Re: [RFC PATCH] ipc, mqueue: lazy call kern_mount_data in new namespaces

[Andrew Morton]

On Wed, 29 Nov 2017 11:33:28 +0100 Giuseppe Scrivano <gscrivan@xxxxxxxxxx> wrote:

> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> 
> > OK, but this simply moves the expense so it happens later on.  Why is
> > that better?
> 
> the optimization is for new IPC namespaces that don't use mq_open.  In
> this case there won't be any kern_mount_data cost at all.
> 

Fair enough.  Please add this paragraph (or similar) to the changelog:

: This is a net saving for new IPC namespaces that don't use mq_open().  In
: this case there won't be any kern_mount_data() cost at all

And..  the patch calls
kern_mount_data()->vfs_kern_mount()->...->kmem_cache_zalloc(GFP_KERNEL)
under spin_lock().  This should have created a might_sleep() warning in
your testing, but obviously did not.

Could you please find out why?  Do you have
CONFIG_DEBUG_ATOMIC_SLEEP=n, I hope?  Please peruse
Documentation/process/submit-checklist.rst, section 12...

I assume a suitable fix would be to create a new mutex (static to
do_mq_open()) to prevent concurrent mounting.