Re: [RFC][overlayfs] do we still need d_instantiate_anon() and export of d_alloc_anon()?

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

On Sat, Nov 11, 2023 at 08:31:11PM +0200, Amir Goldstein wrote:
> > in ovl_lookup(), and in case we have d_splice_alias() return a non-NULL
> > dentry we can simply copy it there.  Sure, somebody might race with
> > us, pick dentry from hash and call ->d_revalidate() before we notice that
> > DCACHE_OP_REVALIDATE could be cleaned.  So what?  That call of ->d_revalidate()
> > will find nothing to do and return 1.  Which is the effect of having
> > DCACHE_OP_REVALIDATE cleared, except for pointless method call.  Anyone
> > who finds that dentry after the flag is cleared will skip the call.
> > IOW, that race is harmless.
> 
> Just a minute.
> Do you know that ovl_obtain_alias() is *only* used to obtain a disconnected
> non-dir overlayfs dentry?

D'oh...

> I think that makes all the analysis regarding race with d_splice_alias()
> moot. Right?

Right you are.

> Do DCACHE_OP_*REVALIDATE even matter for a disconnected
> non-dir dentry?

As long as nothing picks it via d_find_any_alias() and moves it somewhere
manually.  The former might happen, the latter, AFAICS, doesn't - nothing
like d_move() anywhere in sight...

> You are missing that the OVL_E_UPPER_ALIAS flag is a property of
> the overlay dentry, not a property of the inode.
> 
> N lower hardlinks, the first copy up created an upper inode
> all the rest of the N upper aliases to that upper inode are
> created lazily.
> 
> However, for obvious reasons, OVL_E_UPPER_ALIAS is not
> well defined for a disconnected overlay dentry.
> There should not be any code (I hope) that cares about
> OVL_E_UPPER_ALIAS for a disconnected overlay dentry,
> so I *think* ovl_dentry_set_upper_alias() in this code is moot.
> 
> I need to look closer to verify, but please confirm my assumption
> regarding the irrelevance of  DCACHE_OP_*REVALIDATE for a
> disconnected non-dir dentry.

Correct; we only care if it gets reconnected to the main tree.
The fact that it's only for non-directories simplifies life a lot
there.  Sorry, got confused by the work you do with ->d_flags
and hadn't stopped to ask whether it's needed in the first place
in there.

OK, so... are there any reasons why simply calling d_obtain_alias()
wouldn't do the right thing these days?