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

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Message-ID: <20111201221940.GD10552@Krystal>
Date:	Thu, 1 Dec 2011 17:19:40 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Greg KH <greg@...ah.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

* 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.

This is a point I'm really anal about: the tracer should _never_ crash
the traced system, _ever_, in any foreseeable condition.

Thanks,

Mathieu

> 
> thanks,
> 
> greg k-h

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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