On 11/27/2015 02:12 AM, Thomas Gleixner wrote:
> On Mon, 16 Nov 2015, Dave Hansen wrote:
>> +static inline int pte_allows_gup(pte_t pte, int write)
>> +{
>> + /*
>> + * 'entry_flags' can come from a pte, pmd or pud. Make
>> + * sure to only check flags here that are valid *and* the
>> + * same value on all 3 types. (PAT is currently the only
>> + * one where that is true and is not checked here).
>
> I have a hard time to understand that comment.
>
> /*
> * 'entry_flags' can come from a pte, pmd or pud. We only check
> * _PAGE_PRESENT, _PAGE_USER and _PAGE_RW here, which are the
> * same for all types.
> */
>
> Is that what you wanted to say?
Yeah, that's a much better way to say it. I'll fix it up.
>> + */
>> + if (!(pte_flags(pte) & (_PAGE_PRESENT|_PAGE_USER)))
>> + return 0;
>> + if (write && !(pte_write(pte)))
>> + return 0;
>> + return 1;
>> +}
>> +
>> +static inline int pmd_allows_gup(pmd_t pmd, int write)
>> +{
>> + return pte_allows_gup(*(pte_t *)&pmd, write);
>> +}
>> +
>> +static inline int pud_allows_gup(pud_t pud, int write)
>> +{
>> + return pte_allows_gup(*(pte_t *)&pud, write);
>> +}
>> +
>
>> static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
>> unsigned long end, int write, struct page **pages, int *nr)
>> {
>> - unsigned long mask;
>> struct page *head, *page;
>> int refs;
>>
>> - mask = _PAGE_PRESENT|_PAGE_USER;
>> - if (write)
>> - mask |= _PAGE_RW;
>> - if ((pmd_flags(pmd) & mask) != mask)
>> + if (!pmd_allows_gup(pmd, write))
>
> Why do you need that extra indirection here of rereading the pmd?
> You have the pmd already.
The intention there was not to re-read the PMD, but only to cast the
structure over to a pte_t. I expected the compiler to be able to figure
out what was going on and not actually re-read anything.
Is that a bad assumption?
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