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From: ebiederm@xmission.com (Eric W. Biederman)
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	Hugh Dickins <hugh@veritas.com>, Tejun Heo <tj@kernel.org>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@suse.de>,
	Nick Piggin <npiggin@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 0/23] File descriptor hot-unplug support v2
Date: Mon, 08 Jun 2009 23:22:50 -0700
Message-ID: <m1k53mylhh.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20090606080334.GA15204@ZenIV.linux.org.uk> (Al Viro's message of "Sat\, 6 Jun 2009 09\:03\:34 +0100")

Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Mon, Jun 01, 2009 at 02:45:17PM -0700, Eric W. Biederman wrote:
>> 
>> I found myself looking at the uio, seeing that it does not support pci
>> hot-unplug, and thinking "Great yet another implementation of
>> hotunplug logic that needs to be added".
>> 
>> I decided to see what it would take to add a generic implementation of
>> the code we have for supporting hot unplugging devices in sysfs, proc,
>> sysctl, tty_io, and now almost in the tun driver.
>> 
>> Not long after I touched the tun driver and made it safe to delete the
>> network device while still holding it's file descriptor open I someone
>> else touch the code adding a different feature and my careful work
>> went up in flames.  Which brought home another point at the best of it
>> this is ultimately complex tricky code that subsystems should not need
>> to worry about.
>> 
>> What makes this even more interesting is that in the presence of pci
>> hot-unplug it looks like most subsystems and most devices will have to
>> deal with the issue one way or another.
>> 
>> This infrastructure could also be used to implement both force
>> unmounts and sys_revoke.  When I could not think of a better name for
>> I have drawn on that and used revoke.
>
> To be honest, the longer I'm looking at it, the less I like the approach...
> It really looks as if we'd be much better off with functionality sitting
> in a set of library helpers to be used by instances that need this stuff.
> Do we really want it for generic case?

I think so.  I do know I have seen enough weird cases actually being
used and not being done correctly we want a clean pattern for handling
the general case that works and is complete.

The problem seems to break up into several pieces.
- unmap support.
- Getting a list of the files that are open for an inode.
- Waking up interruptible sleepers.
- A test to see if we are executing any of the functions in 
  the file_operations structure. (needed before we can free state)
- Calling frelease and generally releasing of the state held by the
  file.

It might be possible to solve the entire problem outside of the vfs

> Note that "we might someday implement real force-umount" doesn't count;
> the same kind of arguments had been given nine years ago in case of AIO
> ("oh, sure, we'll eventually cover foo_get_block() too - it will all be
> a state machine, fully asynchronous; whaddya mean 'it's not feasible'?").
> Of course, it was _not_ feasible and had never been implemented.

> Frankly, I very much suspect that force-umount is another case like that;
> we'll need a *lot* of interesting cooperation from fs for that to work and
> to be useful.  I'd be delighted to be proven incorrect on that one, so
> if you have anything serious in that direction, please share the details.

So far nothing but thought experiments, but you have a good point at
least a proof of concept should be done of the various pieces.  To flush
out some niggling little detail that messes up the design.

So I hereby sign up for writing a sys_revoke patch, a forced umount patch
and a writing a patch to ext2 to support it.  Supporting proc and
sysfs while easy is not really the common case of an nfs exportable block
filesystem so it is not complete.

> As for the patchset in the current form...  Could you explain what's to prevent
> POSIX locks and dnotify entries from outliving a struct file you'd revoked,
> seeing that filp_close() will skip killing them in that case.

Good catch that looks like a big fat bug to me.  It seems I overlooked
the fact that we actually free things in filp_close.

Given that posix_remove_file calls vfs_lock_file which calls
file->f_op->lock it looks like something really needs to be done here.

dnotify_flush doesn't look to hard to spin a special case for revoke.

I am going to have to spend I while longer studying the rest of the
code in filp_close.  I hope I don't need to figure out the various
fl_owner_t values to safely revoke a file, but it looks like I might.

