linux-kernel - Re: bootmem allocator

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Message-ID: <20080409060854.GB19010@one.firstfloor.org>
Date:	Wed, 9 Apr 2008 08:08:54 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mike Travis <travis@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: bootmem allocator

On Tue, Apr 08, 2008 at 11:00:28PM -0700, Mike Travis wrote:
> Ingo Molnar wrote:
> > * Andi Kleen <andi@...stfloor.org> wrote:
> > 
> >>> hm, bootmem allocator is supposed to clear memory. We have a couple 
> >>> of places that rely on that.
> >> I was actually considering to change that for the GB pages hugetlbfs 
> >> patchkit, because memset for 1G is a little slow and not needed (will 
> >> be cleared later anyways) and it might be a problem for very large 
> >> systems with a lot of such pages at boot.
> > 
> > changing the default behavior of bootmem alloc to be non-clearing is a 
> > really bad idea that will only cause unrobustness. The proper approach 
> > is to add an _opt-in_ API that does not clear memory 
> > (bootmem_alloc_dontclear() or whatever), available to callers that know 
> > it for sure that they dont need the clearing.
> 
> Yes, changing the default of bootmem_alloc is a bad idea.  I just changed
> a bunch of static arrays to bootmem alloc's and it was pointed out early
> that not only does bootmem_alloc clear memory, but also panics if it's not
> available.

There are more and more bootmem calls that don't want the panic actually.
That is why _nopanic was invented (and gets more and more variants)
At some point the default could be even switched.

I think the right way would be to survey the callers (there are not
that many) and then come up with a sane single API that caters to the
majority of them by default and passes flags for the special cases.

-Andi
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