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Message-ID: <20260108122713.00007e54@huawei.com>
Date: Thu, 8 Jan 2026 12:27:13 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: John Groves <John@...ves.net>
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 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.
> + 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.
> struct dax_device *inode_dax(struct inode *inode);
> #endif
> void *dax_holder(struct dax_device *dax_dev);
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