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

[Balbir Singh]

On 19/11/16 05:18, JÃrÃme Glisse wrote:
> This patch add a new memory migration helpers, which migrate memory
             adds                       helper         migrates
> 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.
> 
> Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Signed-off-by: Jatin Kumar <jakumar@xxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
> Signed-off-by: Sherry Cheung <SCheung@xxxxxxxxxx>
> Signed-off-by: Subhash Gutti <sgutti@xxxxxxxxxx>
> ---
>  include/linux/hmm.h |  54 ++++-
>  mm/migrate.c        | 584 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 635 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index c79abfc..9777309 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -101,10 +101,13 @@ struct hmm;
>   * HMM_PFN_EMPTY: corresponding CPU page table entry is none (pte_none() true)
>   * HMM_PFN_FAULT: use by hmm_vma_fault() to signify which address need faulting
>   * HMM_PFN_DEVICE: this is device memory (ie a ZONE_DEVICE page)
> + * HMM_PFN_LOCKED: underlying struct page is lock
>   * HMM_PFN_SPECIAL: corresponding CPU page table entry is special ie result of
>   *      vm_insert_pfn() or vm_insert_page() and thus should not be mirror by a
>   *      device (the entry will never have HMM_PFN_VALID set and the pfn value
>   *      is undefine)
> + * HMM_PFN_MIGRATE: use by hmm_vma_migrate() to signify which address can be
> + *      migrated
>   * HMM_PFN_UNADDRESSABLE: unaddressable device memory (ZONE_DEVICE)
>   */
>  typedef unsigned long hmm_pfn_t;
> @@ -116,9 +119,11 @@ typedef unsigned long hmm_pfn_t;
>  #define HMM_PFN_EMPTY (1 << 4)
>  #define HMM_PFN_FAULT (1 << 5)
>  #define HMM_PFN_DEVICE (1 << 6)
> -#define HMM_PFN_SPECIAL (1 << 7)
> -#define HMM_PFN_UNADDRESSABLE (1 << 8)
> -#define HMM_PFN_SHIFT 9
> +#define HMM_PFN_LOCKED (1 << 7)
> +#define HMM_PFN_SPECIAL (1 << 8)
> +#define HMM_PFN_MIGRATE (1 << 9)
> +#define HMM_PFN_UNADDRESSABLE (1 << 10)
> +#define HMM_PFN_SHIFT 11
>  
>  static inline struct page *hmm_pfn_to_page(hmm_pfn_t pfn)
>  {
> @@ -323,6 +328,49 @@ bool hmm_vma_fault(struct vm_area_struct *vma,
>  		   hmm_pfn_t *pfns);
>  
>  
> +/*
> + * struct hmm_migrate_ops - migrate operation callback
> + *
> + * @alloc_and_copy: alloc destination memoiry and copy source to it
> + * @finalize_and_map: allow caller to inspect successfull migrated page
> + *
> + * The new HMM migrate helper hmm_vma_migrate() allow memory migration to use
> + * device DMA engine to perform copy from source to destination memory it also
> + * allow caller to use its own memory allocator for destination memory.
