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

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Message-ID: <20200213222350.GU23230@ZenIV.linux.org.uk>
Date:   Thu, 13 Feb 2020 22:23:50 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Security Module <linux-security-module@...r.kernel.org>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Solar Designer <solar@...nwall.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs
 instances

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@...iv.linux.org.uk> 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.