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

[Linus Torvalds]

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?

Am I misunderstanding what you suggest?

            Linus