linux-kernel - Re: 2.6.25-rc3: 34TB vmalloc total -- overflow in /proc/meminfo?

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Date:	Wed, 5 Mar 2008 21:36:37 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Pavel Machek <pavel@....cz>
cc:	Christian Kujau <lists@...dbynature.de>,
	kernel list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: 2.6.25-rc3: 34TB vmalloc total -- overflow in /proc/meminfo?

On Wed, 5 Mar 2008, Pavel Machek wrote:
> > > > CommitLimit:   4132360 kB
> > > > Committed_AS:    27684 kB
> > > > VmallocTotal: 34359738367 kB
> > > > VmallocUsed:     18112 kB
> > > > VmallocChunk: 34359720115 kB
> > 
> > I don't see what Pavel's issue is with this: it's simply a fact that
> > with a 64-bit kernel, we've lots of virtual address space to spare
> > for vmalloc.  What would be surprising is for VmallocUsed to get up
> > as high as that.
> 
> Hmm... ok, I see, I thought "clearly this overflowed somewhere", and I

The (mis)alignment does makes it look that way,
but no, it's not an overflow in this case.

> was wrong, it is expected result.
> 
> Still.... what is 34TB of vmalloc space good for when we can only ever
> allocate 4GB (because that is how much physical memory we have?)? To
> prevent fragmentation? 

Well, what else would you want to use that space for?  If there were
a compelling reason to tune it according to how much physical memory
you have (and you're right, that we want a good surplus of address
space so as to avoid silly limitations by fragmentation), I guess
that could have been done.  But why bother if there's no reason?

It's a hard life, there's just too much room to spare in 64-bit ;)

Hugh
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