach?
> > > 
> > > Of course I explored that kind of approach. The easiest thing to do
> > > is fell back to opening without FMODE_EXCL if FMODE_EXCL open failed.
> > > 
> > > But I remind you once again that in this case, without changing
> > > the code of each target, we will change the behavior of each.
> > > Any target will open the device without the FMODE_EXCL flag if the device
> > > is already busy. This can cause errors and cause data loss.
> > > I would not want the device mapper to get worse when adding new functionality.
> > 
> > Right, but I'm not talking about a blind fallback that strips FMODE_EXCL
> > if FMODE_EXCL open failed.
> >  
> > > I will do so in the next patch, as you are sure that it is better... Or
> > > I'll think about it again and try to suggest a better implementation.
> > > 
> > > Thank you, Mike.
> > > 
> > > > 
> > > > You _should_ be able to infer that interposer is being used given the
> > > > requirement to use an explicit remap ioctl to establish the use of
> > > > interposer.
> > 
> > I'm suggesting that open_table_device and close_table_device be made
> > aware of the fact that they are operating on behalf of your remap ioctl
> > (interpose).  So store state in the mapped_device that reflects a remap
> > was used.
> > 
> > Still clunky but at least it confines it to an implementation detail
> > managed by DM core rather than imposing awkward interface changes in
> > both DM core and the DM targets.
> > 
> > Mike
> > 
> 
> Based on your requirements, I conclude that the knowledge about the use
> of interposer should be passed when creating target, since this is where
> the open_table_device function is called.
> This means that the 'noexcl' parameter is no longer needed, but will be
> replaced with 'interposer'.
> The ioctl is no longer needed, the target already knows that it works
> through the interposer, and we can attach it already when creating
> the target.
> 
> I like this logic, and I will implement it.

Yes, I never understood why a new ioctl was introduced.  But please be
aware that this should _not_ be implemented in terms of each DM target
needing to handle 'interposer' being passed as a text arg to the .ctr().

It should be an optional DM_INTERPOSER_FLAG added to DM_DEV_CREATE_CMD
(much like optional DM_NOFLUSH_FLAG can be used with DM_DEV_SUSPEND_CMD).

Mike