[PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Vivek Goyal]

Hi,

We want to be able to compile in virtiofs/9pfs in kernel and then
boot kernel and mount virtiofs/9pfs as root filesystem.

Currently there does not seem to be any good way to be able to do
that. There seem to be some hacky ways like prefixing filesystem
tag with "mtd" or naming the filesystem tag as "/dev/root" to
mount viritofs.

Both viritofs and 9pfs have the notion of a "tag" to mount a filesystem
and they take this "tag" as a source argument of the mount. Filesystem
understands how to handle the tag.

Current code already has hooks to mount mtd/ubi/cifs/nfs root
filesystems (apart of regular block based filesystems). So intead
of creating two separate hooks for two filesystems, I have tried
creating a hook for tag based filesystems. And now both the filesystems
benefit from it.

This is generic enough that I think many more use cases might be
able to take advantage of it down the line.

Vivek Goyal (2):
  init/do_mounts.c: Add a path to boot from tag based filesystems
  init/do_mounts.c: Add 9pfs to the list of tag based filesystems

 init/do_mounts.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

-- 
2.25.4

[Virtio-fs] [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Vivek Goyal]

Hi,

We want to be able to compile in virtiofs/9pfs in kernel and then
boot kernel and mount virtiofs/9pfs as root filesystem.

Currently there does not seem to be any good way to be able to do
that. There seem to be some hacky ways like prefixing filesystem
tag with "mtd" or naming the filesystem tag as "/dev/root" to
mount viritofs.

Both viritofs and 9pfs have the notion of a "tag" to mount a filesystem
and they take this "tag" as a source argument of the mount. Filesystem
understands how to handle the tag.

Current code already has hooks to mount mtd/ubi/cifs/nfs root
filesystems (apart of regular block based filesystems). So intead
of creating two separate hooks for two filesystems, I have tried
creating a hook for tag based filesystems. And now both the filesystems
benefit from it.

This is generic enough that I think many more use cases might be
able to take advantage of it down the line.

Vivek Goyal (2):
  init/do_mounts.c: Add a path to boot from tag based filesystems
  init/do_mounts.c: Add 9pfs to the list of tag based filesystems

 init/do_mounts.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

-- 
2.25.4

[PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

[Vivek Goyal]

We want to be able to mount virtiofs as rootfs and pass appropriate
kernel command line. Right now there does not seem to be a good way
to do that. If I specify "root=myfs rootfstype=virtiofs", system
panics.

virtio-fs: tag </dev/root> not found
..
..
[ end Kernel panic - not syncing: VFS: Unable to mount root fs on
+unknown-block(0,0) ]

Basic problem here is that kernel assumes that device identifier
passed in "root=" is a block device. But there are few execptions
to this rule to take care of the needs of mtd, ubi, NFS and CIFS.

For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.

"root=mtd:<identifier>" or "root=ubi:<identifier>"

NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
actual root device details come from filesystem specific kernel
command line options.

virtiofs does not seem to fit in any of the above categories. In fact
we have 9pfs which can be used to boot from but it also does not
have a proper syntax to specify rootfs and does not fit into any of
the existing syntax. They both expect a device "tag" to be passed
in a device to be mounted. And filesystem knows how to parse and
use "tag".

So there seem to be a class of filesystems which specify root device
using a "tag" which is understood by the filesystem. And filesystem
simply expects that "tag" to be passed as "source" of mount and
how to mount filesystem using that "tag" is responsibility of filesystem.

This patch proposes that we internally create a list of filesystems
which pass a "tag" in "root=<tag>" and expect that tag to be passed
as "source" of mount. With this patch I can boot into virtiofs rootfs
with following syntax.

"root=myfs rootfstype=virtiofs rw"

This patch adds support for virtiofs and next patch adds 9p to the
list.