> + *
> + * Note that in alloc_and_copy device driver can decide not to migrate some of
> + * the entry, for those it must clear the HMM_PFN_MIGRATE flag. The destination
> + * page must lock and the corresponding hmm_pfn_t value in the array updated
> + * with the HMM_PFN_MIGRATE and HMM_PFN_LOCKED flag set (and of course be a
> + * valid entry). It is expected that the page allocated will have an elevated
> + * refcount and that a put_page() will free the page. Device driver might want
> + * to allocate with an extra-refcount if they want to control deallocation of
> + * failed migration inside the finalize_and_map() callback.
> + *
> + * Inside finalize_and_map() device driver must use the HMM_PFN_MIGRATE flag to
> + * determine which page have been successfully migrated.
> + */
> +struct hmm_migrate_ops {
> +	void (*alloc_and_copy)(struct vm_area_struct *vma,
> +			       unsigned long start,
> +			       unsigned long end,
> +			       hmm_pfn_t *pfns,
> +			       void *private);
> +	void (*finalize_and_map)(struct vm_area_struct *vma,
> +				 unsigned long start,
> +				 unsigned long end,
> +				 hmm_pfn_t *pfns,
> +				 void *private);
> +};
> +
> +int hmm_vma_migrate(const struct hmm_migrate_ops *ops,
> +		    struct vm_area_struct *vma,
> +		    unsigned long start,
> +		    unsigned long end,
> +		    hmm_pfn_t *pfns,
> +		    void *private);
> +
> +
>  /* Below are for HMM internal use only ! Not to be use by device driver ! */
>  void hmm_mm_destroy(struct mm_struct *mm);
>  
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d9ce8db..393d592 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -41,6 +41,7 @@
>  #include <linux/page_idle.h>
>  #include <linux/page_owner.h>
>  #include <linux/memremap.h>
> +#include <linux/hmm.h>
>  
>  #include <asm/tlbflush.h>
>  
> @@ -421,6 +422,14 @@ int migrate_page_move_mapping(struct address_space *mapping,
>  	int expected_count = 1 + extra_count;
>  	void **pslot;
>  
> +	/*
> +	 * ZONE_DEVICE pages have 1 refcount always held by their device
> +	 *
> +	 * Note that DAX memory will never reach that point as it does not have
> +	 * the MEMORY_MOVABLE flag set (see include/linux/memory_hotplug.h).
> +	 */
> +	expected_count += is_zone_device_page(page);
> +
>  	if (!mapping) {
>  		/* Anonymous page without mapping */
>  		if (page_count(page) != expected_count)
> @@ -2087,3 +2096,578 @@ out_unlock:
>  #endif /* CONFIG_NUMA_BALANCING */
>  
>  #endif /* CONFIG_NUMA */
> +
> +
> +#if defined(CONFIG_HMM)
> +struct hmm_migrate {
> +	struct vm_area_struct	*vma;
> +	unsigned long		start;
> +	unsigned long		end;
> +	unsigned long		npages;
> +	hmm_pfn_t		*pfns;

