Re: [patch 00/18] multi size, and giant hugetlb page support, 1GB hugetlb for x86

[Nishanth Aravamudan <nacc@us.ibm.com>]

On 23.04.2008 [18:02:10 +0200], Andi Kleen wrote:
> > No, it can generally determine the size of the hugepages. It would
> > be more wrong (but probably more common) for portable code to assume
> 
> For compatibility we have to assume code does that.
> 
> > 2MB hugepages.
> 
> Well then it should just run with 2MB pages on a kernel where both
> 1G and 2M are configured. Does it not do that? 
> 
> > If you want your legacy userspace to have 2MB hugepages, then you would
> 
> I think all legacy user space should only use 2MB huge pages.

Even with what you're saying (that 1G implies 2M is also there), let's
say a legacy app just looks in /proc/mounts for hugetlbfs mountpoints
and then creates a file in the first one it finds. If the system
administrator mounted a 1G hugetlbfs first, then the legacy app is going
to get 1G pages, regardless of whether or not 2M are presented to
userspace. So that legacy app just broke -- I don't see any way of
preventing that.

I think Nick's method is sane and reasonable. Do you know of specific
legacy apps that require what you're saying?

Thanks,
Nish

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