And any other file system which is fine with these semantics,
should be able to work with it easily.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 init/do_mounts.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index a78e44ee6adb..2a18238f4962 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -31,6 +31,12 @@
 int root_mountflags = MS_RDONLY | MS_SILENT;
 static char * __initdata root_device_name;
 static char __initdata saved_root_name[64];
+static char *__initdata tag_based_rootfs[] = {
+#if IS_BUILTIN(CONFIG_VIRTIO_FS)
+	"virtiofs",
+#endif
+};
+static bool __initdata tag_based_root;
 static int root_wait;
 
 dev_t ROOT_DEV;
@@ -552,6 +558,14 @@ void __init mount_root(void)
 		return;
 	}
 #endif
+	if (tag_based_root) {
+		if (!do_mount_root(root_device_name, root_fs_names,
+				   root_mountflags, root_mount_data))
+			return;
+		panic("VFS: Unable to mount root \"%s\" via \"%s\"\n",
+		      root_device_name, root_fs_names);
+	}
+
 #ifdef CONFIG_BLOCK
 	{
 		int err = create_dev("/dev/root", ROOT_DEV);
@@ -563,6 +577,20 @@ void __init mount_root(void)
 #endif
 }
 
+static bool is_tag_based_rootfs(char *name)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(tag_based_rootfs); i++) {
+		int name_len = strlen(tag_based_rootfs[i]) + 1;
+
+		if (!strncmp(tag_based_rootfs[i], name, name_len))
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * Prepare the namespace - decide what/where to mount, load ramdisks, etc.
  */
@@ -593,6 +621,10 @@ void __init prepare_namespace(void)
 			goto out;
 		}
 		ROOT_DEV = name_to_dev_t(root_device_name);
+		if (ROOT_DEV == 0 && root_fs_names) {
+			if (is_tag_based_rootfs(root_fs_names))
+				tag_based_root = true;
+		}
 		if (strncmp(root_device_name, "/dev/", 5) == 0)
 			root_device_name += 5;
 	}
-- 
2.25.4

[Virtio-fs] [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

[Vivek Goyal]

We want to be able to mount virtiofs as rootfs and pass appropriate
kernel command line. Right now there does not seem to be a good way
to do that. If I specify "root=myfs rootfstype=virtiofs", system
panics.

virtio-fs: tag </dev/root> not found
..
..
[ end Kernel panic - not syncing: VFS: Unable to mount root fs on
+unknown-block(0,0) ]

Basic problem here is that kernel assumes that device identifier
passed in "root=" is a block device. But there are few execptions
to this rule to take care of the needs of mtd, ubi, NFS and CIFS.

For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.

"root=mtd:<identifier>" or "root=ubi:<identifier>"

NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
actual root device details come from filesystem specific kernel
command line options.

virtiofs does not seem to fit in any of the above categories. In fact
we have 9pfs which can be used to boot from but it also does not
have a proper syntax to specify rootfs and does not fit into any of
the existing syntax. They both expect a device "tag" to be passed
in a device to be mounted. And filesystem knows how to parse and
use "tag".

So there seem to be a class of filesystems which specify root device
using a "tag" which is understood by the filesystem. And filesystem
simply expects that "tag" to be passed as "source" of mount and
how to mount filesystem using that "tag" is responsibility of filesystem.

This patch proposes that we internally create a list of filesystems
which pass a "tag" in "root=<tag>" and expect that tag to be passed
as "source" of mount. With this patch I can boot into virtiofs rootfs
with following syntax.

"root=myfs rootfstype=virtiofs rw"

This patch adds support for virtiofs and next patch adds 9p to the
list.

And any other file system which is fine with these semantics,
should be able to work with it easily.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 init/do_mounts.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index a78e44ee6adb..2a18238f4962 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -31,6 +31,12 @@
 int root_mountflags = MS_RDONLY | MS_SILENT;
 static char * __initdata root_device_name;
 static char __initdata saved_root_name[64];
+static char *__initdata tag_based_rootfs[] = {
+#if IS_BUILTIN(CONFIG_VIRTIO_FS)
+	"virtiofs",
+#endif
+};
+static bool __initdata tag_based_root;
 static int root_wait;
 
 dev_t ROOT_DEV;
@@ -552,6 +558,14 @@ void __init mount_root(void)
 		return;
 	}
 #endif
+	if (tag_based_root) {
+		if (!do_mount_root(root_device_name, root_fs_names,
+				   root_mountflags, root_mount_data))
+			return;
+		panic("VFS: Unable to mount root \"%s\" via \"%s\"\n",
+		      root_device_name, root_fs_names);
+	}
+
 #ifdef CONFIG_BLOCK
 	{
 		int err = create_dev("/dev/root", ROOT_DEV);
@@ -563,6 +577,20 @@ void __init mount_root(void)
 #endif
 }
 
