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

[Baokun Li]

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@xxxxxxxxxx>
> > 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.
Thank you for your feedback!

If I understand you correctly, you mean to remove erofs_pseudo_mnt
directly to avoid this false positive, and use anon_inode_create_getfile()
to create the required anonymous inode.

--
With Best Regards,
Baokun Li
.