[PATCH v9 2/8] mm/huge_memory: add two new (not yet used) functions for folio_split()

Zi Yan
2025-02-26T21:00:50Z

This is a preparation patch, both added functions are not used yet.

The added __split_unmapped_folio() is able to split a folio with its
mapping removed in two manners: 1) uniform split (the existing way), and
2) buddy allocator like split.

The added __split_folio_to_order() can split a folio into any lower order.
For uniform split, __split_unmapped_folio() calls it once to split the
given folio to the new order. For buddy allocator split,
__split_unmapped_folio() calls it (folio_order - new_order) times and each
time splits the folio containing the given page to one lower order.

Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Kairui Song <kasong@tencent.com>
---
mm/huge_memory.c | 338 ++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 337 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 468e8ea76bc9..b0105ba6db94 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3267,7 +3267,6 @@ static void remap_page(struct folio *folio, unsigned long nr, int flags)
static void lru_add_page_tail(struct folio *folio, struct page *tail,
struct lruvec *lruvec, struct list_head *list)
{
- VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
VM_BUG_ON_FOLIO(PageLRU(tail), folio);
lockdep_assert_held(&lruvec->lru_lock);

@@ -3511,6 +3510,343 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
caller_pins;
}

+/*
+ * It splits @folio into @new_order folios and copies the @folio metadata to
+ * all the resulting folios.
+ */
+static void __split_folio_to_order(struct folio *folio, int new_order)
+{
+ long nr_pages = folio_nr_pages(folio);
+ long new_nr_pages = 1 << new_order;
+ long index;
+
+ /*
+ * Skip the first new_nr_pages, since the new folio from them have all
+ * the flags from the original folio.
+ */
+ for (index = new_nr_pages; index < nr_pages; index += new_nr_pages) {
+ struct page *head = &folio->page;
+ struct page *new_head = head + index;
+
+ /*
+ * Careful: new_folio is not a "real" folio before we cleared PageTail.
+ * Don't pass it around before clear_compound_head().
+ */
+ struct folio *new_folio = (struct folio *)new_head;
+
+ VM_BUG_ON_PAGE(atomic_read(&new_head->_mapcount) != -1, new_head);
+
+ /*
+ * Clone page flags before unfreezing refcount.
+ *
+ * After successful get_page_unless_zero() might follow flags change,
+ * for example lock_page() which set PG_waiters.
+ *
+ * Note that for mapped sub-pages of an anonymous THP,
+ * PG_anon_exclusive has been cleared in unmap_folio() and is stored in
+ * the migration entry instead from where remap_page() will restore it.
+ * We can still have PG_anon_exclusive set on effectively unmapped and
+ * unreferenced sub-pages of an anonymous THP: we can simply drop
+ * PG_anon_exclusive (-> PG_mappedtodisk) for these here.
+ */
+ new_head->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
+ new_head->flags |= (head->flags &
+ ((1L << PG_referenced) |
+ (1L << PG_swapbacked) |
+ (1L << PG_swapcache) |
+ (1L << PG_mlocked) |
+ (1L << PG_uptodate) |
+ (1L << PG_active) |
+ (1L << PG_workingset) |
+ (1L << PG_locked) |
+ (1L << PG_unevictable) |
+#ifdef CONFIG_ARCH_USES_PG_ARCH_2
+ (1L << PG_arch_2) |
+#endif
+#ifdef CONFIG_ARCH_USES_PG_ARCH_3
+ (1L << PG_arch_3) |
+#endif
+ (1L << PG_dirty) |
+ LRU_GEN_MASK | LRU_REFS_MASK));
+
+ /* ->mapping in first and second tail page is replaced by other uses */
+ VM_BUG_ON_PAGE(new_nr_pages > 2 && new_head->mapping != TAIL_MAPPING,
+ new_head);
+ new_head->mapping = head->mapping;
+ new_head->index = head->index + index;
+
+ /*
+ * page->private should not be set in tail pages. Fix up and warn once
+ * if private is unexpectedly set.
+ */
+ if (unlikely(new_head->private)) {
+ VM_WARN_ON_ONCE_PAGE(true, new_head);
+ new_head->private = 0;
+ }
+
+ if (folio_test_swapcache(folio))
+ new_folio->swap.val = folio->swap.val + index;
+
+ /* Page flags must be visible before we make the page non-compound. */
+ smp_wmb();
+
+ /*
+ * Clear PageTail before unfreezing page refcount.
+ *
+ * After successful get_page_unless_zero() might follow put_page()
+ * which needs correct compound_head().
+ */
+ clear_compound_head(new_head);
+ if (new_order) {
+ prep_compound_page(new_head, new_order);
+ folio_set_large_rmappable(new_folio);
+
+ folio_set_order(folio, new_order);
+ }
+
+ if (folio_test_young(folio))
+ folio_set_young(new_folio);
+ if (folio_test_idle(folio))
+ folio_set_idle(new_folio);
+
+ folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio));
+ }
+
+ if (!new_order)
+ ClearPageCompound(&folio->page);
+}
+
+/*
+ * It splits an unmapped @folio to lower order smaller folios in two ways.
+ * @folio: the to-be-split folio
+ * @new_order: the smallest order of the after split folios (since buddy
+ * allocator like split generates folios with orders from @folio's
+ * order - 1 to new_order).