+static bool is_tag_based_rootfs(char *name)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(tag_based_rootfs); i++) {
+		int name_len = strlen(tag_based_rootfs[i]) + 1;
+
+		if (!strncmp(tag_based_rootfs[i], name, name_len))
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * Prepare the namespace - decide what/where to mount, load ramdisks, etc.
  */
@@ -593,6 +621,10 @@ void __init prepare_namespace(void)
 			goto out;
 		}
 		ROOT_DEV = name_to_dev_t(root_device_name);
+		if (ROOT_DEV == 0 && root_fs_names) {
+			if (is_tag_based_rootfs(root_fs_names))
+				tag_based_root = true;
+		}
 		if (strncmp(root_device_name, "/dev/", 5) == 0)
 			root_device_name += 5;
 	}
-- 
2.25.4

[PATCH v2 2/2] init/do_mounts.c: Add 9pfs to the list of tag based filesystems

[Vivek Goyal]

Add 9p to the list of tag based filesystems. I tested basic testing
with kernel command line "root=hostShared rootfstype=9p rw" and it
works for me.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 init/do_mounts.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 2a18238f4962..7c86bfdab15b 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -35,6 +35,9 @@ static char *__initdata tag_based_rootfs[] = {
 #if IS_BUILTIN(CONFIG_VIRTIO_FS)
 	"virtiofs",
 #endif
+#if IS_BUILTIN(CONFIG_9P_FS)
+	"9p",
+#endif
 };
 static bool __initdata tag_based_root;
 static int root_wait;
-- 
2.25.4

[Virtio-fs] [PATCH v2 2/2] init/do_mounts.c: Add 9pfs to the list of tag based filesystems

[Vivek Goyal]

Add 9p to the list of tag based filesystems. I tested basic testing
with kernel command line "root=hostShared rootfstype=9p rw" and it
works for me.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 init/do_mounts.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 2a18238f4962..7c86bfdab15b 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -35,6 +35,9 @@ static char *__initdata tag_based_rootfs[] = {
 #if IS_BUILTIN(CONFIG_VIRTIO_FS)
 	"virtiofs",
 #endif
+#if IS_BUILTIN(CONFIG_9P_FS)
+	"9p",
+#endif
 };
 static bool __initdata tag_based_root;
 static int root_wait;
-- 
2.25.4

Re: [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Christoph Hellwig]

Why not something like the version below that should work for all nodev
file systems?

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 74aede860de7..3c5676603fef 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -530,6 +530,39 @@ static int __init mount_cifs_root(void)
 }
 #endif
 
+static int __init mount_nodev_root(void)
+{
+	struct file_system_type *fs = get_fs_type(root_fs_names);
+	char *fs_names, *p;
+	int err = -ENODEV;
+
+	if (!fs)
+		goto out;
+	if (fs->fs_flags & FS_REQUIRES_DEV)
+		goto out_put_filesystem;
+
+	fs_names = (void *)__get_free_page(GFP_KERNEL);
+	if (!fs_names)
+		goto out_put_filesystem;
+	get_fs_names(fs_names);
+
+	for (p = fs_names; *p; p += strlen(p) + 1) {
+		err = do_mount_root(root_device_name, p, root_mountflags,
+					root_mount_data);
+		if (!err)
+			break;
+		if (err != -EACCES && err != -EINVAL)
+			panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
+				      root_device_name, p, err);
+	}
+
+	free_page((unsigned long)fs_names);
+out_put_filesystem:
+	put_filesystem(fs);
+out:
+	return err;
+}
+
 void __init mount_root(void)
 {
 #ifdef CONFIG_ROOT_NFS
@@ -546,6 +579,8 @@ void __init mount_root(void)
 		return;
 	}
 #endif
+	if (ROOT_DEV == 0 && mount_nodev_root() == 0)
+		return;
 #ifdef CONFIG_BLOCK
 	{
 		int err = create_dev("/dev/root", ROOT_DEV);

Re: [Virtio-fs] [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Christoph Hellwig]

Why not something like the version below that should work for all nodev
file systems?

