linux-kernel - Re: ima: use of radix tree cache indexing == massive waste of memory?

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Message-ID: <20101018181916.GB12372@elte.hu>
Date:	Mon, 18 Oct 2010 20:19:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Paris <eparis@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kyle McMartin <kyle@...artin.ca>,
	James Morris <jmorris@...ei.org>,
	Christoph Hellwig <hch@...radead.org>,
	kernel@...ts.fedoraproject.org, Mimi Zohar <zohar@...ibm.com>,
	warthog9@...nel.org, Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Serge Hallyn <serue@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: ima: use of radix tree cache indexing == massive waste of memory?


* Eric Paris <eparis@...hat.com> wrote:

> On Mon, 2010-10-18 at 10:56 -0700, Linus Torvalds wrote:
> > On Mon, Oct 18, 2010 at 9:48 AM, Eric Paris <eparis@...hat.com> wrote:
> > >
> > > 1) IMA uses radix trees which end up wasting 500 bytes per inode because
> > > the key is too sparse.  I've got a patch which uses an rbtree instead
> > > I'm testing and will send along shortly.  I found it funny working on
> > > the patch to see that Documentation/rbtree.txt says "This differs from
> > > radix trees (which are used to efficiently store sparse arrays and thus
> > > use long integer indexes to insert/access/delete nodes)"  Which flys in
> > > the face of this report.
> > 
> > Please. Look at the report more carefully.
> > 
> > The radix tree memory use is disgusting. Yes. But it is absolutely NOT
> > sufficient to try to just fix that part. Go back, look at the original
> > report email, and this line in particular:
> > 
> >    2235648 2069791  92%    0.12K  69864       32    279456K iint_cache
> > 
> > There's 2.2 million iint_cache allocations too, each 128 bytes in
> > size. That's still a quarter _gigabyte_ of crap that adds zero value
> > at all.
> 
> That was #2 in my list of things to fix:
>
> 2) IMA creates an entire integrity structure for every inode even when most or all 
> of this structure will not be needed.
> 
> I'm stating with #1 since that was 2G of wasted space (thus far my switch to 
> rbtree seems to be surviving an xfstest) so I expect to send the patch this 
> afternoon.  #2 should attack the size of the iint_cache entries.  #3 should attack 
> the scalability.  I'm certainly hoping I didn't miss part of the report....

I think it would be fair to argue that #2 is the thing that should be fixed first 
and foremost - before touching any data structure details.

Because if you fix #2 then all the other items will become no-op to 99.9% of the 
people who are affected by this bug today.

It's also probably a much simpler fix for -stable, so should be done first, etc.

If you do the data structure changes first then #2 will likely not be backportable 
standalone and #1 will be risky to backport - creating nasty dependencies.

Thanks,

	Ingo
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