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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: Wed, 12 Feb 2020 20:03:35 +0000
Message-ID: <20200212200335.GO23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=wgwmu4jpmOqW0+Lz0dcem1Fub=ThLHvmLobf_WqCq7bwg@mail.gmail.com>

On Wed, Feb 12, 2020 at 11:49:58AM -0800, Linus Torvalds wrote:

> I wonder if we could split up d_invalidate(). It already ends up being
> two phases: first the unhashing under the d_lock, and then the
> recursive shrinking of parents and children.
> 
> The recursive shrinking of the parent isn't actually interesting for
> the proc shrinking case: we just looked up one child, after all. So we
> only care about the d_walk of the children.
> 
> So if we only did the first part under the RCU lock, and just
> collected the dentries (can we perhaps then re-use the hash list to
> collect them to another list?) and then did the child d_walk
> afterwards?

What's to prevent racing with fs shutdown while you are doing the second part?
We could, after all, just have them[*] on procfs-private list (anchored in
task_struct) from the very beginning; evict on ->d_prune(), walk the list
on exit...  How do you make sure the fs instance won't go away right under
you while you are doing the real work?  Suppose you are looking at one
of those dentries and you've found something blocking to do.  You can't
pin that dentry; you can pin ->s_active on its superblock (if it's already
zero, you can skip it - fs shutdown already in progress will take care of
the damn thing), but that will lead to quite a bit of cacheline pingpong...

[*] only /proc/<pid> and /proc/*/task/<pid> dentries, obviously.

  reply index

Thread overview: 74+ 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 [this message]
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
2020-02-13 22:47                                     ` Linus Torvalds
2020-02-14 14:15                                       ` ebiederm
2020-02-14  3:48                                 ` ebiederm
2020-02-20 20:46                               ` [PATCH 0/7] proc: Dentry flushing without proc_mnt ebiederm
2020-02-20 20:47                                 ` [PATCH 1/7] proc: Rename in proc_inode rename sysctl_inodes sibling_inodes ebiederm
2020-02-20 20:48                                 ` [PATCH 2/7] proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache ebiederm
2020-02-20 20:49                                 ` [PATCH 3/7] proc: Mov rcu_read_(lock|unlock) in proc_prune_siblings_dcache ebiederm
2020-02-20 22:33                                   ` Linus Torvalds
2020-02-20 20:49                                 ` [PATCH 4/7] proc: Use d_invalidate " ebiederm
2020-02-20 22:43                                   ` Linus Torvalds
2020-02-20 22:54                                   ` Al Viro
2020-02-20 23:00                                     ` Linus Torvalds
2020-02-20 23:03                                     ` Al Viro
2020-02-20 23:39                                       ` ebiederm
2020-02-20 20:51                                 ` [PATCH 5/7] proc: Clear the pieces of proc_inode that proc_evict_inode cares about ebiederm
2020-02-20 20:52                                 ` [PATCH 6/7] proc: Use a list of inodes to flush from proc ebiederm
2020-02-20 20:52                                 ` [PATCH 7/7] proc: Ensure we see the exit of each process tid exactly once ebiederm
2020-02-21 16:50                                   ` Oleg Nesterov
2020-02-22 15:46                                     ` ebiederm
2020-02-20 23:02                                 ` [PATCH 0/7] proc: Dentry flushing without proc_mnt Linus Torvalds
2020-02-20 23:07                                   ` Al Viro
2020-02-20 23:37                                     ` ebiederm
2020-02-24 16:25                                 ` [PATCH v2 0/6] " ebiederm
2020-02-24 16:26                                   ` [PATCH v2 1/6] proc: Rename in proc_inode rename sysctl_inodes sibling_inodes ebiederm
2020-02-24 16:27                                   ` [PATCH v2 2/6] proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache ebiederm
2020-02-24 16:27                                   ` [PATCH v2 3/6] proc: In proc_prune_siblings_dcache cache an aquired super block ebiederm
2020-02-24 16:28                                   ` [PATCH v2 4/6] proc: Use d_invalidate in proc_prune_siblings_dcache ebiederm
2020-02-24 16:28                                   ` [PATCH v2 5/6] proc: Clear the pieces of proc_inode that proc_evict_inode cares about ebiederm
2020-02-24 16:29                                   ` [PATCH v2 6/6] proc: Use a list of inodes to flush from proc ebiederm
2020-02-14  3:49                     ` [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances 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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