Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

[Al Viro]

On Thu, Feb 13, 2020 at 01:30:11PM -0800, Linus Torvalds wrote:
> On Wed, Feb 12, 2020 at 9:55 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > What I don't understand is the insistence on getting those dentries
> > via dcache lookups.
> 
> I don't think that's an "insistence", it's more of a "historical
> behavior" together with "several changes over the years to deal with
> dentry-level cleanups and updates".
> 
> > _IF_ we are willing to live with cacheline
> > contention (on ->d_lock of root dentry, if nothing else), why not
> > do the following:
> >         * put all dentries of such directories ([0-9]* and [0-9]*/task/*)
> > into a list anchored in task_struct; have non-counting reference to
> > task_struct stored in them (might simplify part of get_proc_task() users,
> 
> Hmm.
> 
> Right now I don't think we actually create any dentries at all for the
> short-lived process case.
> 
> Wouldn't your suggestion make fork/exit rather worse?
> 
> Or would you create the dentries dynamically still at lookup time, and
> then attach them to the process at that point?
> 
> What list would you use for the dentry chaining? Would you play games
> with the dentry hashing, and "hash" them off the process, and never
> hit in the lookup cache?

I'd been thinking of ->d_fsdata pointing to a structure with list_head
and a (non-counting) task_struct pointer for those guys.  Allocated
on lookup, of course (as well as readdir ;-/) and put on the list
at the same time.

IOW, for short-lived process we simply have an empty (h)list anchored
in task_struct and that's it.