Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked_super

[Al Viro]

On Tue, Sep 26, 2023 at 11:38:34AM +0200, Christoph Hellwig wrote:

> How?
> 
> Old sequence before his patch:
> 
> 	deactivate_locked_super()
> 	  -> kill_anon_super()
> 	    -> generic_shutdown_super()
> 	    -> kill_super_notify()
> 	    -> free_anon_bdev()
> 	  -> kill_super_notify()
> 
> New sequence with this patch:
> 
> 	deactivate_locked_super()
> 	  -> generic_shutdown_super()
> 	    -> kill_super_notify()
> 	    -> free_anon_bdev()
> 

Before your patch: foo_kill_super() calls kill_anon_super(),
which calls kill_super_notify(), which removes the sucker from
the list, then frees ->s_fs_info.  After your patch:
removal from the lists happens via the call of kill_super_notify()
*after* both of your methods had been called, while freeing
->s_fs_info happens from the method call.  IOW, you've restored
the situation prior to "super: ensure valid info".  The whole
point of that commit had been to make sure that we have nothing
in the lists with ->s_fs_info pointing to a freed object.

It's not about free_anon_bdev(); that part is fine - it's the
"we can drop the weird second call site of kill_super_notify()"
thing that is broken.

Al, still slogging through the rcu pathwalk races in the methods...
The latest catch: nfs_set_verifier() can get called on a dentry
that had just been seen to have positive parent, but is not
pinned down.
	grab ->d_lock; OK, we know that dentry won't get freed under us
	fetch ->d_parent->d_inode
	pass that to nfs_verify_change_attribute()
... which assumes that inode it's been given is not NULL.  Normally it
would've been - ->d_lock stabilizes ->d_parent, and negative dentries
obviously have no children.  Except that we might've been just hit
by dentry_kill() due to eviction on memory pressure, got ->d_lock
right after that and proceeded to play with ->d_parent, just as
that parent is going through dentry_kill() from the same eviction on
memory pressure...  If it gets to dentry_unlink_inode() before we get to
fetching ->d_parent->d_inode, nfs_verify_change_attribute(NULL, whatever)
is going to oops...