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 74aede860de7..3c5676603fef 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -530,6 +530,39 @@ static int __init mount_cifs_root(void)
 }
 #endif
 
+static int __init mount_nodev_root(void)
+{
+	struct file_system_type *fs = get_fs_type(root_fs_names);
+	char *fs_names, *p;
+	int err = -ENODEV;
+
+	if (!fs)
+		goto out;
+	if (fs->fs_flags & FS_REQUIRES_DEV)
+		goto out_put_filesystem;
+
+	fs_names = (void *)__get_free_page(GFP_KERNEL);
+	if (!fs_names)
+		goto out_put_filesystem;
+	get_fs_names(fs_names);
+
+	for (p = fs_names; *p; p += strlen(p) + 1) {
+		err = do_mount_root(root_device_name, p, root_mountflags,
+					root_mount_data);
+		if (!err)
+			break;
+		if (err != -EACCES && err != -EINVAL)
+			panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
+				      root_device_name, p, err);
+	}
+
+	free_page((unsigned long)fs_names);
+out_put_filesystem:
+	put_filesystem(fs);
+out:
+	return err;
+}
+
 void __init mount_root(void)
 {
 #ifdef CONFIG_ROOT_NFS
@@ -546,6 +579,8 @@ void __init mount_root(void)
 		return;
 	}
 #endif
+	if (ROOT_DEV == 0 && mount_nodev_root() == 0)
+		return;
 #ifdef CONFIG_BLOCK
 	{
 		int err = create_dev("/dev/root", ROOT_DEV);

Re: [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Vivek Goyal]

On Thu, Jun 17, 2021 at 11:14:00AM +0100, Christoph Hellwig wrote:
> Why not something like the version below that should work for all nodev
> file systems?

Hi Christoph,

Thanks for this patch. It definitely looks much better. I had a fear
of breaking something if I were to go through this path of using
FS_REQUIRES_DEV.

This patch works for me with "root=myfs rootfstype=virtiofs rw". Have
few thoughts inline.
> 
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index 74aede860de7..3c5676603fef 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -530,6 +530,39 @@ static int __init mount_cifs_root(void)
>  }
>  #endif
>  
> +static int __init mount_nodev_root(void)
> +{
> +	struct file_system_type *fs = get_fs_type(root_fs_names);

get_fs_type() assumes root_fs_names is not null. So if I pass
"root=myfs rw", it crashes with null pointer dereference.


> +	char *fs_names, *p;
> +	int err = -ENODEV;
> +
> +	if (!fs)
> +		goto out;
> +	if (fs->fs_flags & FS_REQUIRES_DEV)
> +		goto out_put_filesystem;
> +
> +	fs_names = (void *)__get_free_page(GFP_KERNEL);
> +	if (!fs_names)
> +		goto out_put_filesystem;
> +	get_fs_names(fs_names);

I am wondering what use case we are trying to address by calling
get_fs_names() and trying do_mount_root() on all filesystems
returned by get_fs_names(). I am assuming following use cases
you have in mind.

A. User passes a single filesystem in rootfstype.
   
   root=myfs rootfstype=virtiofs rw

B. User passes multiple filesystems in rootfstype and kernel tries all
   of them one after the other

   root=myfs, rootfstype=9p,virtiofs rw

C. User does not pass a filesystem type at all. And kernel will get a
   list of in-built filesystems and will try these one after the other.

   root=myfs rw

If that's the thought, will it make sense to call get_fs_names() first
and then inside the for loop call get_fs_type() and try mounting
only if FS_REQUIRES_DEV is not set, otherwise skip and move onto th
next filesystem in the list (fs_names).

