Re: [RFC] Converging userspace and kernel code

[Eric Sandeen <sandeen@redhat.com>]

On 1/8/17 8:11 PM, Qu Wenruo wrote:
> 
> 
> At 01/08/2017 09:16 PM, Goldwyn Rodrigues wrote:
>>
>> 1. Motivation
>> While fixing user space tools for btrfs-progs, I found a couple of bugs
>> which are already solved in kernel space but were not ported to user
>> space. User space is a little ignored when it comes to fixing bugs in
>> the core functionality. XFS developers have already performed this and
>> the userspace and kernel code walks in lockstep for libxfs.
> 
> Personally speaking, I'm not a fan of re-using kernel code in btrfs-progs.

But it already does re-use kernel code, it's just that the re-use is
extremely stale, with unfixed bugs in both directions as a result
(at least last time I looked.)

> In fact, in btrfs-progs, we don't need a lot of kernel facilities,
> like page/VFS/lock(btrfs-progs works in single thread under most
> case).
> 
> And that should make btrfs-progs easier to maintain.

But as Goldwyn already pointed out, many bugs have gone un-fixed
in userspace, in code which was forked long ago from kernelspace.

For things like locking it's trivial to define that away.

xfsprogs does i.e. -

/* miscellaneous kernel routines not in user space */
#define down_read(a)            ((void) 0)
#define up_read(a)              ((void) 0)
#define spin_lock_init(a)       ((void) 0)
#define spin_lock(a)            ((void) 0)
#define spin_unlock(a)          ((void) 0)
#define likely(x)               (x)
#define unlikely(x)             (x)
#define rcu_read_lock()         ((void) 0)
#define rcu_read_unlock()       ((void) 0)
#define WARN_ON_ONCE(expr)      ((void) 0)


> Furthermore, there are cases while kernel is doing things wrong while
> btrfs-progs does it right.

All the more reason to sync it up, fixes should always be in both
places, right?

I had looked at this a few years ago, and started trying to sync things
up, but got daunted and busy and never completed anything.  :(  I sent
a few fixups back in April 2013 to get things /slightly/ closer.

The libxfs sync in xfs has borne fruit; I'm of the opinion that similar
work would help btrfs too, though it can be a long road.

(e2fsprogs has gone the other way, and has a completely separate
re-implementation in userspace; it works, I guess, but I have to say
that I really like the code commonality in xfs.)

Thanks,
-Eric