Re: [PATCH v4 1/3] ext4: add discard/zeroout flags to journal flush

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, May 13, 2021 at 04:27:23PM -0400, Leah Rumancik wrote:
> 
> Just to make sure I understand correctly, the explicit __u32 is critical
> due to the size being read in by the ioctl, specifically through
> copy_from_user? When do you switch from __u32 to unsigned long? I don't
> see the __* types being carried throughout.

What I was thinking was something like this:

static int ext4_foo_ioctl(struct super_block *sb, unsigned long arg)
{
	__u32 flags;
	unsigned int f = 0;

	if (get_user(flags, (__u32 __user *) arg))
		return -EFAULT;

	if (flags & EXT4_IOC_FOO_AAA)
		f |= JBD2_FLAGS_FOO_AAA;
		
	if (flags & EXT4_IOC_FOO_BBB)
		f |= JBD2_FLAGS_FOO_BBB;
	...

	jbd_foo(sb, f);
}

So there are two separate flag namespaces, one exported to userspace
which is __u32, and which are the EXT4_IOC_FOO_*, defined in
fs/ext4/ext4.h.  (Actually, we should move the ext4 ioctls to a header
file like under include/uapi/..., maybe include/uapi/linux/ext4.h, or
include/uapi/fs/ext4.h, but that's a different patch series.)


And then there is a second flag namespace, JBD2_FLAGS_FOO_*, which is
defined in include/linux/jbd2.h, which is an unsigned int, and which
is a kernel-internal interface.

> (Also, just got Darrick's reply about the 32 vs. 64. Yes, originally
> went with 64 because there was an argument for it. I believe the 32
> is likely sufficient, but I don't feel that strongly about this matter)

Sure, but for EXT4_IOC_SHUTDOWN?  We have 3 flags defined today, and
I'm not convinced we'll have 12 more flags defined, let alone enough
that we really need a 64-bit flags.

> > What you probably want is something like:
> > 
> > #define JBD2_JOURNAL_FLUSH_DISCARD	0x0001
> > #define JBD2_JOURNAL_FLUSH_ZEROOUT	0x0002
> > #define JBD2_JOURNAL_FLUSH_VALID	0x0003
> > 
> 
> Why call them JBD2_JOURNAL_FLUSH* instead of JBD2_JOURNAL_ERASE* since
> they get passed directly through to the erase function? I feel like it
> would be weird if someone wanted to use the erase function directly but
> had to use JBD2_JOURNAL_FLUSH* flags.

The erase function is a static function that's not exported outside of
fs/jbd2.  The interface which is exposed to kernel callers outside of
fs/jbd2 is jbd2_journal_flush().  Since that's the public interface,
the flags should be similarly defined that way.

Cheers,

					- Ted