I presume the destination is pfns[] or is the source?

> +};
> +
> +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;
> +

OK., so we always split THP before migration


> +	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);

Why hard code this, in general the ability to migrate a VMA
start/end range seems like a useful API.

> +				continue;
> +			}
> +
> +			flags = HMM_PFN_DEVICE | HMM_PFN_UNADDRESSABLE;

Currently 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;

Didn't we split the THP above?

> +
> +		*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);

put_page()?

> +		} 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++;
> +		}
> +	}
> +
> +	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;
> +}
> +
> +static void hmm_migrate_collect(struct hmm_migrate *migrate)
> +{
> +	struct mm_walk mm_walk;
> +
> +	mm_walk.pmd_entry = hmm_collect_walk_pmd;
> +	mm_walk.pte_entry = NULL;
> +	mm_walk.pte_hole = NULL;
> +	mm_walk.hugetlb_entry = NULL;
> +	mm_walk.test_walk = NULL;
> +	mm_walk.vma = migrate->vma;
> +	mm_walk.mm = migrate->vma->vm_mm;
> +	mm_walk.private = migrate;
> +
> +	mmu_notifier_invalidate_range_start(mm_walk.mm,
> +					    migrate->start,
> +					    migrate->end);
> +	walk_page_range(migrate->start, migrate->end, &mm_walk);
> +	mmu_notifier_invalidate_range_end(mm_walk.mm,
> +					  migrate->start,
> +					  migrate->end);
> +}
> +
> +static inline bool hmm_migrate_page_check(struct page *page, int extra)
> +{
> +	/*
> +	 * FIXME support THP (transparent huge page), it is bit more complex to
> +	 * check them then regular page because they can be map with a pmd or
> +	 * with a pte (split pte mapping).
> +	 */
> +	if (PageCompound(page))
> +		return false;

PageTransCompound()?

