Re: [patch 14/21] x86: add hugepagesz option on 64-bit

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Tue, 2008-06-03 at 20:00 +1000, npiggin@suse.de wrote:
> +static __init int setup_hugepagesz(char *opt)
> +{
> +       unsigned long ps = memparse(opt, &opt);
> +       if (ps == PMD_SIZE) {
> +               hugetlb_add_hstate(PMD_SHIFT - PAGE_SHIFT);
> +       } else if (ps == PUD_SIZE && cpu_has_gbpages) {
> +               hugetlb_add_hstate(PUD_SHIFT - PAGE_SHIFT);
> +       } else {
> +               printk(KERN_ERR "hugepagesz: Unsupported page size %lu M\n",
> +                       ps >> 20);
> +               return 0;
> +       }
> +       return 1;
> +}
> +__setup("hugepagesz=", setup_hugepagesz);
> +#endif

Hi Nick,

I was talking to Nish a bit about these bits.  I'm worried that this
setup isn't going to be very user friendly.

First of all, it seems a bit silly to require that users spell out all
of the huge page sizes at boot.  Shouldn't we allow the small sizes to
be runtime-added as well?

This also requires that users know at boot time which page sizes are
supported, and that might not be horribly feasible if you wanted to have
a common setup among a bunch of different machines, or if the supported
sizes can change with things as insignificant as firmware revisions
(they can on ppc).

So, here's what I propose.  At boot, the architecture calls
hugetlb_add_hstate() for every hardware-supported huge page size.  This,
of course, gets enumerated in Nish's  new sysfs interface.

Then, give the boot-time large page reservations either to hugepages= or
a new boot option.  But, instead of doing it in number of hpages, do it
in sizes like hugepages=10G.  Bootmem-alloc that area, and make it
available to the first hugetlbfs users that come along, regardless of
their hpage size.  Do whatever is simplest here when breaking down the
large area.

I *think* this could all be done on top of what you have here.

-- Dave

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