On Fri, Nov 30, 2007 at 10:14:18AM +0100, Ingo Molnar wrote:
>
> * Matt Mackall <[email protected]> wrote:
>
> > > plus, and this is a slob question i guess, how come we drop into
> > > clear_highpage() for a kzalloc()??
> >
> > Good question. Looks like kzalloc switched from doing a memset to
> > passing a GFP_ZERO flag down to kmalloc. Slob didn't get completely
> > updated to reflect this, so it blindly propagates the flag onto
> > __alloc_pages and does a harmless double-clear.
> >
> > Someone should remind us what the point of moving the kzalloc memset
> > down into the allocators was. We now have all three allocators doing:
> >
> > if (unlikely((flags & __GFP_ZERO) && ptr))
> > memset(ptr, 0, obj_size(cachep));
> >
> > and needing to mask flags before passing them to page allocators,
> > which hardly seems better than kzalloc unconditionally doing the
> > memset. Wouldn't it be better/faster/smaller to make kzalloc a
> > non-inline?
> >
> > Slob also has a nice second path for large kmallocs where it just
> > calls the page allocator directly which also needs this treatment.
> > Which does the right thing with non-highmem systems, but can hit this
> > bug otherwise.
> >
> > Below is a totally untested patch. Alternately, we could simply tweak
> > clear_highpage to remove the limitation, but that would leave slob
> > doing an extra clear.
>
> ok, this fixes the debug warning.
>
But the question remains: is this the right fix? The commit in
question is here:
http://www.kernel.org/hg/linux-2.6/rev/13683609d67a
Christoph, remind us what's the upside here? It seems to me it would
be better to have separate non-inline kzalloc and kcalloc functions
that did the memset instead.
Another smaller open question is whether we want to remove the
in_interrupt restriction from clear_pagehigh.
--
Mathematics is the supreme nostalgia of our time.
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