> +
> +	if (is_zone_device_page(page))
> +		extra++;
> +
> +	if ((page_count(page) - extra) > page_mapcount(page))
> +		return false;
> +
> +	return true;
> +}
> +
> +static void hmm_migrate_lock_and_isolate(struct hmm_migrate *migrate)
> +{
> +	unsigned long addr = migrate->start, i = 0;
> +	struct mm_struct *mm = migrate->vma->vm_mm;
> +	struct vm_area_struct *vma = migrate->vma;
> +	unsigned long restore = 0;
> +	bool allow_drain = true;
> +
> +	lru_add_drain();
> +
> +again:
> +	for (; addr < migrate->end; addr += PAGE_SIZE, i++) {
> +		struct page *page = hmm_pfn_to_page(migrate->pfns[i]);
> +
> +		if (!page)
> +			continue;
> +
> +		if (!(migrate->pfns[i] & HMM_PFN_LOCKED)) {
> +			lock_page(page);
> +			migrate->pfns[i] |= HMM_PFN_LOCKED;
> +		}
> +
> +		/* ZONE_DEVICE page are not on LRU */
> +		if (is_zone_device_page(page))
> +			goto check;
> +
> +		if (!PageLRU(page) && allow_drain) {
> +			/* Drain CPU's pagevec so page can be isolated */
> +			lru_add_drain_all();
> +			allow_drain = false;
> +			goto again;
> +		}
> +
> +		if (isolate_lru_page(page)) {
> +			migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
> +			migrate->npages--;
> +			put_page(page);
> +			restore++;
> +		} else
> +			/* Drop the reference we took in collect */
> +			put_page(page);
> +
> +check:
> +		if (!hmm_migrate_page_check(page, 1)) {
> +			migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
> +			migrate->npages--;
> +			restore++;
> +		}
> +	}
> +
> +	if (!restore)
> +		return;
> +
> +	for (addr = migrate->start, i = 0; addr < migrate->end;) {
> +		struct page *page = hmm_pfn_to_page(migrate->pfns[i]);
> +		unsigned long next, restart;
> +		spinlock_t *ptl;
> +		pgd_t *pgdp;
> +		pud_t *pudp;
> +		pmd_t *pmdp;
> +		pte_t *ptep;
> +
> +		if (!page || !(migrate->pfns[i] & HMM_PFN_MIGRATE)) {
> +			addr += PAGE_SIZE;
> +			i++;
> +			continue;
> +		}
> +
> +		restart = addr;
> +		pgdp = pgd_offset(mm, addr);
> +		if (!pgdp || pgd_none_or_clear_bad(pgdp)) {
> +			addr = pgd_addr_end(addr, migrate->end);
> +			i = (addr - migrate->start) >> PAGE_SHIFT;
> +			continue;
> +		}
> +		pudp = pud_offset(pgdp, addr);
> +		if (!pudp || pud_none(*pudp)) {
> +			addr = pgd_addr_end(addr, migrate->end);
> +			i = (addr - migrate->start) >> PAGE_SHIFT;
> +			continue;
> +		}
> +		pmdp = pmd_offset(pudp, addr);
> +		next = pmd_addr_end(addr, migrate->end);
> +		if (!pmdp || pmd_none(*pmdp) || pmd_trans_huge(*pmdp)) {
> +			addr = next;
> +			i = (addr - migrate->start) >> PAGE_SHIFT;
> +			continue;
> +		}
> +		ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +		for (; addr < next; addr += PAGE_SIZE, i++, ptep++) {
> +			swp_entry_t entry;
> +			bool write;
> +			pte_t pte;
> +
> +			page = hmm_pfn_to_page(migrate->pfns[i]);
> +			if (!page || (migrate->pfns[i] & HMM_PFN_MIGRATE))
> +				continue;
> +
> +			write = migrate->pfns[i] & HMM_PFN_WRITE;
> +			write &= (vma->vm_flags & VM_WRITE);
> +
> +			/* Here it means pte must be a valid migration entry */
> +			pte = ptep_get_and_clear(mm, addr, ptep);
> +			if (pte_none(pte) || pte_present(pte))
> +				/* SOMETHING BAD IS GOING ON ! */
> +				continue;
> +			entry = pte_to_swp_entry(pte);
> +			if (!is_migration_entry(entry))
> +				/* SOMETHING BAD IS GOING ON ! */
> +				continue;
> +
> +			if (is_zone_device_page(page) &&
> +			    !is_addressable_page(page)) {
> +				entry = make_device_entry(page, write);
> +				pte = swp_entry_to_pte(entry);
> +			} else {
> +				pte = mk_pte(page, vma->vm_page_prot);
> +				pte = pte_mkold(pte);
> +				if (write)
> +					pte = pte_mkwrite(pte);
> +			}
> +			if (pte_swp_soft_dirty(*ptep))
> +				pte = pte_mksoft_dirty(pte);
> +
> +			get_page(page);
> +			set_pte_at(mm, addr, ptep, pte);
> +			if (PageAnon(page))
> +				page_add_anon_rmap(page, vma, addr, false);
> +			else
> +				page_add_file_rmap(page, false);

