[PATCH kernel] powerpc/iommu: Do not call PageTransHuge() on tail pages

[Alexey Kardashevskiy]

On 04/04/17 19:26, Aneesh Kumar K.V wrote:
> Alexey Kardashevskiy <aik at ozlabs.ru> writes:
> 
>> The CMA pages migration code does not support compound pages at
>> the moment so it performs few tests before proceeding to actual page
>> migration.
>>
>> One of the tests - PageTransHuge() - has VM_BUG_ON_PAGE(PageTail()) as
>> it should be called on head pages. Since we also test for PageCompound(),
>> and it contains PageTail(), we can simply move PageCompound() in front
>> of PageTransHuge() and therefore avoid possible VM_BUG_ON_PAGE.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>> ---
>>
>> Some of actual POWER8 systems do crash on that BUG_ON.
>> ---
>>  arch/powerpc/mm/mmu_context_iommu.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
>> index 497130c5c742..ba7fccf993b3 100644
>> --- a/arch/powerpc/mm/mmu_context_iommu.c
>> +++ b/arch/powerpc/mm/mmu_context_iommu.c
>> @@ -81,7 +81,7 @@ struct page *new_iommu_non_cma_page(struct page *page, unsigned long private,
>>  	gfp_t gfp_mask = GFP_USER;
>>  	struct page *new_page;
>>
>> -	if (PageHuge(page) || PageTransHuge(page) || PageCompound(page))
>> +	if (PageCompound(page) || PageHuge(page) || PageTransHuge(page))
> 
> 
> A checked for compound page should be sufficient here, because a
> Huge/TransHuge page is also marked compound.


But PageCompound() calls PageTail() so PageTail() will be called on a trans
page which is BUG_ON in PageTransHuge but it is not in PageCompound() -
this inconsistency is bothering me. Does not this BUG_ON tell us that we
should not be calling PageTail() on _any_ page?

In other words, should I get a head page (via compound_head()) first and
only then test the head page if it is thp/huge (as you suggested in a chat)?


> If we want to indicate that
> we don't handle hugetlb and THP pages, we can write that as a comment ?
> 
> 
> 
>>  		return NULL;
>>
>>  	if (PageHighMem(page))
>> @@ -100,7 +100,7 @@ static int mm_iommu_move_page_from_cma(struct page *page)
>>  	LIST_HEAD(cma_migrate_pages);
>>
>>  	/* Ignore huge pages for now */
>> -	if (PageHuge(page) || PageTransHuge(page) || PageCompound(page))
>> +	if (PageCompound(page) || PageHuge(page) || PageTransHuge(page))
>>  		return -EBUSY;
>>
>>  	lru_add_drain();
>> -- 
>> 2.11.0
> 


-- 
Alexey