Re: [RFC PATCH 1/5] fs: introduce notifier list for vfs inode

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

On Thu, Oct 15, 2020 at 11:42:51AM +0800, Chengguang Xu wrote:
>  ---- 在 星期四, 2020-10-15 11:25:01 Al Viro <viro@zeniv.linux.org.uk> 撰写 ----
>  > On Sat, Oct 10, 2020 at 10:23:51PM +0800, Chengguang Xu wrote:
>  > > Currently there is no notification api for kernel about modification
>  > > of vfs inode, in some use cases like overlayfs, this kind of notification
>  > > will be very helpful to implement containerized syncfs functionality.
>  > > As the first attempt, we introduce marking inode dirty notification so that
>  > > overlay's inode could mark itself dirty as well and then only sync dirty
>  > > overlay inode while syncfs.
>  > 
>  > Who's responsible for removing the crap from notifier chain?  And how does
>  > that affect the lifetime of inode?
>  
> In this case, overlayfs unregisters call back from the notifier chain of upper inode
> when evicting it's own  inode. It will not affect the lifetime of upper inode because
> overlayfs inode holds a reference of upper inode that means upper inode will not be
> evicted while overlayfs inode is still alive.

Let me see if I've got it right:
	* your chain contains 1 (for upper inodes) or 0 (everything else, i.e. the
vast majority of inodes) recepients
	* recepient pins the inode for as long as the recepient exists

That looks like a massive overkill, especially since all you are propagating is
dirtying the suckers.  All you really need is one bit in your inode + hash table
indexed by the address of struct inode (well, middle bits thereof, as usual).
With entries embedded into overlayfs-private part of overlayfs inode.  And callback
to be called stored in that entry...