Why do we do the rmap bits here?


> +		}
> +		pte_unmap_unlock(ptep - 1, ptl);
> +
> +		addr = restart;
> +		i = (addr - migrate->start) >> PAGE_SHIFT;
> +		for (; addr < next && restore; addr += PAGE_SHIFT, i++) {
> +			page = hmm_pfn_to_page(migrate->pfns[i]);
> +			if (!page || (migrate->pfns[i] & HMM_PFN_MIGRATE))
> +				continue;
> +
> +			migrate->pfns[i] = 0;
> +			unlock_page(page);
> +			restore--;
> +
> +			if (is_zone_device_page(page)) {
> +				put_page(page);
> +				continue;
> +			}
> +
> +			putback_lru_page(page);
> +		}
> +
> +		if (!restore)
> +			break;
> +	}
> +}
> +
> +static void hmm_migrate_unmap(struct hmm_migrate *migrate)
> +{
> +	int flags = TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
> +	unsigned long addr = migrate->start, i = 0, restore = 0;
> +
> +	for (; addr < migrate->end; addr += PAGE_SIZE, i++) {
> +		struct page *page = hmm_pfn_to_page(migrate->pfns[i]);
> +
> +		if (!page || !(migrate->pfns[i] & HMM_PFN_MIGRATE))
> +			continue;
> +
> +		try_to_unmap(page, flags);
> +		if (page_mapped(page) || !hmm_migrate_page_check(page, 1)) {
> +			migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
> +			migrate->npages--;
> +			restore++;
> +		}
> +	}
> +
> +	for (; (addr < migrate->end) && restore; addr += PAGE_SIZE, i++) {
> +		struct page *page = hmm_pfn_to_page(migrate->pfns[i]);
> +
> +		if (!page || (migrate->pfns[i] & HMM_PFN_MIGRATE))
> +			continue;
> +
> +		remove_migration_ptes(page, page, false);
> +
> +		migrate->pfns[i] = 0;
> +		unlock_page(page);
> +		restore--;
> +
> +		if (is_zone_device_page(page)) {
> +			put_page(page);
> +			continue;
> +		}
> +
> +		putback_lru_page(page);
> +	}
> +}
> +
> +static void hmm_migrate_struct_page(struct hmm_migrate *migrate)
> +{
> +	unsigned long addr = migrate->start, i = 0;
> +	struct mm_struct *mm = migrate->vma->vm_mm;
> +
> +	for (; addr < migrate->end;) {
> +		unsigned long next;
> +		pgd_t *pgdp;
> +		pud_t *pudp;
> +		pmd_t *pmdp;
> +		pte_t *ptep;
> +
> +		pgdp = pgd_offset(mm, addr);
> +		if (!pgdp || pgd_none_or_clear_bad(pgdp)) {
> +			addr = pgd_addr_end(addr, migrate->end);
> +			i = (addr - migrate->start) >> PAGE_SHIFT;
> +			continue;
> +		}
> +		pudp = pud_offset(pgdp, addr);
> +		if (!pudp || pud_none(*pudp)) {
> +			addr = pgd_addr_end(addr, migrate->end);
> +			i = (addr - migrate->start) >> PAGE_SHIFT;
> +			continue;
> +		}
> +		pmdp = pmd_offset(pudp, addr);
> +		next = pmd_addr_end(addr, migrate->end);
> +		if (!pmdp || pmd_none(*pmdp) || pmd_trans_huge(*pmdp)) {
> +			addr = next;
> +			i = (addr - migrate->start) >> PAGE_SHIFT;
> +			continue;
> +		}
> +
> +		/* No need to lock nothing can change from under us */
> +		ptep = pte_offset_map(pmdp, addr);
> +		for (; addr < next; addr += PAGE_SIZE, i++, ptep++) {
> +			struct address_space *mapping;
> +			struct page *newpage, *page;
> +			swp_entry_t entry;
> +			int r;
> +
> +			newpage = hmm_pfn_to_page(migrate->pfns[i]);
> +			if (!newpage || !(migrate->pfns[i] & HMM_PFN_MIGRATE))
> +				continue;
> +			if (pte_none(*ptep) || pte_present(*ptep)) {
> +				/* This should not happen but be nice */
> +				migrate->pfns[i] = 0;
> +				put_page(newpage);
> +				continue;
> +			}
> +			entry = pte_to_swp_entry(*ptep);
> +			if (!is_migration_entry(entry)) {
> +				/* This should not happen but be nice */
> +				migrate->pfns[i] = 0;
> +				put_page(newpage);
> +				continue;
> +			}
> +
> +			page = migration_entry_to_page(entry);
> +			mapping = page_mapping(page);
> +
> +			/*
> +			 * For now only support private anonymous when migrating
> +			 * to un-addressable device memory.

