Andrea Arcangeli <aarcange@xxxxxxxxxx> writes:
> Hi Aneesh,
>
> On Mon, May 13, 2013 at 07:18:57PM +0530, Aneesh Kumar K.V wrote:
>>
>> updated one fixing a compile warning.
>>
>> From f721c77eb0d6aaf75758e8e93991a05207680ac8 Mon Sep 17 00:00:00 2001
>> From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>> Date: Sun, 12 May 2013 01:59:00 +0530
>> Subject: [PATCH] mm/THP: Use pmd_populate to update the pmd with pgtable_t
>> pointer
>>
>> We should not use set_pmd_at to update pmd_t with pgtable_t pointer. set_pmd_at
>> is used to set pmd with huge pte entries and architectures like ppc64, clear
>> few flags from the pte when saving a new entry. Without this change we observe
>> bad pte errors like below on ppc64 with THP enabled.
>>
>> BUG: Bad page map in process ld mm=0xc000001ee39f4780 pte:7fc3f37848000001 pmd:c000001ec0000000
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>> ---
>> mm/huge_memory.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 03a89a2..f0bad1f 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2325,7 +2325,7 @@ static void collapse_huge_page(struct mm_struct *mm,
>> pte_unmap(pte);
>> spin_lock(&mm->page_table_lock);
>> BUG_ON(!pmd_none(*pmd));
>> - set_pmd_at(mm, address, pmd, _pmd);
>> + pmd_populate(mm, pmd, (pgtable_t)_pmd);
>> spin_unlock(&mm->page_table_lock);
>> anon_vma_unlock_write(vma->anon_vma);
>> goto out;
>
> Great, looks like you found the ppc problem with gcc builds and that
> explains also why it cannot happen on x86.
yes. That was the reason for the failure.
>
> But about the fix, did you test it? The above should be:
> pmd_populate(mm, pmd, pmd_pgtable(_pmd)) instead.
>
Yes and it worked in powerpc because we have for ppc64
static inline pgtable_t pmd_pgtable(pmd_t pmd)
{
return (pgtable_t)(pmd_val(pmd) & ~PMD_MASKED_BITS);
}
That is because we share the PTE page with multiple pmds
> _pmd is not a pointer to a page struct and the cast seems to be hiding
> a bug. _pmd if something is a physical address potentially with some
> high bit set not making it a good physical address either.
>
> So you can only use set_pmd_at when establishing hugepmds, and never
> for establishing regular pmds that points to regular pagetables. I
> guess a comment would be good to add too.
>
I will send an updated patch.
-aneesh
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>