Re: [PATCH] Limit number of concurrent hotplug processes

[Paul Jackson <pj@sgi.com>]

Andrew wrote:
> We'd allocate the same amount of memory if we were sending
> messages up a socket/pipe to userspace, which is what we should always have
> done, instead of the direct-exec - it's caused tons of trouble.

This touches on a question I have, off-topic to the discussion of
the patch proposed by Hannes.

Doesn't doing a direct-exec have one powerful advantage over sending
some message or kevent to userspace by socket/pipe/d-bus, in that
sending the message requires that there is some userspace code already
running that is competent to receive the message?

Doing the usermodehelper direct-exec _both_ provides the thread context
in which to execute, _and_ the code to be executed.  The alternative
seems to require long running threads, primed and ready to accept
particular events, cluttering up the system.

I will readily grant that this usermodehelper direct kernel exec thing
seems weird as all heck to me.  But I predict that in a couple of weeks
lkml will be seeing a patch from me (the next version of the 'cpuset'
patch I'm working with Simon and Sylvain of Bull) using it -- because
the alternative of a long running, rarely used, user thread just for one
obscure particular event that required user code sucked worse.

Am I missing something?  Are there _always_ better solutions than
usermodehelper's kernel direct-exec?

Or perhaps am I confusing what Andrew was referring to and the various
mechanisms available here in unfortunate ways?

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373


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