Date:   Sun, 21 Jun 2020 19:20:57 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Ralph Campbell <rcampbell@...dia.com>
CC:     <nouveau@...ts.freedesktop.org>, <linux-rdma@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kselftest@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Jerome Glisse <jglisse@...hat.com>,
        "John Hubbard" <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        "Jason Gunthorpe" <jgg@...lanox.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 13/16] mm: support THP migration to device private memory

On 19 Jun 2020, at 17:56, Ralph Campbell wrote:

> Support transparent huge page migration to ZONE_DEVICE private memory.
> A new flag (MIGRATE_PFN_COMPOUND) is added to the input PFN array to
> indicate the huge page was fully mapped by the CPU.
> Export prep_compound_page() so that device drivers can create huge
> device private pages after calling memremap_pages().
>
> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
> ---
>  include/linux/migrate.h |   1 +
>  include/linux/mm.h      |   1 +
>  mm/huge_memory.c        |  30 ++++--
>  mm/internal.h           |   1 -
>  mm/memory.c             |  10 +-
>  mm/memremap.c           |   9 +-
>  mm/migrate.c            | 226 ++++++++++++++++++++++++++++++++--------
>  mm/page_alloc.c         |   1 +
>  8 files changed, 226 insertions(+), 53 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 3e546cbf03dd..f6a64965c8bd 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -166,6 +166,7 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  #define MIGRATE_PFN_MIGRATE	(1UL << 1)
>  #define MIGRATE_PFN_LOCKED	(1UL << 2)
>  #define MIGRATE_PFN_WRITE	(1UL << 3)
> +#define MIGRATE_PFN_COMPOUND	(1UL << 4)
>  #define MIGRATE_PFN_SHIFT	6
>
>  static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dc7b87310c10..020b9dd3cddb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -932,6 +932,7 @@ static inline unsigned int page_shift(struct page *page)
>  }
>
>  void free_compound_page(struct page *page);
> +void prep_compound_page(struct page *page, unsigned int order);
>
>  #ifdef CONFIG_MMU
>  /*
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 78c84bee7e29..25d95f7b1e98 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1663,23 +1663,35 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  	} else {
>  		struct page *page = NULL;
>  		int flush_needed = 1;
> +		bool is_anon = false;
>
>  		if (pmd_present(orig_pmd)) {
>  			page = pmd_page(orig_pmd);
> +			is_anon = PageAnon(page);
>  			page_remove_rmap(page, true);
>  			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
>  			VM_BUG_ON_PAGE(!PageHead(page), page);
>  		} else if (thp_migration_supported()) {
>  			swp_entry_t entry;
>
> -			VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
>  			entry = pmd_to_swp_entry(orig_pmd);
> -			page = pfn_to_page(swp_offset(entry));
> +			if (is_device_private_entry(entry)) {
> +				page = device_private_entry_to_page(entry);
> +				is_anon = PageAnon(page);
> +				page_remove_rmap(page, true);
> +				VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
> +				VM_BUG_ON_PAGE(!PageHead(page), page);
> +				put_page(page);

Why do you hide this code behind thp_migration_supported()? It seems that you just need
pmd swap entry not pmd migration entry. Also the condition is not consistent with the code
in __handle_mm_fault(), in which you handle is_device_private_entry() directly without
checking thp_migration_support().

Do we need to support split_huge_pmd() if a page is migrated to device? Any new code
needed in split_huge_pmd()?



