linux-kernel - Re: [PATCH 01/11] mm: export vmalloc_sync

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Message-ID: <20111201224128.GB3716@kroah.com>
Date:	Thu, 1 Dec 2011 14:41:28 -0800
From:	Greg KH <greg@...ah.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Christoph Hellwig <hch@...radead.org>, devel@...verdev.osuosl.org,
	lttng-dev@...ts.lttng.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, David Howells <dhowells@...hat.com>,
	David McCullough <davidm@...pgear.com>,
	D Jeff Dionne <jeff@...inux.org>,
	Greg Ungerer <gerg@...pgear.com>,
	Paul Mundt <lethal@...ux-sh.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/11] mm: export vmalloc_sync_all symbol to GPL modules

On Thu, Dec 01, 2011 at 05:19:40PM -0500, Mathieu Desnoyers wrote:
> * Greg KH (greg@...ah.com) wrote:
> > On Thu, Dec 01, 2011 at 04:57:00PM -0500, Christoph Hellwig wrote:
> > > On Thu, Dec 01, 2011 at 04:41:13PM -0500, Mathieu Desnoyers wrote:
> > > > LTTng needs this symbol exported. It calls it to ensure its tracing
> > > > buffers and allocated data structures never trigger a page fault. This
> > > > is required to handle page fault handler tracing and NMI tracing
> > > > gracefully.
> > > 
> > > We:
> > > 
> > >  a) don't export symbols unless they have an intree-user
> > 
> > lttng is now in-tree in the drivers/staging/ area.  See linux-next for
> > details if you are curious.
> > 
> > >  b) especially don't export something as lowlevel as this one.
> > 
> > Mathieu, there's nothing else you can do to get this information?  Or
> > does lttng really want such lowlevel data?
> 
> LTTng calls vmalloc_sync_all() to make sure it won't crash the system
> (due to recursive page fault) when hooking on the page fault handler and
> on any hook that would happen to sit in a function hit by NMI context.
> So it really goes beyond just extracting information for this one I'm
> afraid: it's a matter of execution correctness.

Ok, fair enough.

Christoph, is there any other way to achive something like this without
this symbol being exported that you know of?

thanks,

greg k-h
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