Date:   Wed, 31 May 2017 13:59:54 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Dan Williams <dan.j.williams@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        Evgeny Baskakov <ebaskakov@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM 12/15] mm/migrate: new memory migration helper for use
 with device memory v4

On Wed, 24 May 2017 13:20:21 -0400
Jérôme Glisse <jglisse@...hat.com> wrote:

> 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 useful for device that
> have large pool of such like GPU, NVidia plans to use HMM for that.
> 

It is helpful, for HMM-CDM however we would like to avoid the downsides
of MIGRATE_SYNC_NOCOPY

> Changes since v3:
>   - Rebase
> 
> Changes since v2:
>   - droped HMM prefix and HMM specific code
> Changes since v1:
>   - typos fix
>   - split early unmap optimization for page with single mapping
> 
> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> Signed-off-by: Evgeny Baskakov <ebaskakov@...dia.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> Signed-off-by: Mark Hairgrove <mhairgrove@...dia.com>
> Signed-off-by: Sherry Cheung <SCheung@...dia.com>
> Signed-off-by: Subhash Gutti <sgutti@...dia.com>
> ---
>  include/linux/migrate.h | 104 ++++++++++++
>  mm/migrate.c            | 444 ++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 548 insertions(+)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 78a0fdc..576b3f5 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -127,4 +127,108 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  }
>  #endif /* CONFIG_NUMA_BALANCING && CONFIG_TRANSPARENT_HUGEPAGE*/
>  
> +
> +#ifdef CONFIG_MIGRATION
> +
> +#define MIGRATE_PFN_VALID	(1UL << 0)
> +#define MIGRATE_PFN_MIGRATE	(1UL << 1)
> +#define MIGRATE_PFN_LOCKED	(1UL << 2)
> +#define MIGRATE_PFN_WRITE	(1UL << 3)
> +#define MIGRATE_PFN_ERROR	(1UL << 4)
> +#define MIGRATE_PFN_SHIFT	5
> +
> +static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> +{
> +	if (!(mpfn & MIGRATE_PFN_VALID))
> +		return NULL;
> +	return pfn_to_page(mpfn >> MIGRATE_PFN_SHIFT);
> +}
> +
> +static inline unsigned long migrate_pfn(unsigned long pfn)
> +{
> +	return (pfn << MIGRATE_PFN_SHIFT) | MIGRATE_PFN_VALID;
> +}
> +
> +/*
> + * struct migrate_vma_ops - migrate operation callback
> + *
> + * @alloc_and_copy: alloc destination memory and copy source memory to it
> + * @finalize_and_map: allow caller to map the successfully migrated pages
> + *
> + *
> + * The alloc_and_copy() callback happens once all source pages have been locked,
> + * unmapped and checked (checked whether pinned or not). All pages that can be
> + * migrated will have an entry in the src array set with the pfn value of the
> + * page and with the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set (other
> + * flags might be set but should be ignored by the callback).
> + *
> + * The alloc_and_copy() callback can then allocate destination memory and copy
> + * source memory to it for all those entries (ie with MIGRATE_PFN_VALID and
> + * MIGRATE_PFN_MIGRATE flag set). Once these are allocated and copied, the
> + * callback must update each corresponding entry in the dst array with the pfn
> + * value of the destination page and with the MIGRATE_PFN_VALID and