> +			} else {
> +				VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
> +				page = pfn_to_page(swp_offset(entry));
> +				is_anon = PageAnon(page);
> +			}
>  			flush_needed = 0;
>  		} else
>  			WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
>
> -		if (PageAnon(page)) {
> +		if (is_anon) {
>  			zap_deposited_table(tlb->mm, pmd);
>  			add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
>  		} else {
> @@ -1778,8 +1790,8 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>  /*
>   * Returns
>   *  - 0 if PMD could not be locked
> - *  - 1 if PMD was locked but protections unchange and TLB flush unnecessary
> - *  - HPAGE_PMD_NR is protections changed and TLB flush necessary
> + *  - 1 if PMD was locked but protections unchanged and TLB flush unnecessary
> + *  - HPAGE_PMD_NR if protections changed and TLB flush necessary
>   */
>  int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  		unsigned long addr, pgprot_t newprot, unsigned long cp_flags)
> @@ -2689,7 +2701,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
>  		if (!list_empty(page_deferred_list(head))) {
>  			ds_queue->split_queue_len--;
> -			list_del(page_deferred_list(head));
> +			list_del_init(page_deferred_list(head));
>  		}
>  		spin_unlock(&ds_queue->split_queue_lock);
>  		if (mapping) {
> @@ -2743,7 +2755,7 @@ void free_transhuge_page(struct page *page)
>  	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>  	if (!list_empty(page_deferred_list(page))) {
>  		ds_queue->split_queue_len--;
> -		list_del(page_deferred_list(page));
> +		list_del_init(page_deferred_list(page));
>  	}
>  	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>  	free_compound_page(page);
> @@ -2963,6 +2975,10 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>  		pmde = pmd_mksoft_dirty(pmde);
>  	if (is_write_migration_entry(entry))
>  		pmde = maybe_pmd_mkwrite(pmde, vma);
> +	if (unlikely(is_device_private_page(new))) {
> +		entry = make_device_private_entry(new, pmd_write(pmde));
> +		pmde = swp_entry_to_pmd(entry);
> +	}
>
>  	flush_cache_range(vma, mmun_start, mmun_start + HPAGE_PMD_SIZE);
>  	if (PageAnon(new))
> diff --git a/mm/internal.h b/mm/internal.h
> index 9886db20d94f..58f051a14dae 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -196,7 +196,6 @@ extern void __putback_isolated_page(struct page *page, unsigned int order,
>  extern void memblock_free_pages(struct page *page, unsigned long pfn,
>  					unsigned int order);
>  extern void __free_pages_core(struct page *page, unsigned int order);
> -extern void prep_compound_page(struct page *page, unsigned int order);
>  extern void post_alloc_hook(struct page *page, unsigned int order,
>  					gfp_t gfp_flags);
>  extern int user_min_free_kbytes;
> diff --git a/mm/memory.c b/mm/memory.c
> index dc7f3543b1fd..4e03d1a2ead5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4322,9 +4322,15 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>
>  		barrier();
>  		if (unlikely(is_swap_pmd(orig_pmd))) {
> +			swp_entry_t entry = pmd_to_swp_entry(orig_pmd);
> +
> +			if (is_device_private_entry(entry)) {
> +				vmf.page = device_private_entry_to_page(entry);
> +				return vmf.page->pgmap->ops->migrate_to_ram(&vmf);
> +			}
>  			VM_BUG_ON(thp_migration_supported() &&
> -					  !is_pmd_migration_entry(orig_pmd));
> -			if (is_pmd_migration_entry(orig_pmd))
> +					  !is_migration_entry(entry));
