Re: [RFC PATCH kernel vfio] mm: vfio: Move pages out of CMA before pinning

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 08/05/2015 06:08 PM, Alexey Kardashevskiy wrote:
> This is about VFIO aka PCI passthrough used from QEMU.
> KVM is irrelevant here.


Anyone, any idea? Or the question is way too stupid? :)


>
> QEMU is a machine emulator. It allocates guest RAM from anonymous memory
> and these pages are movable which is ok. They may happen to be allocated
> from the contiguous memory allocation zone (CMA). Which is also ok as
> long they are movable.
>
> However if the guest starts using VFIO (which can be hotplugged into
> the guest), in most cases it involves DMA which requires guest RAM pages
> to be pinned and not move once their addresses are programmed to
> the hardware for DMA.
>
> So we end up in a situation when quite many pages in CMA are not movable
> anymore. And we get bunch of these:
>
> [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
> [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
> [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
>
> This is a very rough patch to start the conversation about how to move
> pages properly. mm/page_alloc.c does this and
> arch/powerpc/mm/mmu_context_iommu.c exploits it.
>
> Please do not comment on the style and code placement,
> this is just to give some context :)
>
> Obviously, this does not work well - it manages to migrate only few pages
> and crashes as it is missing locks/disabling interrupts and I probably
> should not just remove pages from LRU list (normally, I guess, only these
> can migrate) and a million of other things.
>
> The questions are:
>
> - what is the correct way of telling if the page is in CMA?
> is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
>
> - how to tell MM to move page away? I am calling migrate_pages() with
> an get_new_page callback which allocates a page with GFP_USER but without
> GFP_MOVABLE which should allocate new page out of CMA which seems ok but
> there is a little convern that we might want to add MOVABLE back when
> VFIO device is unplugged from the guest.
>
> - do I need to isolate pages by using isolate_migratepages_range,
> reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
> I dropped them for now and the patch uses only @migratepages from
> the compact_control struct.
>
> - are there any flags in madvise() to address this (could not
> locate any relevant)?
>
> - what else is missing? disabled interrupts? locks?
>
> Thanks!
>
>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>   arch/powerpc/mm/mmu_context_iommu.c | 40 +++++++++++++++++++++++++++++++------
>   mm/page_alloc.c                     | 36 +++++++++++++++++++++++++++++++++
>   2 files changed, 70 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
> index da6a216..bf6850e 100644
> --- a/arch/powerpc/mm/mmu_context_iommu.c
> +++ b/arch/powerpc/mm/mmu_context_iommu.c
> @@ -72,12 +72,15 @@ bool mm_iommu_preregistered(void)
>   }
>   EXPORT_SYMBOL_GPL(mm_iommu_preregistered);
>
> +extern int mm_iommu_move_page(unsigned long pfn);
> +
>   long mm_iommu_get(unsigned long ua, unsigned long entries,
>   		struct mm_iommu_table_group_mem_t **pmem)
>   {
>   	struct mm_iommu_table_group_mem_t *mem;
>   	long i, j, ret = 0, locked_entries = 0;
>   	struct page *page = NULL;
> +	unsigned long moved = 0, tried = 0;
>
>   	if (!current || !current->mm)
>   		return -ESRCH; /* process exited */
> @@ -122,15 +125,29 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
>   	}
>
>   	for (i = 0; i < entries; ++i) {
> +		unsigned long pfn;
> +
>   		if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
>   					1/* pages */, 1/* iswrite */, &page)) {
> -			for (j = 0; j < i; ++j)
> -				put_page(pfn_to_page(
> -						mem->hpas[j] >> PAGE_SHIFT));
> -			vfree(mem->hpas);
> -			kfree(mem);
>   			ret = -EFAULT;
> -			goto unlock_exit;
> +			goto put_exit;
> +		}
> +
> +		pfn = page_to_pfn(page);
> +		if (get_pageblock_migratetype(page) == MIGRATE_CMA)
> +		{
> +			unsigned long pfnold = pfn;
> +			put_page(page);
> +			page = NULL;
> +			mm_iommu_move_page(pfn);
> +			if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> +						1/* pages */, 1/* iswrite */, &page)) {
> +				ret = -EFAULT;
> +				goto put_exit;
> +			}
> +			pfn = page_to_pfn(page);
> +			if (pfn != pfnold)
> +				++moved;
>   		}
>
>   		mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
> @@ -144,6 +161,17 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
>
>   	list_add_rcu(&mem->next, &current->mm->context.iommu_group_mem_list);
>
> +	printk("***K*** %s %u: tried %lu, moved %lu of %lu\n", __func__, __LINE__,
> +			tried, moved, entries);
> +
> +put_exit:
> +	if (ret) {
> +		for (j = 0; j < i; ++j)
> +			put_page(pfn_to_page(mem->hpas[j] >> PAGE_SHIFT));
> +		vfree(mem->hpas);
> +		kfree(mem);
> +	}
> +
>   unlock_exit:
>   	if (locked_entries && ret)
>   		mm_iommu_adjust_locked_vm(current->mm, locked_entries, false);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ef19f22..0639cce 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7008,3 +7008,39 @@ bool is_free_buddy_page(struct page *page)
>   	return order < MAX_ORDER;
>   }
>   #endif
> +
> +static struct page *mm_iommu_new_page(struct page *page, unsigned long private,
> +				int **reason)
> +{
> +	 /*
> +	  * Anything but not GFP_MOVABLE!
> +	  */
> +	return alloc_page(GFP_USER);
> +}
> +
> +static void mm_iommu_free_page(struct page *page, unsigned long private)
> +{
> +	free_page(page_to_pfn(page) << PAGE_SHIFT);
> +}
> +
> +int mm_iommu_move_page(unsigned long pfn)
> +{
> +	unsigned long ret, nr_reclaimed;
> +	struct compact_control cc = {
> +		.nr_migratepages = 0,
> +		.order = -1,
> +		.zone = page_zone(pfn_to_page(pfn)),
> +		.mode = MIGRATE_SYNC,
> +		.ignore_skip_hint = true,
> +	};
> +	struct page *page = pfn_to_page(pfn);
> +	INIT_LIST_HEAD(&cc.migratepages);
> +
> +	if (PageLRU(page)) {
> +		list_del(&page->lru);
> +	}
> +	ret = migrate_pages(&cc.migratepages, mm_iommu_new_page,
> +			mm_iommu_free_page, 0, cc.mode, MR_CMA);
> +
> +	return ret;
> +}
>


-- 
Alexey

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