Thanks
Vivek

> +
> +	for (p = fs_names; *p; p += strlen(p) + 1) {
> +		err = do_mount_root(root_device_name, p, root_mountflags,
> +					root_mount_data);
> +		if (!err)
> +			break;
> +		if (err != -EACCES && err != -EINVAL)
> +			panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
> +				      root_device_name, p, err);
> +	}
> +
> +	free_page((unsigned long)fs_names);
> +out_put_filesystem:
> +	put_filesystem(fs);
> +out:
> +	return err;
> +}
> +
>  void __init mount_root(void)
>  {
>  #ifdef CONFIG_ROOT_NFS
> @@ -546,6 +579,8 @@ void __init mount_root(void)
>  		return;
>  	}
>  #endif
> +	if (ROOT_DEV == 0 && mount_nodev_root() == 0)
> +		return;
>  #ifdef CONFIG_BLOCK
>  	{
>  		int err = create_dev("/dev/root", ROOT_DEV);
> 

Re: [Virtio-fs] [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Vivek Goyal]

On Thu, Jun 17, 2021 at 11:14:00AM +0100, Christoph Hellwig wrote:
> Why not something like the version below that should work for all nodev
> file systems?

Hi Christoph,

Thanks for this patch. It definitely looks much better. I had a fear
of breaking something if I were to go through this path of using
FS_REQUIRES_DEV.

This patch works for me with "root=myfs rootfstype=virtiofs rw". Have
few thoughts inline.
> 
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index 74aede860de7..3c5676603fef 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -530,6 +530,39 @@ static int __init mount_cifs_root(void)
>  }
>  #endif
>  
> +static int __init mount_nodev_root(void)
> +{
> +	struct file_system_type *fs = get_fs_type(root_fs_names);

get_fs_type() assumes root_fs_names is not null. So if I pass
"root=myfs rw", it crashes with null pointer dereference.


> +	char *fs_names, *p;
> +	int err = -ENODEV;
> +
> +	if (!fs)
> +		goto out;
> +	if (fs->fs_flags & FS_REQUIRES_DEV)
> +		goto out_put_filesystem;
> +
> +	fs_names = (void *)__get_free_page(GFP_KERNEL);
> +	if (!fs_names)
> +		goto out_put_filesystem;
> +	get_fs_names(fs_names);

I am wondering what use case we are trying to address by calling
get_fs_names() and trying do_mount_root() on all filesystems
returned by get_fs_names(). I am assuming following use cases
you have in mind.

A. User passes a single filesystem in rootfstype.
   
   root=myfs rootfstype=virtiofs rw

B. User passes multiple filesystems in rootfstype and kernel tries all
   of them one after the other

   root=myfs, rootfstype=9p,virtiofs rw

C. User does not pass a filesystem type at all. And kernel will get a
   list of in-built filesystems and will try these one after the other.

   root=myfs rw

If that's the thought, will it make sense to call get_fs_names() first
and then inside the for loop call get_fs_type() and try mounting
only if FS_REQUIRES_DEV is not set, otherwise skip and move onto th
next filesystem in the list (fs_names).

Thanks
Vivek

> +
> +	for (p = fs_names; *p; p += strlen(p) + 1) {
> +		err = do_mount_root(root_device_name, p, root_mountflags,
> +					root_mount_data);
> +		if (!err)
> +			break;
> +		if (err != -EACCES && err != -EINVAL)
> +			panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
> +				      root_device_name, p, err);
> +	}
> +
> +	free_page((unsigned long)fs_names);
> +out_put_filesystem:
> +	put_filesystem(fs);
> +out:
> +	return err;
> +}
> +
>  void __init mount_root(void)
>  {
>  #ifdef CONFIG_ROOT_NFS
> @@ -546,6 +579,8 @@ void __init mount_root(void)
>  		return;
>  	}
>  #endif
> +	if (ROOT_DEV == 0 && mount_nodev_root() == 0)
> +		return;
>  #ifdef CONFIG_BLOCK
>  	{
>  		int err = create_dev("/dev/root", ROOT_DEV);
> 

Re: [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Christoph Hellwig]

On Thu, Jun 17, 2021 at 09:30:52AM -0400, Vivek Goyal wrote:
> > +static int __init mount_nodev_root(void)
> > +{
> > +	struct file_system_type *fs = get_fs_type(root_fs_names);
> 
> get_fs_type() assumes root_fs_names is not null. So if I pass
> "root=myfs rw", it crashes with null pointer dereference.

