Re: [perf-discuss] NUMA ptools and ISM segments

[Joe Bonasera]

The x86 HAT (D)ISM code is tricky. Here's why:

pagesize is 4K, a large page is 2Meg (usually).

So a pagetable covers either an entire 512 4K pages
aligned at 2M or 512 2M pages aligned at 1Gig.

To share a page table, the (D)ISM segment has to be
either a multiple of (512) 4K pages aligned to a 2Meg
boundar or a multiple of 512 2Meg pages aligned to
a 1Gig boundary.

When those alignment / size restrictions aren't met, then
we can't really share the pagetables. So when you attach
to the (d)ISM segment, we copy the values needed for the
mappings into process local pagetables - ie. not really shared.

I suspect the minor fault stuff doesn't work right, because to
unload the mappings you'd have to hat_unshare() in all processes
that have the DISM mapped in order to migrate it.

Joe

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