Subject: Re: [PATCH v3] KVM: PPC: Book3S HV: Migrate pinned pages out of CMA

From: Anshuman Khandual <[email-masked]>
Date: Tue, 06 Sep 2016 17:24:40 +0530

On 09/06/2016 11:57 AM, Balbir Singh wrote:
> 
> When PCI Device pass-through is enabled via VFIO, KVM-PPC will
> pin pages using get_user_pages_fast(). One of the downsides of
> the pinning is that the page could be in CMA region. The CMA
> region is used for other allocations like the hash page table.
> Ideally we want the pinned pages to be from non CMA region.
> 
> This patch (currently only for KVM PPC with VFIO) forcefully
> migrates the pages out (huge pages are omitted for the moment).
> There are more efficient ways of doing this, but that might
> be elaborate and might impact a larger audience beyond just
> the kvm ppc implementation.
> 
> The magic is in new_iommu_non_cma_page() which allocates the
> new page from a non CMA region.
> 
> I've tested the patches lightly at my end. The full solution
> requires migration of THP pages in the CMA region. That work
> will be done incrementally on top of this.
> 
> Previous discussion was at
> http://permalink.gmane.org/gmane.linux.kernel.mm/136738
> 
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Alexey Kardashevskiy <[email-masked]>
> 
> Signed-off-by: Balbir Singh <[email-masked]>
> Acked-by: Alexey Kardashevskiy <[email-masked]>
> ---
>  arch/powerpc/include/asm/mmu_context.h |  1 +
>  arch/powerpc/mm/mmu_context_iommu.c    | 81 ++++++++++++++++++++++++++++++++--
>  2 files changed, 78 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 9d2cd0c..475d1be 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -18,6 +18,7 @@ extern void destroy_context(struct mm_struct *mm);
>  #ifdef CONFIG_SPAPR_TCE_IOMMU
>  struct mm_iommu_table_group_mem_t;
> 
> +extern int isolate_lru_page(struct page *page);	/* from internal.h */

Small nit, cant we just add "mm/internal.h" header here with full path ?

>  extern bool mm_iommu_preregistered(void);
>  extern long mm_iommu_get(unsigned long ua, unsigned long entries,
>  		struct mm_iommu_table_group_mem_t **pmem);
> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
> index da6a216..e0f1c33 100644
> --- a/arch/powerpc/mm/mmu_context_iommu.c
> +++ b/arch/powerpc/mm/mmu_context_iommu.c
> @@ -15,6 +15,9 @@
>  #include <linux/rculist.h>
>  #include <linux/vmalloc.h>
>  #include <linux/mutex.h>
> +#include <linux/migrate.h>
> +#include <linux/hugetlb.h>
> +#include <linux/swap.h>
>  #include <asm/mmu_context.h>
> 
>  static DEFINE_MUTEX(mem_list_mutex);
> @@ -72,6 +75,55 @@ bool mm_iommu_preregistered(void)
>  }
>  EXPORT_SYMBOL_GPL(mm_iommu_preregistered);
> 
> +/*
> + * Taken from alloc_migrate_target with changes to remove CMA allocations
> + */
> +struct page *new_iommu_non_cma_page(struct page *page, unsigned long private,
> +					int **resultp)
> +{
> +	gfp_t gfp_mask = GFP_USER;
> +	struct page *new_page;
> +
> +	if (PageHuge(page) || PageTransHuge(page) || PageCompound(page))
> +		return NULL;
> +
> +	if (PageHighMem(page))
> +		gfp_mask |= __GFP_HIGHMEM;
> +
> +	/*
> +	 * We don't want the allocation to force an OOM if possibe
> +	 */
> +	new_page = alloc_page(gfp_mask | __GFP_NORETRY | __GFP_NOWARN);

So what guarantees that the new page too wont come from MIGRATE_CMA
page block ? Is absence of __GFP_MOVABLE flag enough. Also should not
we be checking that migrate type of the new allocated page is indeed
not MIGRATE_CMA ?