[PATCH review 0/7] Bind mount escape fixes

[Eric W. Biederman]

Linus Torvalds <torvalds at linux-foundation.org> writes:

> On Sat, Aug 15, 2015 at 7:12 PM, Al Viro <viro at zeniv.linux.org.uk> wrote:
>>
>> I think you are underestimating the frequency of .. traversals.  Any build
>> process that creates relative symlinks will be hitting it all the time,
>> for one thing.
>
> I suspect you're over-estimating how expensive it is to just walk down
> to the mount-point. It's just a few pointer traversals.
>
> Realistically, we probably do more than that for a *regular* path
> component lookup, when we follow the hash chains. Following a d_parent
> chain for ".." isn't that different.
>
> Just looking at the last patch Eric sent, that one looks _trivial_. It
> didn't need *any* preparation or new rules. Compared to the mess with
> marking things MNT_DIR_ESCAPED etc, I know which approach I'd prefer.
>
> But hey, if you think you can simplify it... I just don't think that
> even totally ignoring the d_splice_alias() things, and totally
> ignoring any locking around __d_move(), the whole "mark things
> MNT_DIR_ESCAPED" is a lot more complex.

It occurs to me that there is a fairly simple way we can emperically
test to see how expensive calling is_subdir for every .. on a bind mount
is in practice.

- Take my last patch
- run a benchmark outside of a bind mount (perhaps a kernel compile).
- run the same benchmark inside of a bind mount.

See if the performance differs.

I am going to try to find time to do this, but I am travelling for the
next couple of days.

If someone who has a bit more time wants to try it and beats me to that
would be great.

I think having some emperical numbers would be nice in this part of the
conversation.

Eric