On Sat, Jul 22, 2017 at 9:13 AM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, 26 Jun 2017 16:38:33 +1000 "Oliver O'Halloran" <oohall@xxxxxxxxx> wrote:
>
>> These functions are the only bits of generic code that use
>> {pud,pmd}_pfn() without checking for CONFIG_TRANSPARENT_HUGEPAGE.
>> This works fine on x86, the only arch with devmap support, since the
>> *_pfn() functions are always defined there, but this isn't true for
>> every architecture.
>>
>> Signed-off-by: Oliver O'Halloran <oohall@xxxxxxxxx>
>> ---
>> mm/gup.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index d9e6fddcc51f..04cf79291321 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -1287,7 +1287,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>> }
>> #endif /* __HAVE_ARCH_PTE_SPECIAL */
>>
>> -#ifdef __HAVE_ARCH_PTE_DEVMAP
>> +#if defined(__HAVE_ARCH_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
>> static int __gup_device_huge(unsigned long pfn, unsigned long addr,
>> unsigned long end, struct page **pages, int *nr)
>> {
>
> (cc Kirill)
>
> Please provide a full description of the bug which is being fixed. I
> assume it's a build error. What are the error messages and under what
> circumstances.
>
> Etcetera. Enough info for me (and others) to decide which kernel
> version(s) need the fix.
It fixes a build breakage that you will only ever see when enabling
the devmap pte bit for another architecture. Given it requires new
code to hit the bug I don't see much point in backporting it to 4.12,
but taking it as a fix for 4.13 wouldn't hurt.
The root problem is that the arch doesn't need to provide pmd_pfn()
and friends when THP is disabled. They're provided unconditionally by
x86 and ppc, but I did a cursory check and found that mips only
defines pmd_pfn() when THP is enabled so I figured this should be
fixed. Making each arch provide them unconditionally might be a better
idea, but that seemed like it'd be a lot of churn for a minor bug.
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