Re: [PATCH v5 6/6] mtrr, mm, x86: Enhance MTRR checks for KVA huge page mapping

Toshi Kani
2015-05-18T22:12:37Z

On Mon, 2015-05-18 at 22:51 +0200, Borislav Petkov wrote:
> On Mon, May 18, 2015 at 02:21:08PM -0600, Toshi Kani wrote:
> > The caller is the one who makes the condition checks necessary to create
> > a huge page mapping.
>
> How? It would go and change MTRRs configuration and ranges and their
> memory types so that a huge mapping succeeds?
>
> Or go and try a different range?

Try with a smaller page size.

The callers, pud_set_huge() and pmd_set_huge(), check if the given range
is safe with MTRRs for creating a huge page mapping. If not, they fail
the request, which leads their callers, ioremap_pud_range() and
ioremap_pmd_range(), to retry with a smaller page size, i.e. 1GB -> 2MB
-> 4KB. 4KB may not have overlap with MTRRs (hence no checking is
necessary), which will succeed as before.

Thanks,
-Toshi