Re: [RFC][PATCH] btrfs_get_tree_subvol(): switch from fc_mount() to vfs_create_mount()

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, May 06, 2025 at 07:47:29PM +0200, Klara Modin wrote:
> On 2025-05-06 18:25:39 +0100, Al Viro wrote:
> > On Tue, May 06, 2025 at 03:36:03PM +0200, Klara Modin wrote:
> > 
> > >   25:	49 8b 44 24 60       	mov    0x60(%r12),%rax
> > 	rax = fc->root
> > >   2a:*	48 8b 78 68          	mov    0x68(%rax),%rdi		<-- trapping instruction
> > 	rdi = rax->d_sb, hitting rax == 0
> > 
> > > > -	mnt = fc_mount(dup_fc);
> > > > -	if (IS_ERR(mnt)) {
> > > > -		put_fs_context(dup_fc);
> > > > -		return PTR_ERR(mnt);
> > > > +	ret = vfs_get_tree(dup_fc);
> > > > +	if (!ret) {
> > > > +		ret = btrfs_reconfigure_for_mount(dup_fc);
> > > > +		up_write(&fc->root->d_sb->s_umount);
> > 
> > ... here.  D'oh...  Should be dup_fc, obviously - fc->root hadn't been
> > set yet.  Make that line
> > 		up_write(&dup_fc->root->d_sb->s_umount);
> > and see if it helps.  Sorry about the braino...
> 
> Thanks, that fixes the oops for me.
> 
> Though now I hit another issue which I don't know if it's related or
> not. I'm using an overlay mount with squashfs as lower and btrfs as
> upper. The mount fails with invalid argument and I see this in the log:
> 
> overlayfs: failed to clone upperpath

Seeing that you already have a kernel with that thing reverted, could
you check if the problem exists there?