> +			if (is_migration_entry(entry))
>  				pmd_migration_entry_wait(mm, vmf.pmd);
>  			return 0;
>  		}
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 03e38b7a38f1..4231054188b4 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -132,8 +132,13 @@ void memunmap_pages(struct dev_pagemap *pgmap)
>  	int nid;
>
>  	dev_pagemap_kill(pgmap);
> -	for_each_device_pfn(pfn, pgmap)
> -		put_page(pfn_to_page(pfn));
> +	for_each_device_pfn(pfn, pgmap) {
> +		struct page *page = pfn_to_page(pfn);
> +		unsigned int order = compound_order(page);
> +
> +		put_page(page);
> +		pfn += (1U << order) - 1;
> +	}
>  	dev_pagemap_cleanup(pgmap);
>
>  	/* make sure to access a memmap that was actually initialized */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 87c52e0ee580..1536677cefc9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -51,6 +51,7 @@
>  #include <linux/oom.h>
>
>  #include <asm/tlbflush.h>
> +#include <asm/pgalloc.h>
>
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/migrate.h>
> @@ -2185,6 +2186,8 @@ static int migrate_vma_collect_hole(unsigned long start,
>
>  	for (addr = start; addr < end; addr += PAGE_SIZE) {
>  		migrate->src[migrate->npages] = flags;
> +		if ((addr & ~PMD_MASK) == 0 && (end & ~PMD_MASK) == 0)
> +			migrate->src[migrate->npages] |= MIGRATE_PFN_COMPOUND;
>  		migrate->dst[migrate->npages] = 0;
>  		migrate->npages++;
>  		migrate->cpages++;
> @@ -2219,48 +2222,87 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  	unsigned long addr = start, unmapped = 0;
>  	spinlock_t *ptl;
>  	pte_t *ptep;
> +	pmd_t pmd;
>
>  again:
> -	if (pmd_none(*pmdp))
> +	pmd = READ_ONCE(*pmdp);
> +	if (pmd_none(pmd))
>  		return migrate_vma_collect_hole(start, end, -1, walk);
>
> -	if (pmd_trans_huge(*pmdp)) {
> +	if (pmd_trans_huge(pmd) || !pmd_present(pmd)) {
>  		struct page *page;
> +		unsigned long write = 0;
> +		int ret;
>
>  		ptl = pmd_lock(mm, pmdp);
> -		if (unlikely(!pmd_trans_huge(*pmdp))) {
> -			spin_unlock(ptl);
> -			goto again;
> -		}
> +		if (pmd_trans_huge(*pmdp)) {
> +			page = pmd_page(*pmdp);
> +			if (is_huge_zero_page(page)) {
> +				spin_unlock(ptl);
> +				return migrate_vma_collect_hole(start, end, -1,
> +								walk);
> +			}
> +			if (pmd_write(*pmdp))
> +				write = MIGRATE_PFN_WRITE;
> +		} else if (!pmd_present(*pmdp)) {
> +			swp_entry_t entry = pmd_to_swp_entry(*pmdp);
>
> -		page = pmd_page(*pmdp);
> -		if (is_huge_zero_page(page)) {
> -			spin_unlock(ptl);
> -			split_huge_pmd(vma, pmdp, addr);
> -			if (pmd_trans_unstable(pmdp))
> +			if (!is_device_private_entry(entry)) {
> +				spin_unlock(ptl);
>  				return migrate_vma_collect_skip(start, end,
>  								walk);
> +			}
> +			page = device_private_entry_to_page(entry);
> +			if (is_write_device_private_entry(entry))
> +				write = MIGRATE_PFN_WRITE;
>  		} else {
> -			int ret;
> +			spin_unlock(ptl);
> +			goto again;
> +		}
>
> -			get_page(page);
> +		get_page(page);
> +		if (unlikely(!trylock_page(page))) {
>  			spin_unlock(ptl);
> -			if (unlikely(!trylock_page(page)))
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -			ret = split_huge_page(page);
> -			unlock_page(page);
>  			put_page(page);
> -			if (ret)
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -			if (pmd_none(*pmdp))
> -				return migrate_vma_collect_hole(start, end, -1,
> -								walk);
> +			return migrate_vma_collect_skip(start, end, walk);
> +		}
