Re: [RFC PATCH 4/6] mm: thp: add support for split huge page to any lower order pages.

[Zi Yan <ziy@nvidia.com>]

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On 13 Nov 2020, at 19:52, Roman Gushchin wrote:

> On Wed, Nov 11, 2020 at 03:40:06PM -0500, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> To split a THP to any lower order pages, we need to reform THPs on
>> subpages at given order and add page refcount based on the new page
>> order. Also we need to reinitialize page_deferred_list after removing
>> the page from the split_queue, otherwise a subsequent split will see
>> list corruption when checking the page_deferred_list again.
>>
>> It has many uses, like minimizing the number of pages after
>> truncating a pagecache THP. For anonymous THPs, we can only split them
>> to order-0 like before until we add support for any size anonymous THPs.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>  include/linux/huge_mm.h |  8 +++++
>>  mm/huge_memory.c        | 78 +++++++++++++++++++++++++++++------------
>>  mm/swap.c               |  1 -
>>  3 files changed, 63 insertions(+), 24 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 60a907a19f7d..9819cd9b4619 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -189,6 +189,8 @@ bool is_transparent_hugepage(struct page *page);
>>
>>  bool can_split_huge_page(struct page *page, int *pextra_pins);
>>  int split_huge_page_to_list(struct page *page, struct list_head *list);
>> +int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>> +		unsigned int new_order);
>>  static inline int split_huge_page(struct page *page)
>>  {
>>  	return split_huge_page_to_list(page, NULL);
>> @@ -396,6 +398,12 @@ split_huge_page_to_list(struct page *page, struct list_head *list)
>>  {
>>  	return 0;
>>  }
>> +static inline int
>> +split_huge_page_to_order_to_list(struct page *page, struct list_head *list,
>> +		unsigned int new_order)
>
> It was
> int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> 		unsigned int new_order);
> above.

Right. It should be split_huge_page_to_list_to_order. Will fix it.