> + * MIGRATE_PFN_LOCKED flags set (destination pages must have their struct pages
> + * locked, via lock_page()).
> + *
> + * At this point the alloc_and_copy() callback is done and returns.
> + *
> + * Note that the callback does not have to migrate all the pages that are
> + * marked with MIGRATE_PFN_MIGRATE flag in src array unless this is a migration
> + * from device memory to system memory (ie the MIGRATE_PFN_DEVICE flag is also
> + * set in the src array entry). If the device driver cannot migrate a device
> + * page back to system memory, then it must set the corresponding dst array
> + * entry to MIGRATE_PFN_ERROR. This will trigger a SIGBUS if CPU tries to
> + * access any of the virtual addresses originally backed by this page. Because
> + * a SIGBUS is such a severe result for the userspace process, the device
> + * driver should avoid setting MIGRATE_PFN_ERROR unless it is really in an
> + * unrecoverable state.
> + *
> + * THE alloc_and_copy() CALLBACK MUST NOT CHANGE ANY OF THE SRC ARRAY ENTRIES
> + * OR BAD THINGS WILL HAPPEN !
> + *
> + *
> + * The finalize_and_map() callback happens after struct page migration from
> + * source to destination (destination struct pages are the struct pages for the
> + * memory allocated by the alloc_and_copy() callback).  Migration can fail, and
> + * thus the finalize_and_map() allows the driver to inspect which pages were
> + * successfully migrated, and which were not. Successfully migrated pages will
> + * have the MIGRATE_PFN_MIGRATE flag set for their src array entry.
> + *
> + * It is safe to update device page table from within the finalize_and_map()
> + * callback because both destination and source page are still locked, and the
> + * mmap_sem is held in read mode (hence no one can unmap the range being
> + * migrated).
> + *
> + * Once callback is done cleaning up things and updating its page table (if it
> + * chose to do so, this is not an obligation) then it returns. At this point,
> + * the HMM core will finish up the final steps, and the migration is complete.
> + *
> + * THE finalize_and_map() CALLBACK MUST NOT CHANGE ANY OF THE SRC OR DST ARRAY
> + * ENTRIES OR BAD THINGS WILL HAPPEN !
> + */
> +struct migrate_vma_ops {
> +	void (*alloc_and_copy)(struct vm_area_struct *vma,
> +			       const unsigned long *src,
> +			       unsigned long *dst,
> +			       unsigned long start,
> +			       unsigned long end,
> +			       void *private);
> +	void (*finalize_and_map)(struct vm_area_struct *vma,
> +				 const unsigned long *src,
> +				 const unsigned long *dst,
> +				 unsigned long start,
> +				 unsigned long end,
> +				 void *private);
> +};
> +
> +int migrate_vma(const struct migrate_vma_ops *ops,
> +		struct vm_area_struct *vma,
> +		unsigned long start,
> +		unsigned long end,
> +		unsigned long *src,
> +		unsigned long *dst,
> +		void *private);
> +
> +#endif /* CONFIG_MIGRATION */
> +
>  #endif /* _LINUX_MIGRATE_H */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 66410fc..12063f3 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -397,6 +397,14 @@ 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_DEVICE_ALLOW_MIGRATE flag set (see memory_hotplug.h).

