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

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