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From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux Security Module <linux-security-module@vger.kernel.org>,
	Akinobu Mita <akinobu.mita@gmail.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Daniel Micay <danielmicay@gmail.com>,
	Djalal Harouni <tixxdz@gmail.com>,
	"Dmitry V . Levin" <ldv@altlinux.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	"J . Bruce Fields" <bfields@fieldses.org>,
	Jeff Layton <jlayton@poochiereds.net>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Solar Designer <solar@openwall.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances
Date: Thu, 13 Feb 2020 22:23:50 +0000
Message-ID: <20200213222350.GU23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=wgQnNHYxV7-SyRP=g9vTHyNAK9g1juLLB=eho4=DHVZEQ@mail.gmail.com>

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@zeniv.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.

  reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-10 15:05 [PATCH v8 00/11] proc: modernize proc to support multiple private instances Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 01/11] proc: Rename struct proc_fs_info to proc_fs_opts Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 02/11] proc: add proc_fs_info struct to store proc information Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 03/11] proc: move /proc/{self|thread-self} dentries to proc_fs_info Alexey Gladkov
2020-02-10 18:23   ` Andy Lutomirski
2020-02-12 15:00     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 04/11] proc: move hide_pid, pid_gid from pid_namespace " Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 05/11] proc: add helpers to set and get proc hidepid and gid mount options Alexey Gladkov
2020-02-10 18:30   ` Andy Lutomirski
2020-02-12 14:57     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 06/11] proc: support mounting procfs instances inside same pid namespace Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances Alexey Gladkov
2020-02-10 17:46   ` Linus Torvalds
2020-02-10 19:23     ` Al Viro
2020-02-11  1:36   ` ebiederm
2020-02-11  4:01     ` ebiederm
2020-02-12 14:49     ` Alexey Gladkov
2020-02-12 14:59       ` ebiederm
2020-02-12 17:08         ` Alexey Gladkov
2020-02-12 18:45         ` Linus Torvalds
2020-02-12 19:16           ` ebiederm
2020-02-12 19:49             ` Linus Torvalds
2020-02-12 20:03               ` Al Viro
2020-02-12 20:35                 ` Linus Torvalds
2020-02-12 20:38                   ` Al Viro
2020-02-12 20:41                     ` Al Viro
2020-02-12 21:02                       ` Linus Torvalds
2020-02-12 21:46                         ` ebiederm
2020-02-13  0:48                           ` Linus Torvalds
2020-02-13  4:37                             ` ebiederm
2020-02-13  5:55                               ` Al Viro
2020-02-13 21:30                                 ` Linus Torvalds
2020-02-13 22:23                                   ` Al Viro [this message]
2020-02-13 22:47                                     ` Linus Torvalds
2020-02-14 14:15                                       ` ebiederm
2020-02-14  3:48                                 ` ebiederm
2020-02-14  3:49                     ` ebiederm
2020-02-12 19:47           ` Al Viro
2020-02-11 22:45   ` Al Viro
2020-02-12 14:26     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 08/11] proc: instantiate only pids that we can ptrace on 'hidepid=4' mount option Alexey Gladkov
2020-02-10 16:29   ` Jordan Glover
2020-02-12 14:34     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 09/11] proc: add option to mount only a pids subset Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 10/11] docs: proc: add documentation for "hidepid=4" and "subset=pidfs" options and new mount behavior Alexey Gladkov
2020-02-10 18:29   ` Andy Lutomirski
2020-02-12 16:03     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 11/11] proc: Move hidepid values to uapi as they are user interface to mount Alexey Gladkov

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