Eric

      parent reply index

Thread overview: 99+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-04-11 12:01 [RFC][PATCH 0/9] File descriptor hot-unplug support Eric W. Biederman
2009-04-11 12:03 ` [RFC][PATCH 1/9] mm: Introduce remap_file_mappings Eric W. Biederman
2009-04-11 12:05 ` [RFC][PATCH 2/9] mm: Implement generic support for revoking a mapping Eric W. Biederman
2009-04-11 12:06 ` [RFC][PATCH 3/9] sysfs: Use remap_file_mappings Eric W. Biederman
2009-04-11 12:07 ` [RFC][PATCH 4/9] vfs: Generalize the file_list Eric W. Biederman
2009-04-11 12:08 ` [RFC][PATCH 5/9] vfs: Introduce basic infrastructure for revoking a file Eric W. Biederman
2009-04-14 22:12   ` Jonathan Corbet
2009-04-15  2:55     ` Eric W. Biederman
2009-04-11 12:10 ` [RFC][PATCH 6/9] vfs: Utilize fops_read_lock where appropriate Eric W. Biederman
2009-04-11 12:11 ` [RFC][PATCH 7/9] vfs: Optimize fops_read_lock Eric W. Biederman
2009-04-11 12:13 ` [RFC][PATCH 8/9] vfs: Implement generic revoked file operations Eric W. Biederman
2009-04-12 18:56   ` Jamie Lokier
2009-04-12 20:04     ` Eric W. Biederman
2009-04-12 20:31       ` Jamie Lokier
2009-04-12 21:53         ` Eric W. Biederman
2009-04-12 20:54       ` Eric W. Biederman
2009-04-12 21:02         ` Jamie Lokier
2009-04-12 23:06           ` Eric W. Biederman
2009-04-11 12:14 ` [RFC][PATCH 9/9] proc: Use the generic vfs revoke facility that now exists Eric W. Biederman
2009-04-11 15:58 ` [RFC][PATCH 0/9] File descriptor hot-unplug support Al Viro
2009-04-11 16:49   ` Eric W. Biederman
2009-04-11 16:56     ` Al Viro
2009-04-11 23:57       ` Eric W. Biederman
2009-04-12 20:21       ` Eric W. Biederman
2009-04-14  3:16 ` Tejun Heo
2009-04-14  7:39   ` Eric W. Biederman
2009-04-14  7:45     ` Tejun Heo
2009-04-14  8:27       ` Eric W. Biederman
2009-04-14  8:49         ` Tejun Heo
2009-04-14 15:07         ` Jamie Lokier
2009-04-14 19:09           ` Eric W. Biederman
2009-06-01 21:45 ` [PATCH 0/23] File descriptor hot-unplug support v2 Eric W. Biederman
2009-06-01 21:50   ` [PATCH 01/23] mm: Introduce revoke_file_mappings Eric W. Biederman
2009-06-01 22:25     ` Andrew Morton
2009-06-02  0:12       ` Eric W. Biederman
2009-06-01 21:50   ` [PATCH 02/23] vfs: Implement unpoll_file Eric W. Biederman
2009-06-06  8:08     ` Al Viro
2009-06-01 21:50   ` [PATCH 03/23] vfs: Generalize the file_list Eric W. Biederman
2009-06-02  7:06     ` Nick Piggin
2009-06-05 19:33       ` Eric W. Biederman
2009-06-09 10:38         ` Nick Piggin
2009-06-09 18:38           ` Eric W. Biederman
2009-06-10  6:05             ` Nick Piggin
2009-06-01 21:50   ` [PATCH 04/23] vfs: Introduce infrastructure for revoking a file Eric W. Biederman
2009-06-02  5:16     ` Pekka Enberg
2009-06-02  6:51       ` Eric W. Biederman
2009-06-02  7:08         ` Pekka Enberg
2009-06-02  7:14     ` Nick Piggin
2009-06-02 17:06       ` Linus Torvalds
2009-06-02 20:52         ` Eric W. Biederman
2009-06-03  6:37           ` Nick Piggin
2009-06-02 22:56       ` Eric W. Biederman
2009-06-03  6:38         ` Nick Piggin
2009-06-05  9:03     ` Miklos Szeredi
2009-06-05 19:06       ` Eric W. Biederman
2009-06-01 21:50   ` [PATCH 05/23] vfs: Teach lseek to use file_hotplug_lock Eric W. Biederman
2009-06-01 21:50   ` [PATCH 06/23] vfs: Teach read/write to use file_hotplug_read_lock Eric W. Biederman
2009-06-01 21:50   ` [PATCH 07/23] vfs: Teach sendfile,splice,tee,and vmsplice to use file_hotplug_lock Eric W. Biederman
2009-06-03 23:39     ` Badari Pulavarty
2009-06-05 19:37       ` Eric W. Biederman
2009-06-01 21:50   ` [PATCH 08/23] vfs: Teach readdir " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 09/23] vfs: Teach poll and select " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 10/23] vfs: Teach do_path_lookup " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 11/23] mm: Teach mmap " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 12/23] vfs: Teach fcntl " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 13/23] vfs: Teach ioctl " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 14/23] vfs: Teach flock " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 15/23] vfs: Teach fallocate, and filp_close " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 16/23] vfs: Teach fstatfs, fstatfs64, ftruncate, fchdir, fchmod, fchown " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 17/23] proc: Teach /proc/<pid>/fd " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 18/23] vfs: Teach epoll " Eric W. Biederman
2009-06-02 16:51     ` Davide Libenzi
2009-06-02 21:23       ` Eric W. Biederman
2009-06-02 21:52         ` Davide Libenzi
2009-06-02 22:51           ` Eric W. Biederman
2009-06-03 14:57             ` Davide Libenzi
2009-06-03 20:53               ` Eric W. Biederman
2009-06-04  0:50                 ` Davide Libenzi
2009-06-04  1:42                   ` Eric W. Biederman
2009-06-01 21:50   ` [PATCH 19/23] eventpoll: Fix comment Eric W. Biederman
2009-06-01 21:50   ` [PATCH 20/23] vfs: Teach aio to use file_hotplug_lock Eric W. Biederman
2009-06-01 21:50   ` [PATCH 21/23] vfs: Teach fsync " Eric W. Biederman
2009-06-01 21:50   ` [PATCH 22/23] vfs: Teach fadvice to file_hotplug_lock Eric W. Biederman
2009-06-01 21:50   ` [PATCH 23/23] vfs: Teach readahead to use the file_hotplug_lock Eric W. Biederman
2009-06-03 23:25     ` Badari Pulavarty
2009-06-06  8:03   ` [PATCH 0/23] File descriptor hot-unplug support v2 Al Viro
2009-06-08  9:41     ` Miklos Szeredi
2009-06-08 10:24       ` Jamie Lokier
2009-06-08 16:29       ` Al Viro
2009-06-08 16:44         ` Miklos Szeredi
2009-06-08 17:50           ` Al Viro
2009-06-08 18:01             ` Linus Torvalds
2009-06-08 18:50               ` Al Viro
2009-06-08 19:18                 ` Linus Torvalds
2009-06-09  6:42                   ` Eric W. Biederman
2009-06-09 10:52                     ` Nick Piggin
2009-06-09  5:50             ` Miklos Szeredi
2009-06-09  6:31               ` Eric W. Biederman
2009-06-09  6:22     ` Eric W. Biederman [this message]

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