[PATCH] erofs: fix lockdep false positives on initializing erofs_pseudo_mnt

[Christian Brauner]

On Thu, Mar 07, 2024 at 12:18:52PM +0800, Gao Xiang wrote:
> Hi,
> 
> (try to +Cc Christian and Al here...)
> 
> On 2024/3/7 11:41, Jingbo Xu wrote:
> > Hi Baokun,
> > 
> > Thanks for catching this!
> > 
> > 
> > On 3/7/24 10:52 AM, Gao Xiang wrote:
> > > Hi Baokun,
> > > 
> > > On 2024/3/7 10:44, Baokun Li wrote:
> > > > Lockdep reported the following issue when mounting erofs with a
> > > > domain_id:
> > > > 
> > > > ============================================
> > > > WARNING: possible recursive locking detected
> > > > 6.8.0-rc7-xfstests #521 Not tainted
> > > > --------------------------------------------
> > > > mount/396 is trying to acquire lock:
> > > > ffff907a8aaaa0e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
> > > >                          at: alloc_super+0xe3/0x3d0
> > > > 
> > > > but task is already holding lock:
> > > > ffff907a8aaa90e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
> > > >                          at: alloc_super+0xe3/0x3d0
> > > > 
> > > > other info that might help us debug this:
> > > >    Possible unsafe locking scenario:
> > > > 
> > > >          CPU0
> > > >          ----
> > > >     lock(&type->s_umount_key#50/1);
> > > >     lock(&type->s_umount_key#50/1);
> > > > 
> > > >    *** DEADLOCK ***
> > > > 
> > > >    May be due to missing lock nesting notation
> > > > 
> > > > 2 locks held by mount/396:
> > > >    #0: ffff907a8aaa90e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
> > > >              at: alloc_super+0xe3/0x3d0
> > > >    #1: ffffffffc00e6f28 (erofs_domain_list_lock){+.+.}-{3:3},
> > > >              at: erofs_fscache_register_fs+0x3d/0x270 [erofs]
> > > > 
> > > > stack backtrace:
> > > > CPU: 1 PID: 396 Comm: mount Not tainted 6.8.0-rc7-xfstests #521
> > > > Call Trace:
> > > >    <TASK>
> > > >    dump_stack_lvl+0x64/0xb0
> > > >    validate_chain+0x5c4/0xa00
> > > >    __lock_acquire+0x6a9/0xd50
> > > >    lock_acquire+0xcd/0x2b0
> > > >    down_write_nested+0x45/0xd0
> > > >    alloc_super+0xe3/0x3d0
> > > >    sget_fc+0x62/0x2f0
> > > >    vfs_get_super+0x21/0x90
> > > >    vfs_get_tree+0x2c/0xf0
> > > >    fc_mount+0x12/0x40
> > > >    vfs_kern_mount.part.0+0x75/0x90
> > > >    kern_mount+0x24/0x40
> > > >    erofs_fscache_register_fs+0x1ef/0x270 [erofs]
> > > >    erofs_fc_fill_super+0x213/0x380 [erofs]
> > > > 
> > > > This is because the file_system_type of both erofs and the pseudo-mount
> > > > point of domain_id is erofs_fs_type, so two successive calls to
> > > > alloc_super() are considered to be using the same lock and trigger the
> > > > warning above.
> > > > 
> > > > Therefore add a nodev file_system_type named erofs_anon_fs_type to
> > > > silence this complaint. In addition, to reduce code coupling, refactor
> > > > out the erofs_anon_init_fs_context() and erofs_kill_pseudo_sb() functions
> > > > and move the erofs_pseudo_mnt related code to fscache.c.
> > > > 
> > > > Signed-off-by: Baokun Li <libaokun1 at huawei.com>
> > > 
> > > IMHO, in the beginning, I'd like to avoid introducing another fs type
> > > for erofs to share (meta)data between filesystems since it will cause
> > > churn, could we use some alternative way to resolve this?
> > 
> > Yeah as Gao Xiang said, this is initially intended to avoid introducing
> > anothoer file_system_type, say erofs_anon_fs_type.
> > 
> > What we need is actually a method of allocating anonymous inode as a
> > sentinel identifying each blob.  There is indeed a global mount, i.e.
> > anon_inode_mnt, for allocating anonymous inode/file specifically.  At
> > the time the share domain feature is introduced, there's only one
> > anonymous inode, i.e. anon_inode_inode, and all the allocated anonymous
> > files are bound to this single anon_inode_inode.  Thus we decided to
> > implement a erofs internal pseudo mount for this usage.
> > 
> > But I noticed that we can now allocate unique anonymous inodes from
> > anon_inode_mnt since commit e7e832c ("fs: add LSM-supporting anon-inode
> > interface"), though the new interface is initially for LSM usage.
> 
> Yes, as summary, EROFS now maintains a bunch of anon inodes among
> all different filesystem instances, so that like
> 
> blob sharing or
> page cache sharing across filesystems can be done.
> 
> In brief, I think the following patch is a good idea but it
> hasn't been landed until now:
> https://lore.kernel.org/r/20210309155348.974875-3-hch@lst.de
> 
> Other than that, is it a good idea to introduce another fs type
> (like erofs_anon_fs_type) for such usage?

It depends. If you're allocating a lot of inodes then having a separate
filesystem type for erofs makes sense. If it's just a few then it
probably doesn't matter. If you need custom inode operations for these
anonymous inodes then it also makes sense to have a separate filesystem
type.