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

[Jan Kara <jack@suse.cz>]

On Fri 16-10-20 15:09:38, Chengguang Xu wrote:
>  ---- 在 星期四, 2020-10-15 12:57:41 Al Viro <viro@zeniv.linux.org.uk> 撰写 ----
>  > 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...
>  > 
> 
> Hi AI, Jack, Amir
> 
> Based on your feedback, I would to change the inode dirty notification
> something like below, is it acceptable? 
> 
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 1492271..48473d9 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -2249,6 +2249,14 @@ void __mark_inode_dirty(struct inode *inode, int flags)
>  
>         trace_writeback_mark_inode_dirty(inode, flags);
>  
> +       if (inode->state & I_OVL_INUSE) {
> +               struct inode *ovl_inode;
> +
> +               ovl_inode = ilookup5(NULL, (unsigned long)inode, ovl_inode_test, inode);

I don't think this will work - superblock pointer is part of the hash value
inode is hashed with so without proper sb pointer you won't find proper
hash chain.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR