Re: [patch] #ifdef very expensive debug check in page fault path

[Nick Piggin <npiggin@suse.de>]

On Tue, Jan 22, 2008 at 10:35:17PM +0000, Hugh Dickins wrote:
> On Mon, 21 Jan 2008, Carsten Otte wrote:
> > Hugh Dickins wrote:
> > > 
> > > Well: that patch still gets my Nack, but I guess I'm too late.  If
> > > s390 pagetables are better protected than x86 ones, add an s390 ifdef?
> > 
> > The alternative would be to just make
> > #define pfn_valid(pfn) (1)
> > on s390. That would also get _us_ rid of the check while others do benefit. We
> > would trap access to mem_map beyond its limits because we don't have a kernel
> > mapping there. For us, it would not silently corrupt things but crash proper.
> 
> Whilst I quite like the sound of that, I wonder whether it's safe to
> change s390's pfn_valid (rather surprisingly) for all its users.  And
> note that nobody but me has voiced any regret at the loss of the check.

I did want to get rid of the test, but not in a "sneak it in before he
notices" way. So I am disappointed it was merged before you replied.


> My guess is we let it rest for now, and reconsider if a case comes up
> later which would have got caught by the check (but the problem is that
> such a case is much harder to identify than it was).

The only cases I had imagined were repeatable things like a bug in pte
manipulation somewhere, which will hopefully be caught with
CONFIG_DEBUG_VM turned on. 

Are there many other cases where the test is useful? For hardware
failures, I'd say not -- those just tend to waste developers time.

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