linux-kernel - Re: [GIT PULL] Detaching mounts on unlink for 3.15-rc1

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Date:	Wed, 09 Apr 2014 10:32:14 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Karel Zak <kzak@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [GIT PULL] Detaching mounts on unlink for 3.15-rc1

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

> On Wed, Apr 09, 2014 at 03:30:27AM +0100, Al Viro wrote:
>
>> > When renaming or unlinking directory entries that are not mountpoints
>> > no additional locks are taken so no performance differences can result,
>> > and my benchmark reflected that.
>> 
>> It also means that d_invalidate() now might trigger fs shutdown.  Which
>> has bloody huge stack footprint, for obvious reasons.  And d_invalidate()
>> can be called with pretty deep stack - walk into wrong dentry while
>> resolving a deeply nested symlink and there you go...
>
> PS: I thought I actually replied with that point back a month or so ago,
> but having checked sent-mail...  Looks like I had not.  My deep apologies.
>
> FWIW, I think that overall this thing is a good idea, provided that we can
> live with semantics changes.  The implementation is too optimistic, though -
> at the very least, we want this work done upon namespace_unlock() held
> back until we are not too deep in stack.  task_work_add() fodder,
> perhaps?

Hmm.

Just to confirm what I am dealing with I have proceeded to measure the
amount of stack used by these operations.

For resolving a deeply nested symlink that hits the limit of 8 nested
symlinks, I find 4688 bytes left on the stack.  Which means we use
roughly 3504 bytes of stack when stating a deeply nested symlink.

For umount I had a little trouble measuring as typically the work done
by umount was not the largest stack consumer, but I found for a small
ext4 filesystem after the umount operation was complete there were
5152 bytes left on the stack, or umount used roughly 3040 bytes.

3504 + 3040 = 6544 bytes of stack used or 1684 bytes of stack left
unused.  Which certainly isn't a lot of margin but it is not overflowing
the kernel stack either. 

Is there a case that see where umount uses a lot more kernel stack?  Is
your concern an architecture other than x86_64 with different
limitations?

I am quite happy to change my code to avoid stack overflow but I want to
make certain I understand where the stack usage is coming from so that I
actually fix them issue.

Eric

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