linux-kernel - Re: [HMM v13 16/18] mm/hmm/migrate: new memory migration helper for use with device memory

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Message-Id: <87k2c0muhj.fsf@linux.vnet.ibm.com>
Date:   Sat, 19 Nov 2016 01:27:28 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Jérôme Glisse <jglisse@...hat.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jatin Kumar <jakumar@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM v13 16/18] mm/hmm/migrate: new memory migration helper for use with device memory

Jérôme Glisse <jglisse@...hat.com> writes:

> This patch add a new memory migration helpers, which migrate memory
> backing a range of virtual address of a process to different memory
> (which can be allocated through special allocator). It differs from
> numa migration by working on a range of virtual address and thus by
> doing migration in chunk that can be large enough to use DMA engine
> or special copy offloading engine.
>
> Expected users are any one with heterogeneous memory where different
> memory have different characteristics (latency, bandwidth, ...). As
> an example IBM platform with CAPI bus can make use of this feature
> to migrate between regular memory and CAPI device memory. New CPU
> architecture with a pool of high performance memory not manage as
> cache but presented as regular memory (while being faster and with
> lower latency than DDR) will also be prime user of this patch.
>
> Migration to private device memory will be usefull for device that
> have large pool of such like GPU, NVidia plans to use HMM for that.
>



..............


>+
> +static int hmm_collect_walk_pmd(pmd_t *pmdp,
> +				unsigned long start,
> +				unsigned long end,
> +				struct mm_walk *walk)
> +{
> +	struct hmm_migrate *migrate = walk->private;
> +	struct mm_struct *mm = walk->vma->vm_mm;
> +	unsigned long addr = start;
> +	spinlock_t *ptl;
> +	hmm_pfn_t *pfns;
> +	int pages = 0;
> +	pte_t *ptep;
> +
> +again:
> +	if (pmd_none(*pmdp))
> +		return 0;
> +
> +	split_huge_pmd(walk->vma, pmdp, addr);
> +	if (pmd_trans_unstable(pmdp))
> +		goto again;
> +
> +	pfns = &migrate->pfns[(addr - migrate->start) >> PAGE_SHIFT];
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +	arch_enter_lazy_mmu_mode();
> +
> +	for (; addr < end; addr += PAGE_SIZE, pfns++, ptep++) {
> +		unsigned long pfn;
> +		swp_entry_t entry;
> +		struct page *page;
> +		hmm_pfn_t flags;
> +		bool write;
> +		pte_t pte;
> +
> +		pte = ptep_get_and_clear(mm, addr, ptep);
> +		if (!pte_present(pte)) {
> +			if (pte_none(pte))
> +				continue;
> +
> +			entry = pte_to_swp_entry(pte);
> +			if (!is_device_entry(entry)) {
> +				set_pte_at(mm, addr, ptep, pte);
> +				continue;
> +			}
> +
> +			flags = HMM_PFN_DEVICE | HMM_PFN_UNADDRESSABLE;
> +			page = device_entry_to_page(entry);
> +			write = is_write_device_entry(entry);
> +			pfn = page_to_pfn(page);
> +
> +			if (!(page->pgmap->flags & MEMORY_MOVABLE)) {
> +				set_pte_at(mm, addr, ptep, pte);
> +				continue;
> +			}
> +
> +		} else {
> +			pfn = pte_pfn(pte);
> +			page = pfn_to_page(pfn);
> +			write = pte_write(pte);
> +			flags = is_zone_device_page(page) ? HMM_PFN_DEVICE : 0;
> +		}
> +
> +		/* FIXME support THP see hmm_migrate_page_check() */
> +		if (PageTransCompound(page))
> +			continue;
> +
> +		*pfns = hmm_pfn_from_pfn(pfn) | HMM_PFN_MIGRATE | flags;
> +		*pfns |= write ? HMM_PFN_WRITE : 0;
> +		migrate->npages++;
> +		get_page(page);
> +
> +		if (!trylock_page(page)) {
> +			set_pte_at(mm, addr, ptep, pte);
> +		} else {
> +			pte_t swp_pte;
> +
> +			*pfns |= HMM_PFN_LOCKED;
> +
> +			entry = make_migration_entry(page, write);
> +			swp_pte = swp_entry_to_pte(entry);
> +			if (pte_soft_dirty(pte))
> +				swp_pte = pte_swp_mksoft_dirty(swp_pte);
> +			set_pte_at(mm, addr, ptep, swp_pte);
> +
> +			page_remove_rmap(page, false);
> +			put_page(page);
> +			pages++;
> +		}

Can you explain this. What does a failure to lock means here. Also why
convert the pte to migration entries here ? We do that in try_to_unmap right ?


> +	}
> +
> +	arch_leave_lazy_mmu_mode();
> +	pte_unmap_unlock(ptep - 1, ptl);
> +
> +	/* Only flush the TLB if we actually modified any entries */
> +	if (pages)
> +		flush_tlb_range(walk->vma, start, end);
> +
> +	return 0;
> +}
>