Comment 5 for bug 1549601

I'm expecting an upstream fix for this problem soon, based on this discussion (lkml):

From Dexuan Cui:
"d1cd12108346: x86, pageattr: Prevent overflow in slow_virt_to_phys() for X86_PAE"
was unintentionally removed by the recent
"34437e67a672: x86/mm: Fix slow_virt_to_phys() to handle large PAT bit".

And, the variable 'phys_addr' was defined as "unsigned long" by mistake -- it should
be "phys_addr_t".

As a result, Hyper-V network driver in 32-PAE Linux guest can't work again.

Fixes: "commmit 34437e67a672: x86/mm: Fix slow_virt_to_phys() to handle large PAT bit"
Signed-off-by: Dexuan Cui <email address hidden>
Cc: Toshi Kani <email address hidden>
Cc: Andrew Morton <email address hidden>
Cc: Thomas Gleixner <email address hidden>
Cc: K. Y. Srinivasan <email address hidden>
Cc: Haiyang Zhang <email address hidden>
Cc: <email address hidden>
Cc: <email address hidden>
Cc: <email address hidden>
Cc: <email address hidden>
Cc: <email address hidden>
Cc: <email address hidden>
---
 arch/x86/mm/pageattr.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 2440814..9cf96d8 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -419,24 +419,30 @@ pmd_t *lookup_pmd_address(unsigned long address)
 phys_addr_t slow_virt_to_phys(void *__virt_addr)
 {
        unsigned long virt_addr = (unsigned long)__virt_addr;
- unsigned long phys_addr, offset;
+ phys_addr_t phys_addr;
+ unsigned long offset;
        enum pg_level level;
        pte_t *pte;

        pte = lookup_address(virt_addr, &level);
        BUG_ON(!pte);

+ /*
+ * pXX_pfn() returns unsigned long, which must be cast to phys_addr_t
+ * before being left-shifted PAGE_SHIFT bits -- this trick is to
+ * make 32-PAE kernel work correctly.
+ */
        switch (level) {
        case PG_LEVEL_1G:
- phys_addr = pud_pfn(*(pud_t *)pte) << PAGE_SHIFT;
+ phys_addr = (phys_addr_t)pud_pfn(*(pud_t *)pte) << PAGE_SHIFT;
                offset = virt_addr & ~PUD_PAGE_MASK;
                break;
        case PG_LEVEL_2M:
- phys_addr = pmd_pfn(*(pmd_t *)pte) << PAGE_SHIFT;
+ phys_addr = (phys_addr_t)pmd_pfn(*(pmd_t *)pte) << PAGE_SHIFT;
                offset = virt_addr & ~PMD_PAGE_MASK;
                break;
        default:
- phys_addr = pte_pfn(*pte) << PAGE_SHIFT;
+ phys_addr = (phys_addr_t)pte_pfn(*pte) << PAGE_SHIFT;
                offset = virt_addr & ~PAGE_MASK;
        }

From Toshi Kani:
On Thu, 2016-02-25 at 01:58 -0800, Dexuan Cui wrote:
> "d1cd12108346: x86, pageattr: Prevent overflow in slow_virt_to_phys() for
> X86_PAE"
> was unintentionally removed by the recent
> "34437e67a672: x86/mm: Fix slow_virt_to_phys() to handle large PAT bit".
>
> And, the variable 'phys_addr' was defined as "unsigned long" by mistake
> -- it should
> be "phys_addr_t".
>
> As a result, Hyper-V network driver in 32-PAE Linux guest can't work
> again.
>
> Fixes: "commmit 34437e67a672: x86/mm: Fix slow_virt_to_phys() to handle
> large PAT bit"
> Signed-off-by: Dexuan Cui <email address hidden>
> Cc: Toshi Kani <email address hidden>
> Cc: Andrew Morton <email address hidden>
> Cc: Thomas Gleixner <email address hidden>
> Cc: K. Y. Srinivasan <email address hidden>
> Cc: Haiyang Zhang <email address hidden>
> Cc: <email address hidden>
> Cc: <email address hidden>
> Cc: <email address hidden>
> Cc: <email address hidden>
> Cc: <email address hidden>
> Cc: <email address hidden>

Thanks for the fix and adding the comment to explain the trick! The change
looks good to me.

Reviewed-by: Toshi Kani <email address hidden>