Ok, I'll need to fix that.

> > +	int err = -ENODEV;
> > +
> > +	if (!fs)
> > +		goto out;
> > +	if (fs->fs_flags & FS_REQUIRES_DEV)
> > +		goto out_put_filesystem;
> > +
> > +	fs_names = (void *)__get_free_page(GFP_KERNEL);
> > +	if (!fs_names)
> > +		goto out_put_filesystem;
> > +	get_fs_names(fs_names);
> 
> I am wondering what use case we are trying to address by calling
> get_fs_names() and trying do_mount_root() on all filesystems
> returned by get_fs_names(). I am assuming following use cases
> you have in mind.
> 
> A. User passes a single filesystem in rootfstype.
>    
>    root=myfs rootfstype=virtiofs rw
> 
> B. User passes multiple filesystems in rootfstype and kernel tries all
>    of them one after the other
> 
>    root=myfs, rootfstype=9p,virtiofs rw
> 
> C. User does not pass a filesystem type at all. And kernel will get a
>    list of in-built filesystems and will try these one after the other.
> 
>    root=myfs rw
> 
> If that's the thought, will it make sense to call get_fs_names() first
> and then inside the for loop call get_fs_type() and try mounting
> only if FS_REQUIRES_DEV is not set, otherwise skip and move onto th
> next filesystem in the list (fs_names).

I thought of A and B.  I did not think at all of C and think it is
a rather bad idea.  I'll revisit the patch to avoid C and will resend it
as a formal patch.

Re: [Virtio-fs] [PATCH v2 0/2] Add support to boot virtiofs and 9pfs as rootfs

[Christoph Hellwig]

On Thu, Jun 17, 2021 at 09:30:52AM -0400, Vivek Goyal wrote:
> > +static int __init mount_nodev_root(void)
> > +{
> > +	struct file_system_type *fs = get_fs_type(root_fs_names);
> 
> get_fs_type() assumes root_fs_names is not null. So if I pass
> "root=myfs rw", it crashes with null pointer dereference.

Ok, I'll need to fix that.

> > +	int err = -ENODEV;
> > +
> > +	if (!fs)
> > +		goto out;
> > +	if (fs->fs_flags & FS_REQUIRES_DEV)
> > +		goto out_put_filesystem;
> > +
> > +	fs_names = (void *)__get_free_page(GFP_KERNEL);
> > +	if (!fs_names)
> > +		goto out_put_filesystem;
> > +	get_fs_names(fs_names);
> 
> I am wondering what use case we are trying to address by calling
> get_fs_names() and trying do_mount_root() on all filesystems
> returned by get_fs_names(). I am assuming following use cases
> you have in mind.
> 
> A. User passes a single filesystem in rootfstype.
>    
>    root=myfs rootfstype=virtiofs rw
> 
> B. User passes multiple filesystems in rootfstype and kernel tries all
>    of them one after the other
> 
>    root=myfs, rootfstype=9p,virtiofs rw
> 
> C. User does not pass a filesystem type at all. And kernel will get a
>    list of in-built filesystems and will try these one after the other.
> 
>    root=myfs rw
> 
> If that's the thought, will it make sense to call get_fs_names() first
> and then inside the for loop call get_fs_type() and try mounting
> only if FS_REQUIRES_DEV is not set, otherwise skip and move onto th
> next filesystem in the list (fs_names).

I thought of A and B.  I did not think at all of C and think it is
a rather bad idea.  I'll revisit the patch to avoid C and will resend it
as a formal patch.