I couldn't find this flag in memory_hotplug.h? stale comment?

> +	 */
> +	expected_count += is_zone_device_page(page);
> +
>  	if (!mapping) {
>  		/* Anonymous page without mapping */
>  		if (page_count(page) != expected_count)
> @@ -2077,3 +2085,439 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  #endif /* CONFIG_NUMA_BALANCING */
>  
>  #endif /* CONFIG_NUMA */
> +
> +
> +struct migrate_vma {
> +	struct vm_area_struct	*vma;
> +	unsigned long		*dst;
> +	unsigned long		*src;
> +	unsigned long		cpages;
> +	unsigned long		npages;
> +	unsigned long		start;
> +	unsigned long		end;

Could we add a flags that specify if the migration should be MIGRATE_SYNC_NOCOPY or not?
I think the generic routine is helpful outside of the specific HMM use case as well.

> +};
> +
> +static int migrate_vma_collect_hole(unsigned long start,
> +				    unsigned long end,
> +				    struct mm_walk *walk)
> +{
> +	struct migrate_vma *migrate = walk->private;
> +	unsigned long addr, next;
> +
> +	for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE) {
> +		migrate->dst[migrate->npages] = 0;
> +		migrate->src[migrate->npages++] = 0;
> +	}
> +
> +	return 0;
> +}
> +
> +static int migrate_vma_collect_pmd(pmd_t *pmdp,
> +				   unsigned long start,
> +				   unsigned long end,
> +				   struct mm_walk *walk)
> +{
> +	struct migrate_vma *migrate = walk->private;
> +	struct mm_struct *mm = walk->vma->vm_mm;
> +	unsigned long addr = start;
> +	spinlock_t *ptl;
> +	pte_t *ptep;
> +
> +	if (pmd_none(*pmdp) || pmd_trans_unstable(pmdp)) {
> +		/* FIXME support THP */
> +		return migrate_vma_collect_hole(start, end, walk);
> +	}
> +
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +	for (; addr < end; addr += PAGE_SIZE, ptep++) {
> +		unsigned long mpfn, pfn;
> +		struct page *page;
> +		pte_t pte;
> +
> +		pte = *ptep;
> +		pfn = pte_pfn(pte);
> +
> +		if (!pte_present(pte)) {
> +			mpfn = pfn = 0;
> +			goto next;
> +		}
> +
> +		/* FIXME support THP */
> +		page = vm_normal_page(migrate->vma, addr, pte);
> +		if (!page || !page->mapping || PageTransCompound(page)) {
> +			mpfn = pfn = 0;
> +			goto next;
> +		}
> +
> +		/*
> +		 * By getting a reference on the page we pin it and that blocks
> +		 * any kind of migration. Side effect is that it "freezes" the
> +		 * pte.
> +		 *
> +		 * We drop this reference after isolating the page from the lru
> +		 * for non device page (device page are not on the lru and thus
> +		 * can't be dropped from it).
> +		 */
> +		get_page(page);
> +		migrate->cpages++;
> +		mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> +		mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
> +
> +next:
> +		migrate->src[migrate->npages++] = mpfn;
> +	}
> +	pte_unmap_unlock(ptep - 1, ptl);
> +
> +	return 0;
> +}
> +
> +/*
> + * migrate_vma_collect() - collect pages over a range of virtual addresses
> + * @migrate: migrate struct containing all migration information
> + *
> + * This will walk the CPU page table. For each virtual address backed by a
> + * valid page, it updates the src array and takes a reference on the page, in
> + * order to pin the page until we lock it and unmap it.
> + */
> +static void migrate_vma_collect(struct migrate_vma *migrate)
> +{
> +	struct mm_walk mm_walk;
> +
> +	mm_walk.pmd_entry = migrate_vma_collect_pmd;
> +	mm_walk.pte_entry = NULL;
> +	mm_walk.pte_hole = migrate_vma_collect_hole;
> +	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;
> +
> +	walk_page_range(migrate->start, migrate->end, &mm_walk);
> +
> +	migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
> +}
> +
> +/*
> + * migrate_vma_check_page() - check if page is pinned or not
> + * @page: struct page to check
> + *
> + * Pinned pages cannot be migrated. This is the same test as in
> + * migrate_page_move_mapping(), except that here we allow migration of a
> + * ZONE_DEVICE page.
> + */
> +static bool migrate_vma_check_page(struct page *page)
> +{
> +	/*
> +	 * One extra ref because caller holds an extra reference, either from
> +	 * isolate_lru_page() for a regular page, or migrate_vma_collect() for
> +	 * a device page.
> +	 */
> +	int extra = 1;
> +
> +	/*