>
>> +{
>> +	return 0;
>> +}
>>  static inline int split_huge_page(struct page *page)
>>  {
>>  	return 0;
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 8b7d771ee962..88f50da40c9b 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2327,11 +2327,14 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
>>  static void unmap_page(struct page *page)
>>  {
>>  	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
>> -		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
>> +		TTU_RMAP_LOCKED;
>>  	bool unmap_success;
>>
>>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>>
>> +	if (thp_order(page) >= HPAGE_PMD_ORDER)
>> +		ttu_flags |= TTU_SPLIT_HUGE_PMD;
>> +
>>  	if (PageAnon(page))
>>  		ttu_flags |= TTU_SPLIT_FREEZE;
>>
>> @@ -2339,21 +2342,22 @@ static void unmap_page(struct page *page)
>>  	VM_BUG_ON_PAGE(!unmap_success, page);
>>  }
>>
>> -static void remap_page(struct page *page, unsigned int nr)
>> +static void remap_page(struct page *page, unsigned int nr, unsigned int new_nr)
>>  {
>>  	int i;
>> -	if (PageTransHuge(page)) {
>> +	if (thp_nr_pages(page) == nr) {
>>  		remove_migration_ptes(page, page, true);
>>  	} else {
>> -		for (i = 0; i < nr; i++)
>> +		for (i = 0; i < nr; i += new_nr)
>>  			remove_migration_ptes(page + i, page + i, true);
>>  	}
>>  }
>>
>>  static void __split_huge_page_tail(struct page *head, int tail,
>> -		struct lruvec *lruvec, struct list_head *list)
>> +		struct lruvec *lruvec, struct list_head *list, unsigned int new_order)
>>  {
>>  	struct page *page_tail = head + tail;
>> +	unsigned long compound_head_flag = new_order ? (1L << PG_head) : 0;
>>
>>  	VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
>>
>> @@ -2377,6 +2381,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
>>  #ifdef CONFIG_64BIT
>>  			 (1L << PG_arch_2) |
>>  #endif
>> +			 compound_head_flag |
>>  			 (1L << PG_dirty)));
>>
>>  	/* ->mapping in first tail page is compound_mapcount */
>> @@ -2395,10 +2400,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
>>  	 * which needs correct compound_head().
>>  	 */
>>  	clear_compound_head(page_tail);
>> +	if (new_order) {
>> +		prep_compound_page(page_tail, new_order);
>> +		thp_prep(page_tail);
>> +	}
>>
>>  	/* Finally unfreeze refcount. Additional reference from page cache. */
>> -	page_ref_unfreeze(page_tail, 1 + (!PageAnon(head) ||
>> -					  PageSwapCache(head)));
>> +	page_ref_unfreeze(page_tail, 1 + ((!PageAnon(head) ||
>> +					   PageSwapCache(head)) ?
>> +						thp_nr_pages(page_tail) : 0));
>>
>>  	if (page_is_young(head))
>>  		set_page_young(page_tail);
>> @@ -2416,7 +2426,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
>>  }
>>
>>  static void __split_huge_page(struct page *page, struct list_head *list,
>> -		pgoff_t end, unsigned long flags)
>> +		pgoff_t end, unsigned long flags, unsigned int new_order)
>>  {
>>  	struct page *head = compound_head(page);
>>  	pg_data_t *pgdat = page_pgdat(head);
>> @@ -2424,12 +2434,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  	struct address_space *swap_cache = NULL;
>>  	unsigned long offset = 0;
>>  	unsigned int nr = thp_nr_pages(head);
>> +	unsigned int new_nr = 1 << new_order;
>>  	int i;
>>
>>  	lruvec = mem_cgroup_page_lruvec(head, pgdat);
>>
>>  	/* complete memcg works before add pages to LRU */
>> -	mem_cgroup_split_huge_fixup(head, 1);
>> +	mem_cgroup_split_huge_fixup(head, new_nr);
>>
>>  	if (PageAnon(head) && PageSwapCache(head)) {
>>  		swp_entry_t entry = { .val = page_private(head) };
>> @@ -2439,14 +2450,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  		xa_lock(&swap_cache->i_pages);
>>  	}
>>
>> -	for (i = nr - 1; i >= 1; i--) {
>> -		__split_huge_page_tail(head, i, lruvec, list);
>> +	for (i = nr - new_nr; i >= new_nr; i -= new_nr) {
>> +		__split_huge_page_tail(head, i, lruvec, list, new_order);
>>  		/* Some pages can be beyond i_size: drop them from page cache */
>>  		if (head[i].index >= end) {
>>  			ClearPageDirty(head + i);
>>  			__delete_from_page_cache(head + i, NULL);
>>  			if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
>> -				shmem_uncharge(head->mapping->host, 1);
>> +				shmem_uncharge(head->mapping->host, new_nr);
>>  			put_page(head + i);
>>  		} else if (!PageAnon(page)) {
>>  			__xa_store(&head->mapping->i_pages, head[i].index,
>> @@ -2457,28 +2468,31 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  		}
>>  	}
>>
>> -	ClearPageCompound(head);
>> +	if (!new_order)
>> +		ClearPageCompound(head);
>> +	else
>> +		set_compound_order(head, new_order);
>>
>> -	split_page_owner(head, nr, 1);
>> +	split_page_owner(head, nr, new_nr);
>>
>>  	/* See comment in __split_huge_page_tail() */
>>  	if (PageAnon(head)) {
>>  		/* Additional pin to swap cache */
>>  		if (PageSwapCache(head)) {
>> -			page_ref_add(head, 2);
>> +			page_ref_add(head, 1 + new_nr);
>>  			xa_unlock(&swap_cache->i_pages);
>>  		} else {
>>  			page_ref_inc(head);
>>  		}
>>  	} else {
>>  		/* Additional pin to page cache */
>> -		page_ref_add(head, 2);
>> +		page_ref_add(head, 1 + new_nr);
>>  		xa_unlock(&head->mapping->i_pages);
>>  	}
>>
>>  	spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>>
>> -	remap_page(head, nr);
>> +	remap_page(head, nr, new_nr);
>>
>>  	if (PageSwapCache(head)) {
>>  		swp_entry_t entry = { .val = page_private(head) };
>> @@ -2486,7 +2500,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  		split_swap_cluster(entry);
>>  	}
>>
>> -	for (i = 0; i < nr; i++) {
>> +	for (i = 0; i < nr; i += new_nr) {
>>  		struct page *subpage = head + i;
>>  		if (subpage == page)
>>  			continue;
>> @@ -2620,21 +2634,39 @@ bool can_split_huge_page(struct page *page, int *pextra_pins)
>>   * us.
>>   */
>>  int split_huge_page_to_list(struct page *page, struct list_head *list)
>> +{
>> +	return split_huge_page_to_list_to_order(page, list, 0);
>> +}
>> +
>> +int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>> +				     unsigned int new_order)
>>  {
>>  	struct page *head = compound_head(page);
>>  	struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
>>  	struct deferred_split *ds_queue = get_deferred_split_queue(head);
>> -	XA_STATE(xas, &head->mapping->i_pages, head->index);
>> +	/* reset xarray order to new order after split */
>> +	XA_STATE_ORDER(xas, &head->mapping->i_pages, head->index, new_order);
>>  	struct anon_vma *anon_vma = NULL;
>>  	struct address_space *mapping = NULL;
>>  	int count, mapcount, extra_pins, ret;
>>  	unsigned long flags;
>>  	pgoff_t end;
>>
>> +	VM_BUG_ON(thp_order(head) <= new_order);
>>  	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
>>  	VM_BUG_ON_PAGE(!PageLocked(head), head);
>>  	VM_BUG_ON_PAGE(!PageCompound(head), head);
>>
>> +	if (new_order == 1) {
>> +		WARN_ONCE(1, "Cannot split THP to order-1 (no order-1 THPs)");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (PageAnon(head) && new_order) {
>> +		WARN_ONCE(1, "Split anonymous THP to non-zero order not support");
>> +		return -EINVAL;
>> +	}
>
> I'd convert those into VM_BUG_ON()'s. Unlikely they will be hit at arbitrary moments
> by random users.

Sure. Will change them.

Thanks.


—
Best Regards,
Yan Zi

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