Re: [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

[Miklos Szeredi]

On Mon, 14 Jun 2021 at 19:45, Vivek Goyal <vgoyal@redhat.com> wrote:
>
> We want to be able to mount virtiofs as rootfs and pass appropriate
> kernel command line. Right now there does not seem to be a good way
> to do that. If I specify "root=myfs rootfstype=virtiofs", system
> panics.
>
> virtio-fs: tag </dev/root> not found
> ..
> ..
> [ end Kernel panic - not syncing: VFS: Unable to mount root fs on
> +unknown-block(0,0) ]
>
> Basic problem here is that kernel assumes that device identifier
> passed in "root=" is a block device. But there are few execptions
> to this rule to take care of the needs of mtd, ubi, NFS and CIFS.
>
> For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.
>
> "root=mtd:<identifier>" or "root=ubi:<identifier>"
>
> NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
> actual root device details come from filesystem specific kernel
> command line options.
>
> virtiofs does not seem to fit in any of the above categories. In fact
> we have 9pfs which can be used to boot from but it also does not
> have a proper syntax to specify rootfs and does not fit into any of
> the existing syntax. They both expect a device "tag" to be passed
> in a device to be mounted. And filesystem knows how to parse and
> use "tag".
>
> So there seem to be a class of filesystems which specify root device
> using a "tag" which is understood by the filesystem. And filesystem
> simply expects that "tag" to be passed as "source" of mount and
> how to mount filesystem using that "tag" is responsibility of filesystem.
>
> This patch proposes that we internally create a list of filesystems
> which pass a "tag" in "root=<tag>" and expect that tag to be passed
> as "source" of mount. With this patch I can boot into virtiofs rootfs
> with following syntax.
>
> "root=myfs rootfstype=virtiofs rw"

The syntax and the implementation looks good.

Acked-by: Miklos Szeredi <mszeredi@redhat.com>

Re: [Virtio-fs] [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

[Miklos Szeredi]

On Mon, 14 Jun 2021 at 19:45, Vivek Goyal <vgoyal@redhat.com> wrote:
>
> We want to be able to mount virtiofs as rootfs and pass appropriate
> kernel command line. Right now there does not seem to be a good way
> to do that. If I specify "root=myfs rootfstype=virtiofs", system
> panics.
>
> virtio-fs: tag </dev/root> not found
> ..
> ..
> [ end Kernel panic - not syncing: VFS: Unable to mount root fs on
> +unknown-block(0,0) ]
>
> Basic problem here is that kernel assumes that device identifier
> passed in "root=" is a block device. But there are few execptions
> to this rule to take care of the needs of mtd, ubi, NFS and CIFS.
>
> For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.
>
> "root=mtd:<identifier>" or "root=ubi:<identifier>"
>
> NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
> actual root device details come from filesystem specific kernel
> command line options.
>
> virtiofs does not seem to fit in any of the above categories. In fact
> we have 9pfs which can be used to boot from but it also does not
> have a proper syntax to specify rootfs and does not fit into any of
> the existing syntax. They both expect a device "tag" to be passed
> in a device to be mounted. And filesystem knows how to parse and
> use "tag".
>
> So there seem to be a class of filesystems which specify root device
> using a "tag" which is understood by the filesystem. And filesystem
> simply expects that "tag" to be passed as "source" of mount and
> how to mount filesystem using that "tag" is responsibility of filesystem.
>
> This patch proposes that we internally create a list of filesystems
> which pass a "tag" in "root=<tag>" and expect that tag to be passed
> as "source" of mount. With this patch I can boot into virtiofs rootfs
> with following syntax.
>
> "root=myfs rootfstype=virtiofs rw"

The syntax and the implementation looks good.

Acked-by: Miklos Szeredi <mszeredi@redhat.com>

Re: [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

[Vivek Goyal]

