linux-kernel - Re: [PATCH V3 06/21] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

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Message-ID: <d7blicnyehixccalrrmw4wschyngqkjt5jdleesjqcjtwcav4a@wblo3f53shkh>
Date: Thu, 8 Jan 2026 10:45:37 -0600
From: John Groves <John@...ves.net>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Miklos Szeredi <miklos@...redi.hu>, 
	Dan Williams <dan.j.williams@...el.com>, Bernd Schubert <bschubert@....com>, 
	Alison Schofield <alison.schofield@...el.com>, John Groves <jgroves@...ron.com>, 
	Jonathan Corbet <corbet@....net>, Vishal Verma <vishal.l.verma@...el.com>, 
	Dave Jiang <dave.jiang@...el.com>, Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, David Hildenbrand <david@...nel.org>, 
	Christian Brauner <brauner@...nel.org>, "Darrick J . Wong" <djwong@...nel.org>, 
	Randy Dunlap <rdunlap@...radead.org>, Jeff Layton <jlayton@...nel.org>, 
	Amir Goldstein <amir73il@...il.com>, Stefan Hajnoczi <shajnocz@...hat.com>, 
	Joanne Koong <joannelkoong@...il.com>, Josef Bacik <josef@...icpanda.com>, 
	Bagas Sanjaya <bagasdotme@...il.com>, Chen Linxuan <chenlinxuan@...ontech.com>, 
	James Morse <james.morse@....com>, Fuad Tabba <tabba@...gle.com>, 
	Sean Christopherson <seanjc@...gle.com>, Shivank Garg <shivankg@....com>, 
	Ackerley Tng <ackerleytng@...gle.com>, Gregory Price <gourry@...rry.net>, 
	Aravind Ramesh <arramesh@...ron.com>, Ajay Joshi <ajayjoshi@...ron.com>, venkataravis@...ron.com, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, 
	linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V3 06/21] dax: Add fs_dax_get() func to prepare dax for
 fs-dax usage

On 26/01/08 12:27PM, Jonathan Cameron wrote:
> On Wed,  7 Jan 2026 09:33:15 -0600
> John Groves <John@...ves.net> wrote:
> 
> > The fs_dax_get() function should be called by fs-dax file systems after
> > opening a fsdev dax device. This adds holder_operations, which provides
> > a memory failure callback path and effects exclusivity between callers
> > of fs_dax_get().
> > 
> > fs_dax_get() is specific to fsdev_dax, so it checks the driver type
> > (which required touching bus.[ch]). fs_dax_get() fails if fsdev_dax is
> > not bound to the memory.
> > 
> > This function serves the same role as fs_dax_get_by_bdev(), which dax
> > file systems call after opening the pmem block device.
> > 
> > This can't be located in fsdev.c because struct dax_device is opaque
> > there.
> > 
> > This will be called by fs/fuse/famfs.c in a subsequent commit.
> > 
> > Signed-off-by: John Groves <john@...ves.net>
> Hi John,
> 
> A few passing comments on this one.
> 
> Jonathan
> 
> > ---
> 
> >  #define dax_driver_register(driver) \
> > diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> > index ba0b4cd18a77..68c45b918cff 100644
> > --- a/drivers/dax/super.c
> > +++ b/drivers/dax/super.c
> > @@ -14,6 +14,7 @@
> >  #include <linux/fs.h>
> >  #include <linux/cacheinfo.h>
> >  #include "dax-private.h"
> > +#include "bus.h"
> >  
> >  /**
> >   * struct dax_device - anchor object for dax services
> > @@ -121,6 +122,59 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
> >  EXPORT_SYMBOL_GPL(fs_put_dax);
> >  #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> >  
> > +#if IS_ENABLED(CONFIG_DEV_DAX_FS)
> > +/**
> > + * fs_dax_get() - get ownership of a devdax via holder/holder_ops
> > + *
> > + * fs-dax file systems call this function to prepare to use a devdax device for
> > + * fsdax. This is like fs_dax_get_by_bdev(), but the caller already has struct
> > + * dev_dax (and there is no bdev). The holder makes this exclusive.
> > + *
> > + * @dax_dev: dev to be prepared for fs-dax usage
> > + * @holder: filesystem or mapped device inside the dax_device
> > + * @hops: operations for the inner holder
> > + *
> > + * Returns: 0 on success, <0 on failure
> > + */
> > +int fs_dax_get(struct dax_device *dax_dev, void *holder,
> > +	const struct dax_holder_operations *hops)
> > +{
> > +	struct dev_dax *dev_dax;
> > +	struct dax_device_driver *dax_drv;
> > +	int id;
> > +
> > +	id = dax_read_lock();
> 
> Given this is an srcu_read_lock under the hood you could do similar
> to the DEFINE_LOCK_GUARD_1 for the srcu (srcu.h) (though here it's a
> DEFINE_LOCK_GUARD_0 given the lock itself isn't a parameter and then
> use scoped_guard() here.  Might not be worth the hassle and would need
> a wrapper macro to poke &dax_srcu in which means exposing that at least
> a little in a header.
> 
> DEFINE_LOCK_GUARD_0(_T->idx = dax_read_lock, dax_read_lock(_T->idx), idx);
> Based loosely on the irqflags.h irqsave one. 

I'm getting more comfortable with scoped_guard(), but this feels like
a good leanup patch addressing all call sites of dax_read_lock() - after
the famfs dust settles.

If feelings are strong about this I'm open...

> 
> > +	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode)) {
> > +		dax_read_unlock(id);
> > +		return -ENODEV;
> > +	}
> > +	dax_read_unlock(id);
> > +
> > +	/* Verify the device is bound to fsdev_dax driver */
> > +	dev_dax = dax_get_private(dax_dev);
> > +	if (!dev_dax || !dev_dax->dev.driver) {
> > +		iput(&dax_dev->inode);
> > +		return -ENODEV;
> > +	}
> > +
> > +	dax_drv = to_dax_drv(dev_dax->dev.driver);
> > +	if (dax_drv->type != DAXDRV_FSDEV_TYPE) {
> > +		iput(&dax_dev->inode);
> > +		return -EOPNOTSUPP;
> > +	}
> > +
> > +	if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
> > +		iput(&dax_dev->inode);
> > +		return -EBUSY;
> > +	}
> > +
> > +	dax_dev->holder_ops = hops;
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(fs_dax_get);
> > +#endif /* DEV_DAX_FS */
> > +
> >  enum dax_device_flags {
> >  	/* !alive + rcu grace period == no new operations / mappings */
> >  	DAXDEV_ALIVE,
> > diff --git a/include/linux/dax.h b/include/linux/dax.h
> > index 3fcd8562b72b..76f2a75f3144 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -53,6 +53,7 @@ struct dax_holder_operations {
> >  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
> >  
> >  #if IS_ENABLED(CONFIG_DEV_DAX_FS)
> > +int fs_dax_get(struct dax_device *dax_dev, void *holder, const struct dax_holder_operations *hops);
> I'd wrap this.  It's rather long and there isn't a huge readability benefit in keeping
> it on one line.

Done, thanks!

John