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +		if ((start & HPAGE_PMD_MASK) == start &&
> +		    start + HPAGE_PMD_SIZE == end) {
> +			struct page_vma_mapped_walk vmw = {
> +				.vma = vma,
> +				.address = start,
> +				.pmd = pmdp,
> +				.ptl = ptl,
> +			};
> +
> +			migrate->src[migrate->npages] = write |
> +				migrate_pfn(page_to_pfn(page)) |
> +				MIGRATE_PFN_MIGRATE | MIGRATE_PFN_LOCKED |
> +				MIGRATE_PFN_COMPOUND;
> +			migrate->dst[migrate->npages] = 0;
> +			migrate->npages++;
> +			migrate->cpages++;
> +			migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
> +
> +			/* Note that this also removes the page from the rmap. */
> +			set_pmd_migration_entry(&vmw, page);
> +			spin_unlock(ptl);
> +
> +			return 0;
>  		}
> +#endif
> +		spin_unlock(ptl);
> +		ret = split_huge_page(page);
> +		unlock_page(page);
> +		put_page(page);
> +		if (ret)
> +			return migrate_vma_collect_skip(start, end, walk);
> +		if (pmd_none(*pmdp))
> +			return migrate_vma_collect_hole(start, end, -1, walk);
>  	}
>
> -	if (unlikely(pmd_bad(*pmdp)))
> +	if (unlikely(pmd_bad(pmd) || pmd_devmap(pmd)))
>  		return migrate_vma_collect_skip(start, end, walk);
>
>  	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> @@ -2310,8 +2352,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
>  		}
>
> -		/* FIXME support THP */
> -		if (!page || !page->mapping || PageTransCompound(page)) {
> +		if (!page || !page->mapping) {
>  			mpfn = 0;
>  			goto next;
>  		}
> @@ -2420,14 +2461,6 @@ static bool migrate_vma_check_page(struct page *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;
> -
>  	/* Page from ZONE_DEVICE have one extra reference */
>  	if (is_zone_device_page(page)) {
>  		/*
> @@ -2726,13 +2759,115 @@ int migrate_vma_setup(struct migrate_vma *args)
>  }
>  EXPORT_SYMBOL(migrate_vma_setup);
>
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +/*
> + * This code closely follows:
> + * do_huge_pmd_anonymous_page()
> + *   __do_huge_pmd_anonymous_page()
> + * except that the page being inserted is likely to be a device private page
> + * instead of an allocated or zero page.
> + */
> +static int insert_huge_pmd_anonymous_page(struct vm_area_struct *vma,
> +					  unsigned long haddr,
> +					  struct page *page,
> +					  unsigned long *src,
> +					  unsigned long *dst,
> +					  pmd_t *pmdp)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	unsigned int i;
> +	spinlock_t *ptl;
> +	bool flush = false;
> +	pgtable_t pgtable;
> +	gfp_t gfp;
> +	pmd_t entry;
> +
> +	if (WARN_ON_ONCE(compound_order(page) != HPAGE_PMD_ORDER))
> +		goto abort;
> +
> +	if (unlikely(anon_vma_prepare(vma)))
> +		goto abort;
> +
> +	prep_transhuge_page(page);
> +
> +	gfp = GFP_TRANSHUGE_LIGHT;
> +	if (mem_cgroup_charge(page, mm, gfp))
> +		goto abort;
> +
> +	pgtable = pte_alloc_one(mm);
> +	if (unlikely(!pgtable))
> +		goto abort;
> +
> +	__SetPageUptodate(page);
> +
> +	if (is_zone_device_page(page)) {
> +		if (!is_device_private_page(page))
> +			goto pgtable_abort;
> +		entry = swp_entry_to_pmd(make_device_private_entry(page,
> +						vma->vm_flags & VM_WRITE));
> +	} else {
> +		entry = mk_huge_pmd(page, vma->vm_page_prot);
> +		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> +	}
> +
> +	ptl = pmd_lock(mm, pmdp);
> +
> +	if (check_stable_address_space(mm))
> +		goto unlock_abort;
> +
> +	/*
> +	 * Check for userfaultfd but do not deliver the fault. Instead,
> +	 * just back off.