On Fri, Jun 18, 2021 at 09:07:14AM +0200, Miklos Szeredi wrote:
> On Mon, 14 Jun 2021 at 19:45, Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > We want to be able to mount virtiofs as rootfs and pass appropriate
> > kernel command line. Right now there does not seem to be a good way
> > to do that. If I specify "root=myfs rootfstype=virtiofs", system
> > panics.
> >
> > virtio-fs: tag </dev/root> not found
> > ..
> > ..
> > [ end Kernel panic - not syncing: VFS: Unable to mount root fs on
> > +unknown-block(0,0) ]
> >
> > Basic problem here is that kernel assumes that device identifier
> > passed in "root=" is a block device. But there are few execptions
> > to this rule to take care of the needs of mtd, ubi, NFS and CIFS.
> >
> > For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.
> >
> > "root=mtd:<identifier>" or "root=ubi:<identifier>"
> >
> > NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
> > actual root device details come from filesystem specific kernel
> > command line options.
> >
> > virtiofs does not seem to fit in any of the above categories. In fact
> > we have 9pfs which can be used to boot from but it also does not
> > have a proper syntax to specify rootfs and does not fit into any of
> > the existing syntax. They both expect a device "tag" to be passed
> > in a device to be mounted. And filesystem knows how to parse and
> > use "tag".
> >
> > So there seem to be a class of filesystems which specify root device
> > using a "tag" which is understood by the filesystem. And filesystem
> > simply expects that "tag" to be passed as "source" of mount and
> > how to mount filesystem using that "tag" is responsibility of filesystem.
> >
> > This patch proposes that we internally create a list of filesystems
> > which pass a "tag" in "root=<tag>" and expect that tag to be passed
> > as "source" of mount. With this patch I can boot into virtiofs rootfs
> > with following syntax.
> >
> > "root=myfs rootfstype=virtiofs rw"
> 
> The syntax and the implementation looks good.
> 
> Acked-by: Miklos Szeredi <mszeredi@redhat.com>

Thanks Miklos. Now Christoph has come up with another patch series to
achieve the same. And that patches series looks better than my patches.

https://lore.kernel.org/linux-fsdevel/20210617153649.1886693-1-hch@lst.de/

There are couple of minor issues to be taken care of. I am hoping after
that these patches can be merged.

Thanks
Vivek

Re: [Virtio-fs] [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

[Vivek Goyal]

On Fri, Jun 18, 2021 at 09:07:14AM +0200, Miklos Szeredi wrote:
> On Mon, 14 Jun 2021 at 19:45, Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > We want to be able to mount virtiofs as rootfs and pass appropriate
> > kernel command line. Right now there does not seem to be a good way
> > to do that. If I specify "root=myfs rootfstype=virtiofs", system
> > panics.
> >
> > virtio-fs: tag </dev/root> not found
> > ..
> > ..
> > [ end Kernel panic - not syncing: VFS: Unable to mount root fs on
> > +unknown-block(0,0) ]
> >
> > Basic problem here is that kernel assumes that device identifier
> > passed in "root=" is a block device. But there are few execptions
> > to this rule to take care of the needs of mtd, ubi, NFS and CIFS.
> >
> > For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.
> >
> > "root=mtd:<identifier>" or "root=ubi:<identifier>"
> >
> > NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
> > actual root device details come from filesystem specific kernel
> > command line options.
> >
> > virtiofs does not seem to fit in any of the above categories. In fact
> > we have 9pfs which can be used to boot from but it also does not
> > have a proper syntax to specify rootfs and does not fit into any of
> > the existing syntax. They both expect a device "tag" to be passed
> > in a device to be mounted. And filesystem knows how to parse and
> > use "tag".
> >
> > So there seem to be a class of filesystems which specify root device
> > using a "tag" which is understood by the filesystem. And filesystem
> > simply expects that "tag" to be passed as "source" of mount and
> > how to mount filesystem using that "tag" is responsibility of filesystem.
> >
> > This patch proposes that we internally create a list of filesystems
> > which pass a "tag" in "root=<tag>" and expect that tag to be passed
> > as "source" of mount. With this patch I can boot into virtiofs rootfs
> > with following syntax.
> >
> > "root=myfs rootfstype=virtiofs rw"
> 
> The syntax and the implementation looks good.
> 
> Acked-by: Miklos Szeredi <mszeredi@redhat.com>

Thanks Miklos. Now Christoph has come up with another patch series to
achieve the same. And that patches series looks better than my patches.

https://lore.kernel.org/linux-fsdevel/20210617153649.1886693-1-hch@lst.de/

There are couple of minor issues to be taken care of. I am hoping after
that these patches can be merged.

Thanks
Vivek