Re: [PATCH V12 09/12] famfs: Register secondary daxdevs by path (FAMFSIOC_DAXDEV_OPEN)

[John Groves <John@groves.net>]

On 26/08/05 10:29PM, Darrick J. Wong wrote:
> On Mon, Aug 03, 2026 at 02:29:47AM +0000, John Groves wrote:
> > From: John Groves <John@Groves.net>
> > 
> > Famfs file maps (fmaps) may reference multiple daxdevs. Before passing
> > an fmap that references a new daxdev, the daxdev is pushed into the
> > kernel via FAMFSIOC_DAXDEV_OPEN). This adds daxdevs to daxdev_table for
> > index-based resolution from famfs extents to daxdevs.
> > 
> > Signed-off-by: John Groves <john@groves.net>
> > ---
> >  fs/famfs/famfs_file.c            | 74 ++++++++++++++++++++++++++++++++
> >  include/uapi/linux/famfs_ioctl.h | 24 +++++++++++
> >  2 files changed, 98 insertions(+)
> > 
> > diff --git a/fs/famfs/famfs_file.c b/fs/famfs/famfs_file.c
> > index e7f271ce6d03..e11a55ecf8d7 100644
> > --- a/fs/famfs/famfs_file.c
> > +++ b/fs/famfs/famfs_file.c
> > @@ -287,6 +287,76 @@ famfs_file_init_dax(struct file *file, void __user *arg)
> >  	return rc;
> >  }
> >  
> > +/**
> > + * famfs_daxdev_open() - FAMFSIOC_DAXDEV_OPEN ioctl handler
> > + * @file: any file in the famfs mount (the table is per-superblock)
> > + * @arg:  ptr to struct famfs_ioc_daxdev in user space
> > + *
> > + * Register a devdax device (identified by path) into the mount's daxdev table
> > + * at the caller-specified index, so files whose extents reference that index
> > + * can be mapped. The path is resolved by lookup_daxdev() - the same helper the
> > + * mount uses for the primary daxdev - so every slot is resolved identically.
> > + * Registering exposes raw device memory, so it requires CAP_SYS_RAWIO.
> > + */
> > +static int
> > +famfs_daxdev_open(struct file *file, void __user *arg)
> > +{
> > +	struct super_block *sb = file_inode(file)->i_sb;
> > +	struct famfs_fs_info *fsi = sb->s_fs_info;
> > +	struct famfs_ioc_daxdev dd;
> > +	dev_t devno;
> > +	char *path;
> > +	int rc;
> > +
> > +	if (!capable(CAP_SYS_RAWIO))
> > +		return -EPERM;
> > +
> > +	if (copy_from_user(&dd, arg, sizeof(dd)))
> > +		return -EFAULT;
> > +
> > +	/* @flags is reserved; reject non-zero so it stays available */
> > +	if (dd.flags)
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * If this daxdev index is already populated there is nothing to do.
> > +	 * The index is cluster-invariant, so a valid slot already names this
> > +	 * device; skip the path resolution entirely. install_daxdev() rechecks
> > +	 * ->valid under the write lock, so this is purely an optimization.
> > +	 */
> > +	scoped_guard(rwsem_read, &fsi->devlist_sem) {
> > +		if (dd.daxdev_index >= fsi->dax_devlist->nslots)
> > +			return -EINVAL;
> > +		if (fsi->dax_devlist->devlist[dd.daxdev_index].valid)
> > +			return 0;
> > +	}
> > +
> > +	if (dd.daxdev_path_len == 0 || dd.daxdev_path_len >= PATH_MAX)
> > +		return -EINVAL;
> > +
> > +	/* +1 so the terminating NUL is included within the bound */
> > +	path = strndup_user((const char __user *)(uintptr_t)dd.daxdev_path,
> > +			    dd.daxdev_path_len + 1);
> > +	if (IS_ERR(path))
> > +		return PTR_ERR(path);
> > +
> > +	rc = lookup_daxdev(path, &devno);
> > +	if (rc)
> > +		goto out;
> > +
> > +	/*
> > +	 * The daxdev table is allocated at mount time (for the slot-0 primary),
> > +	 * so it is always present here; no need to allocate it.
> > +	 */
> > +	rc = famfs_install_daxdev(fsi, sb, dd.daxdev_index, devno, path);
> > +	if (rc)
> > +		pr_debug("%s: failed to install daxdev index %llu (%s)\n",
> > +		       __func__, dd.daxdev_index, path);
> > +out:
> > +	kfree(path);
> > +	return rc;
> > +}
> > +
> >  /**
> >   * famfs_file_ioctl() - Top-level famfs file ioctl handler
> >   * @file: the file
> > @@ -308,6 +378,10 @@ famfs_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> >  		rc = 0;
> >  		break;
> >  
> > +	case FAMFSIOC_DAXDEV_OPEN:
> > +		rc = famfs_daxdev_open(file, (void __user *)arg);
> > +		break;
> > +
> >  	case FAMFSIOC_MAP_CREATE:
> >  		rc = famfs_file_init_dax(file, (void __user *)arg);
> >  		break;
> > diff --git a/include/uapi/linux/famfs_ioctl.h b/include/uapi/linux/famfs_ioctl.h
> > index b4eb373c1ade..751d8b033c2e 100644
> > --- a/include/uapi/linux/famfs_ioctl.h
> > +++ b/include/uapi/linux/famfs_ioctl.h
> > @@ -77,6 +77,29 @@ struct famfs_ioc_fmap_header {
> >  	__u64 reserved1;
> >  };
> >  
> > +/**
> > + * struct famfs_ioc_daxdev - register an additional backing daxdev by path
> > + * @daxdev_index:    the (cluster-invariant) index this daxdev occupies in
> > + *                   extent dev_index fields. Index 0 is the mount-time primary.
> > + * @daxdev_path:     userspace pointer to the devdax device path (e.g.
> > + *                   "/dev/dax0.0"); resolved in the kernel the same way the
> > + *                   mount primary is.
> > + * @daxdev_path_len: length of the path string, not counting the NUL.
> > + * @flags:           reserved; must be zero.
> > + *
> > + * Standalone famfs registers every daxdev by path: the mount primary comes in
> > + * as the mount device name, and slots 1..n come in here. (This deliberately
> > + * differs from fuse's fd-based FUSE_DEV_IOC_DAXDEV_OPEN; each side is uniform
> > + * within itself.) Passing the path by pointer keeps the struct fixed-size, so
> > + * longer paths never require an ABI change.
> > + */
> > +struct famfs_ioc_daxdev {
> > +	__u64 daxdev_index;
> > +	__u64 daxdev_path;
> > +	__u32 daxdev_path_len;
> 
> Can't we just pass an open fd to a dax device and have it use that
> rather than doing its own path operations?
> 
> --D

Could do, but then there would be two ways to "open" a daxdev, since the primary
is passed by path via the mount command or call.

If anybody thinks that's superior enough to do it 2 different ways, let me know.
Otherwise I'll leave it.

Thanks!
John

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