* Borislav Petkov <b...@alien8.de> wrote:
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -566,19 +566,28 @@ void native_set_fixmap(enum fixed_addresses idx,
> phys_addr_t phys,
> /**
> * pud_set_huge - setup kernel PUD mapping
> *
> - * MTRR can override PAT memory types with 4KiB granularity. Therefore,
> - * this function does not set up a huge page when the range is covered
> - * by a non-WB type of MTRR. MTRR_TYPE_INVALID indicates that MTRR are
> - * disabled.
> + * MTRRs can override PAT memory types with 4KiB granularity. Therefore, this
> + * function sets up a huge page only if any of the following conditions are
> met:
> + *
> + * - MTRRs are disabled, or
> + *
> + * - MTRRs are enabled and the range is completely covered by a single MTRR,
> or
> + *
> + * - MTRRs are enabled and the range is not completely covered by a single
> MTRR
> + * but the memory type of the range is WB, even if covered by multiple
> MTRRs.
> + *
> + * Callers should try to decrease page size (1GB -> 2MB -> 4K) if the bigger
> + * page mapping attempt fails.
This comment should explain why it's ok in the WB case.
Also, the phrase 'the memory type of the range' is ambiguous: it might
mean the partial MTRR's, or the memory type specified via PAT by the
huge-pmd entry.
Thanks,
Ingo
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