On Fri, Feb 11, 2011 at 11:58:58AM -0800, Hugh Dickins wrote:

Oh, I hadn't realized Fedora use it.  I wonder if that's wise, I thought
Nick introduced it partly for the more expensive checks, and there might
be one or two of those around - those bad_range()s in page_alloc.c?

I doubt the more expensive checks are very measurable.. benchmarks
usually run on enterprise distro. I'm sure when they enabled, they
were aware of having to run more expensive runtime checks.

But the patch actually says -1024*1024: either would do.

I actually increased it to -1024*1024 after writing the email ;) sorry
the for the confusion.

Yes, that's fine, 0xfff00000 looks unlikely enough (and my
imagination for "deadbeef"-like magic is too drowsy today).

I used a negative power of two even if I doubt the compiler can make
much use of it.

Okay I suppose: it seems rather laboured to me, I think I'd have just
moved the VM_BUG_ON into rmv_page_order() if I'd done the patch; but
since I was too lazy to do it, I'd better be grateful for yours!

Ok the reason I didn't move the VM_BUG_ON is to be stricter in case
there are more usages of __ClearPageBuddy in the future. I guess it's
not so important, but when I initially implemented it, it wasn't
entirely obvious it would work safe with memory hotplug, compaction
and all other bits using PageBuddy, so...

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