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

[Gao Xiang]

Hi Christian,

On 2024/3/7 17:17, Christian Brauner wrote:
> 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@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.
> >
> > 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@xxxxxx
> >
> > 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.

Yeah, I think some time this year we will finish a formal
page cache sharing design and implementation for both bdev
and fscache mode.

So a separate filesystem type seems more reasonable in the
future,  thanks for your confirmation!

Thanks,
Gao Xiang