linux-kernel - Re: [PATCH] proc: Do not invalidate dentries with submounts

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Date:	Thu, 06 Dec 2007 09:05:02 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Denis V. Lunev" <den@...ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, marcus@...ter.se,
	containers@...ts.osdl.org, devel@...nvz.org,
	linux-kernel@...r.kernel.org, "Denis V. Lunev" <den@...nvz.org>
Subject: Re: [PATCH] proc: Do not invalidate dentries with submounts

"Denis V. Lunev" <den@...ru> writes:

> you have changed the behavior of revalidation by shadows. I think it
> will be better to restore it and keep new one for shadows (and below)
> only, which has been done by my yesterday patch.

- I think it is better to move forward rather then back.
- The old proc dentry caching behavior is actually too aggressive, and has
  problem corner cases.  Keeping the dentries when we have something mounted
  on top is a trade off that is the least of two evils.
- My change fixes the mount leak on all of /proc not just on /proc/generic.

What you did is a hack that restored the old slightly buggy behavior.  Which
is fine if we can't find anything better.  It is not code that is on the
path towards a /proc that properly caches it's dentries.

With the old behavior a random user space application can open a file or
a directory in /proc pinning it's dcache entry.  Then the module supplying
that open file can be removed and reinserted.  Until the user space application
removes reference to that /proc file all you will be able to find is the
version of the file from before /proc was removed.

That sounds like a way to trigger nasty behavior to me.  I would like
to remove that possibility from the kernel if I can.

Eric

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