I thought HMM supported page cache migration as well.

> +			 */
> +			if (mapping && is_zone_device_page(newpage) &&
> +			    !is_addressable_page(newpage)) {
> +				migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
> +				continue;
> +			}
> +
> +			r = migrate_page(mapping, newpage, page,
> +					 MIGRATE_SYNC, false);
> +			if (r != MIGRATEPAGE_SUCCESS)
> +				migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
> +		}
> +		pte_unmap(ptep - 1);
> +	}
> +}
> +
> +static void hmm_migrate_remove_migration_pte(struct hmm_migrate *migrate)
> +{
> +	unsigned long addr = migrate->start, i = 0;
> +	struct mm_struct *mm = migrate->vma->vm_mm;
> +
> +	for (; addr < migrate->end;) {
> +		unsigned long next;
> +		pgd_t *pgdp;
> +		pud_t *pudp;
> +		pmd_t *pmdp;
> +		pte_t *ptep;
> +
> +		pgdp = pgd_offset(mm, addr);
> +		pudp = pud_offset(pgdp, addr);
> +		pmdp = pmd_offset(pudp, addr);
> +		next = pmd_addr_end(addr, migrate->end);
> +
> +		/* No need to lock nothing can change from under us */
> +		ptep = pte_offset_map(pmdp, addr);
> +		for (; addr < next; addr += PAGE_SIZE, i++, ptep++) {
> +			struct page *page, *newpage;
> +			swp_entry_t entry;
> +
> +			if (pte_none(*ptep) || pte_present(*ptep))
> +				continue;
> +			entry = pte_to_swp_entry(*ptep);
> +			if (!is_migration_entry(entry))
> +				continue;
> +
> +			page = migration_entry_to_page(entry);
> +			newpage = hmm_pfn_to_page(migrate->pfns[i]);
> +			if (!newpage)
> +				newpage = page;
> +			remove_migration_ptes(page, newpage, false);
> +
> +			migrate->pfns[i] = 0;
> +			unlock_page(page);
> +			migrate->npages--;
> +
> +			if (is_zone_device_page(page))
> +				put_page(page);
> +			else
> +				putback_lru_page(page);
> +
> +			if (newpage != page) {
> +				unlock_page(newpage);
> +				if (is_zone_device_page(newpage))
> +					put_page(newpage);
> +				else
> +					putback_lru_page(newpage);
> +			}
> +		}
> +		pte_unmap(ptep - 1);
> +	}
> +}
> +
> +/*
> + * hmm_vma_migrate() - migrate a range of memory inside vma using accel copy
> + *
> + * @ops: migration callback for allocating destination memory and copying
> + * @vma: virtual memory area containing the range to be migrated
> + * @start: start address of the range to migrate (inclusive)
> + * @end: end address of the range to migrate (exclusive)
> + * @pfns: array of hmm_pfn_t first containing source pfns then destination
> + * @private: pointer passed back to each of the callback
> + * Returns: 0 on success, error code otherwise
> + *
> + * This will try to migrate a range of memory using callback to allocate and
> + * copy memory from source to destination. This function will first collect,
> + * lock and unmap pages in the range and then call alloc_and_copy() callback
> + * for device driver to allocate destination memory and copy from source.
> + *
> + * Then it will proceed and try to effectively migrate the page (struct page
> + * metadata) a step that can fail for various reasons. Before updating CPU page
> + * table it will call finalize_and_map() callback so that device driver can
> + * inspect what have been successfully migrated and update its own page table
> + * (this latter aspect is not mandatory and only make sense for some user of
> + * this API).
> + *
> + * Finaly the function update CPU page table and unlock the pages before
> + * returning 0.
> + *
> + * It will return an error code only if one of the argument is invalid.
> + */
> +int hmm_vma_migrate(const struct hmm_migrate_ops *ops,
> +		    struct vm_area_struct *vma,
> +		    unsigned long start,
> +		    unsigned long end,
> +		    hmm_pfn_t *pfns,
> +		    void *private)
> +{
> +	struct hmm_migrate migrate;
> +
> +	/* Sanity check the arguments */
> +	start &= PAGE_MASK;
> +	end &= PAGE_MASK;
> +	if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL))
> +		return -EINVAL;
> +	if (!vma || !ops || !pfns || start >= end)
> +		return -EINVAL;
> +	if (start < vma->vm_start || start >= vma->vm_end)
> +		return -EINVAL;
> +	if (end <= vma->vm_start || end > vma->vm_end)
> +		return -EINVAL;
> +
> +	migrate.start = start;
> +	migrate.pfns = pfns;
> +	migrate.npages = 0;
> +	migrate.end = end;
> +	migrate.vma = vma;
> +
> +	/* Collect, and try to unmap source pages */
> +	hmm_migrate_collect(&migrate);
> +	if (!migrate.npages)
> +		return 0;
> +
> +	/* Lock and isolate page */
> +	hmm_migrate_lock_and_isolate(&migrate);
> +	if (!migrate.npages)
> +		return 0;
> +
> +	/* Unmap pages */
> +	hmm_migrate_unmap(&migrate);
> +	if (!migrate.npages)
> +		return 0;
> +
> +	/*
> +	 * At this point pages are lock and unmap and thus they have stable
> +	 * content and can safely be copied to destination memory that is
> +	 * allocated by the callback.
> +	 *
> +	 * Note that migration can fail in hmm_migrate_struct_page() for each
> +	 * individual page.
> +	 */
> +	ops->alloc_and_copy(vma, start, end, pfns, private);

What is the expectation from alloc_and_copy()? Can it fail?

> +
> +	/* This does the real migration of struct page */
> +	hmm_migrate_struct_page(&migrate);
> +
> +	ops->finalize_and_map(vma, start, end, pfns, private);

Is this just notification to the driver or more?

> +
> +	/* Unlock and remap pages */
> +	hmm_migrate_remove_migration_pte(&migrate);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(hmm_vma_migrate);
> +#endif /* CONFIG_HMM */
> 

Balbir Singh