> +	 * FIXME support THP (transparent huge page), it is bit more complex to
> +	 * check them than regular pages, because they can be mapped with a pmd
> +	 * or with a pte (split pte mapping).
> +	 */
> +	if (PageCompound(page))
> +		return false;
> +
> +	if ((page_count(page) - extra) > page_mapcount(page))
> +		return false;
> +
> +	return true;
> +}
> +
> +/*
> + * migrate_vma_prepare() - lock pages and isolate them from the lru
> + * @migrate: migrate struct containing all migration information
> + *
> + * This locks pages that have been collected by migrate_vma_collect(). Once each
> + * page is locked it is isolated from the lru (for non-device pages). Finally,
> + * the ref taken by migrate_vma_collect() is dropped, as locked pages cannot be
> + * migrated by concurrent kernel threads.
> + */
> +static void migrate_vma_prepare(struct migrate_vma *migrate)
> +{
> +	const unsigned long npages = migrate->npages;
> +	const unsigned long start = migrate->start;
> +	unsigned long addr, i, restore = 0;
> +	bool allow_drain = true;
> +
> +	lru_add_drain();
> +
> +	for (i = 0; i < npages; i++) {
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page)
> +			continue;
> +
> +		lock_page(page);
> +		migrate->src[i] |= MIGRATE_PFN_LOCKED;
> +
> +		if (!PageLRU(page) && allow_drain) {
> +			/* Drain CPU's pagevec */
> +			lru_add_drain_all();
> +			allow_drain = false;
> +		}
> +
> +		if (isolate_lru_page(page)) {
> +			migrate->src[i] = 0;
> +			unlock_page(page);
> +			migrate->cpages--;
> +			put_page(page);
> +			continue;
> +		}
> +
> +		if (!migrate_vma_check_page(page)) {
> +			migrate->src[i] = 0;
> +			unlock_page(page);
> +			migrate->cpages--;
> +
> +			putback_lru_page(page);
> +		}
> +	}
> +}
> +
> +/*
> + * migrate_vma_unmap() - replace page mapping with special migration pte entry
> + * @migrate: migrate struct containing all migration information
> + *
> + * Replace page mapping (CPU page table pte) with a special migration pte entry
> + * and check again if it has been pinned. Pinned pages are restored because we
> + * cannot migrate them.
> + *
> + * This is the last step before we call the device driver callback to allocate
> + * destination memory and copy contents of original page over to new page.
> + */
> +static void migrate_vma_unmap(struct migrate_vma *migrate)
> +{
> +	int flags = TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
> +	const unsigned long npages = migrate->npages;
> +	const unsigned long start = migrate->start;
> +	unsigned long addr, i, restore = 0;
> +
> +	for (i = 0; i < npages; i++) {
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page || !(migrate->src[i] & MIGRATE_PFN_MIGRATE))
> +			continue;
> +
> +		try_to_unmap(page, flags);
> +		if (page_mapped(page) || !migrate_vma_check_page(page)) {
> +			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +			migrate->cpages--;
> +			restore++;
> +		}
> +	}
> +
> +	for (addr = start, i = 0; i < npages && restore; addr += PAGE_SIZE, i++) {
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
> +			continue;
> +
> +		remove_migration_ptes(page, page, false);
> +
> +		migrate->src[i] = 0;
> +		unlock_page(page);
> +		restore--;
> +
> +		putback_lru_page(page);
> +	}
> +}
> +
> +/*
> + * migrate_vma_pages() - migrate meta-data from src page to dst page
> + * @migrate: migrate struct containing all migration information
> + *
> + * This migrates struct page meta-data from source struct page to destination
> + * struct page. This effectively finishes the migration from source page to the
> + * destination page.
> + */
> +static void migrate_vma_pages(struct migrate_vma *migrate)
> +{
> +	const unsigned long npages = migrate->npages;
> +	const unsigned long start = migrate->start;
> +	unsigned long addr, i;
> +
> +	for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
> +		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +		struct address_space *mapping;
> +		int r;
> +
> +		if (!page || !newpage)
> +			continue;
> +		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
> +			continue;
> +
> +		mapping = page_mapping(page);
> +
> +		r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);

