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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 04 Apr 2017 14:56:17 +0530

Alexey Kardashevskiy <[email-masked]> 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 <[email-masked]>
> ---
>
> 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. 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