> +	 */
> +	if (userfaultfd_missing(vma))
> +		goto unlock_abort;
> +
> +	if (pmd_present(*pmdp)) {
> +		if (!is_huge_zero_pmd(*pmdp))
> +			goto unlock_abort;
> +		flush = true;
> +	} else if (!pmd_none(*pmdp))
> +		goto unlock_abort;
> +
> +	get_page(page);
> +	page_add_new_anon_rmap(page, vma, haddr, true);
> +	if (!is_device_private_page(page))
> +		lru_cache_add_active_or_unevictable(page, vma);
> +	if (flush) {
> +		pte_free(mm, pgtable);
> +		flush_cache_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
> +		pmdp_invalidate(vma, haddr, pmdp);
> +	} else {
> +		pgtable_trans_huge_deposit(mm, pmdp, pgtable);
> +		mm_inc_nr_ptes(mm);
> +	}
> +	set_pmd_at(mm, haddr, pmdp, entry);
> +	update_mmu_cache_pmd(vma, haddr, pmdp);
> +	add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
> +	spin_unlock(ptl);
> +	count_vm_event(THP_FAULT_ALLOC);
> +	count_memcg_event_mm(mm, THP_FAULT_ALLOC);
> +
> +	return 0;
> +
> +unlock_abort:
> +	spin_unlock(ptl);
> +pgtable_abort:
> +	pte_free(mm, pgtable);
> +abort:
> +	for (i = 0; i < HPAGE_PMD_NR; i++)
> +		src[i] &= ~MIGRATE_PFN_MIGRATE;
> +	return -EINVAL;
> +}
> +#endif
> +
>  /*
>   * This code closely matches the code in:
>   *   __handle_mm_fault()
>   *     handle_pte_fault()
>   *       do_anonymous_page()
> - * to map in an anonymous zero page but the struct page will be a ZONE_DEVICE
> - * private page.
> + * to map in an anonymous zero page except the struct page is already allocated
> + * and will likely be a ZONE_DEVICE private page.
>   */
>  static void migrate_vma_insert_page(struct migrate_vma *migrate,
>  				    unsigned long addr,
> @@ -2766,7 +2901,16 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
>  	if (!pmdp)
>  		goto abort;
>
> -	if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp))
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +	if (*dst & MIGRATE_PFN_COMPOUND) {
> +		int ret = insert_huge_pmd_anonymous_page(vma, addr, page, src,
> +							 dst, pmdp);
> +		if (ret)
> +			goto abort;
> +		return;
> +	}
> +#endif
> +	if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp) || pmd_bad(*pmdp))
>  		goto abort;
>
>  	/*
> @@ -2804,7 +2948,8 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
>
>  			swp_entry = make_device_private_entry(page, vma->vm_flags & VM_WRITE);
>  			entry = swp_entry_to_pte(swp_entry);
> -		}
> +		} else
> +			goto abort;
>  	} else {
>  		entry = mk_pte(page, vma->vm_page_prot);
>  		if (vma->vm_flags & VM_WRITE)
> @@ -2833,10 +2978,10 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
>  		goto unlock_abort;
>
>  	inc_mm_counter(mm, MM_ANONPAGES);
> +	get_page(page);
>  	page_add_new_anon_rmap(page, vma, addr, false);
>  	if (!is_zone_device_page(page))
>  		lru_cache_add_active_or_unevictable(page, vma);
> -	get_page(page);
>
>  	if (flush) {
>  		flush_cache_page(vma, addr, pte_pfn(*ptep));
> @@ -2850,7 +2995,6 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
>  	}
>
>  	pte_unmap_unlock(ptep, ptl);
> -	*src = MIGRATE_PFN_MIGRATE;
>  	return;
>
>  unlock_abort:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48eb0f1410d4..a852ed2f204c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -686,6 +686,7 @@ void prep_compound_page(struct page *page, unsigned int order)
>  	if (hpage_pincount_available(page))
>  		atomic_set(compound_pincount_ptr(page), 0);
>  }
> +EXPORT_SYMBOL_GPL(prep_compound_page);
>
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  unsigned int _debug_guardpage_minorder;
> -- 
> 2.20.1


--
Best Regards,
Yan Zi

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