Could we use a flags field to determine if we should use MIGRATE_SYNC_NO_COPY or not?

> +		if (r != MIGRATEPAGE_SUCCESS)
> +			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +	}
> +}
> +
> +/*
> + * migrate_vma_finalize() - restore CPU page table entry
> + * @migrate: migrate struct containing all migration information
> + *
> + * This replaces the special migration pte entry with either a mapping to the
> + * new page if migration was successful for that page, or to the original page
> + * otherwise.
> + *
> + * This also unlocks the pages and puts them back on the lru, or drops the extra
> + * refcount, for device pages.
> + */
> +static void migrate_vma_finalize(struct migrate_vma *migrate)
> +{
> +	const unsigned long npages = migrate->npages;
> +	unsigned long i;
> +
> +	for (i = 0; i < npages; i++) {
> +		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page)
> +			continue;
> +		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
> +			if (newpage) {
> +				unlock_page(newpage);
> +				put_page(newpage);
> +			}
> +			newpage = page;
> +		}
> +
> +		remove_migration_ptes(page, newpage, false);
> +		unlock_page(page);
> +		migrate->cpages--;
> +
> +		putback_lru_page(page);
> +
> +		if (newpage != page) {
> +			unlock_page(newpage);
> +			putback_lru_page(newpage);
> +		}
> +	}
> +}
> +
> +/*
> + * migrate_vma() - migrate a range of memory inside vma
> + *
> + * @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)
> + * @src: array of hmm_pfn_t containing source pfns
> + * @dst: array of hmm_pfn_t containing destination pfns
> + * @private: pointer passed back to each of the callback
> + * Returns: 0 on success, error code otherwise
> + *
> + * This function tries to migrate a range of memory virtual address range, using
> + * callbacks to allocate and copy memory from source to destination. First it
> + * collects all the pages backing each virtual address in the range, saving this
> + * inside the src array. Then it locks those pages and unmaps them. Once the pages
> + * are locked and unmapped, it checks whether each page is pinned or not. Pages
> + * that aren't pinned have the MIGRATE_PFN_MIGRATE flag set (by this function)
> + * in the corresponding src array entry. It then restores any pages that are
> + * pinned, by remapping and unlocking those pages.
> + *
> + * At this point it calls the alloc_and_copy() callback. For documentation on
> + * what is expected from that callback, see struct migrate_vma_ops comments in
> + * include/linux/migrate.h
> + *
> + * After the alloc_and_copy() callback, this function goes over each entry in
> + * the src array that has the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag
> + * set. If the corresponding entry in dst array has MIGRATE_PFN_VALID flag set,
> + * then the function tries to migrate struct page information from the source
> + * struct page to the destination struct page. If it fails to migrate the struct
> + * page information, then it clears the MIGRATE_PFN_MIGRATE flag in the src
> + * array.
> + *
> + * At this point all successfully migrated pages have an entry in the src
> + * array with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set and the dst
> + * array entry with MIGRATE_PFN_VALID flag set.
> + *
> + * It then calls the finalize_and_map() callback. See comments for "struct
> + * migrate_vma_ops", in include/linux/migrate.h for details about
> + * finalize_and_map() behavior.
> + *
> + * After the finalize_and_map() callback, for successfully migrated pages, this
> + * function updates the CPU page table to point to new pages, otherwise it
> + * restores the CPU page table to point to the original source pages.
> + *
> + * Function returns 0 after the above steps, even if no pages were migrated
> + * (The function only returns an error if any of the arguments are invalid.)
> + *
> + * Both src and dst array must be big enough for (end - start) >> PAGE_SHIFT
> + * unsigned long entries.
> + */
> +int migrate_vma(const struct migrate_vma_ops *ops,
> +		struct vm_area_struct *vma,
> +		unsigned long start,
> +		unsigned long end,
> +		unsigned long *src,
> +		unsigned long *dst,
> +		void *private)
> +{
> +	struct migrate_vma migrate;
> +
> +	/* Sanity check the arguments */
> +	start &= PAGE_MASK;
> +	end &= PAGE_MASK;
> +	if (!vma || is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL))
> +		return -EINVAL;
> +	if (start < vma->vm_start || start >= vma->vm_end)
> +		return -EINVAL;
> +	if (end <= vma->vm_start || end > vma->vm_end)
> +		return -EINVAL;
> +	if (!ops || !src || !dst || start >= end)
> +		return -EINVAL;
> +
> +	memset(src, 0, sizeof(*src) * ((end - start) >> PAGE_SHIFT));
> +	migrate.src = src;
> +	migrate.dst = dst;
> +	migrate.start = start;
> +	migrate.npages = 0;
> +	migrate.cpages = 0;
> +	migrate.end = end;
> +	migrate.vma = vma;
> +
> +	/* Collect, and try to unmap source pages */
> +	migrate_vma_collect(&migrate);
> +	if (!migrate.cpages)
> +		return 0;
> +
> +	/* Lock and isolate page */
> +	migrate_vma_prepare(&migrate);
> +	if (!migrate.cpages)
> +		return 0;
> +
> +	/* Unmap pages */
> +	migrate_vma_unmap(&migrate);
> +	if (!migrate.cpages)
> +		return 0;
> +
> +	/*
> +	 * At this point pages are locked and unmapped, 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 migrate_vma_struct_page() for each
> +	 * individual page.
> +	 */
> +	ops->alloc_and_copy(vma, src, dst, start, end, private);
> +
> +	/* This does the real migration of struct page */
> +	migrate_vma_pages(&migrate);
> +
> +	ops->finalize_and_map(vma, src, dst, start, end, private);
> +
> +	/* Unlock and remap pages */
> +	migrate_vma_finalize(&migrate);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(migrate_vma);

In general, its helpful to have

Acked-by: Balbir Singh <bsingharora@...il.com>

Balbir Singh.

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