+ * @split_at: in buddy allocator like split, the folio containing @split_at
+ * will be split until its order becomes @new_order.
+ * @lock_at: the folio containing @lock_at is left locked for caller.
+ * @list: the after split folios will be added to @list if it is not NULL,
+ * otherwise to LRU lists.
+ * @end: the end of the file @folio maps to. -1 if @folio is anonymous memory.
+ * @xas: xa_state pointing to folio->mapping->i_pages and locked by caller
+ * @mapping: @folio->mapping
+ * @uniform_split: if the split is uniform or not (buddy allocator like split)
+ *
+ *
+ * 1. uniform split: the given @folio into multiple @new_order small folios,
+ * where all small folios have the same order. This is done when
+ * uniform_split is true.
+ * 2. buddy allocator like (non-uniform) split: the given @folio is split into
+ * half and one of the half (containing the given page) is split into half
+ * until the given @page's order becomes @new_order. This is done when
+ * uniform_split is false.
+ *
+ * The high level flow for these two methods are:
+ * 1. uniform split: a single __split_folio_to_order() is called to split the
+ * @folio into @new_order, then we traverse all the resulting folios one by
+ * one in PFN ascending order and perform stats, unfreeze, adding to list,
+ * and file mapping index operations.
+ * 2. non-uniform split: in general, folio_order - @new_order calls to
+ * __split_folio_to_order() are made in a for loop to split the @folio
+ * to one lower order at a time. The resulting small folios are processed
+ * like what is done during the traversal in 1, except the one containing
+ * @page, which is split in next for loop.
+ *
+ * After splitting, the caller's folio reference will be transferred to the
+ * folio containing @page. The other folios may be freed if they are not mapped.
+ *
+ * In terms of locking, after splitting,
+ * 1. uniform split leaves @page (or the folio contains it) locked;
+ * 2. buddy allocator like (non-uniform) split leaves @folio locked.
+ *
+ *
+ * For !uniform_split, when -ENOMEM is returned, the original folio might be
+ * split. The caller needs to check the input folio.
+ */
+static int __split_unmapped_folio(struct folio *folio, int new_order,
+ struct page *split_at, struct page *lock_at,
+ struct list_head *list, pgoff_t end,
+ struct xa_state *xas, struct address_space *mapping,
+ bool uniform_split)
+{
+ struct lruvec *lruvec;
+ struct address_space *swap_cache = NULL;
+ struct folio *origin_folio = folio;
+ struct folio *next_folio = folio_next(folio);
+ struct folio *new_folio;
+ struct folio *next;
+ int order = folio_order(folio);
+ int split_order;
+ int start_order = uniform_split ? new_order : order - 1;
+ int nr_dropped = 0;
+ int ret = 0;
+ bool stop_split = false;
+
+ if (folio_test_anon(folio) && folio_test_swapcache(folio)) {
+ /* a swapcache folio can only be uniformly split to order-0 */
+ if (!uniform_split || new_order != 0)
+ return -EINVAL;
+
+ swap_cache = swap_address_space(folio->swap);
+ xa_lock(&swap_cache->i_pages);
+ }
+
+ if (folio_test_anon(folio))
+ mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
+
+ /* lock lru list/PageCompound, ref frozen by page_ref_freeze */
+ lruvec = folio_lruvec_lock(folio);
+
+ folio_clear_has_hwpoisoned(folio);
+
+ /*
+ * split to new_order one order at a time. For uniform split,
+ * folio is split to new_order directly.
+ */
+ for (split_order = start_order;
+ split_order >= new_order && !stop_split;
+ split_order--) {
+ int old_order = folio_order(folio);
+ struct folio *release;
+ struct folio *end_folio = folio_next(folio);
+
+ /* order-1 anonymous folio is not supported */
+ if (folio_test_anon(folio) && split_order == 1)
+ continue;
+ if (uniform_split && split_order != new_order)
+ continue;
+
+ if (mapping) {
+ /*
+ * uniform split has xas_split_alloc() called before
+ * irq is disabled to allocate enough memory, whereas
+ * non-uniform split can handle ENOMEM.
+ */
+ if (uniform_split)
+ xas_split(xas, folio, old_order);
+ else {
+ xas_set_order(xas, folio->index, split_order);
+ xas_try_split(xas, folio, old_order);
+ if (xas_error(xas)) {
+ ret = xas_error(xas);
+ stop_split = true;
+ goto after_split;
+ }
+ }
+ }
+
+ /* complete memcg works before add pages to LRU */
+ split_page_memcg(&folio->page, old_order, split_order);
+ split_page_owner(&folio->page, old_order, split_order);
+ pgalloc_tag_split(folio, old_order, split_order);
+
+ __split_folio_to_order(folio, split_order);
+
+after_split:
+ /*
+ * Iterate through after-split folios and perform related
+ * operations. But in buddy allocator like split, the folio
+ * containing the specified page is skipped until its order
+ * is new_order, since the folio will be worked on in next
+ * iteration.
+ */
+ for (release = folio, next = folio_next(folio);
+ release != end_folio;
+ release = next, next = folio_next(next)) {
+ /*
+ * for buddy allocator like split, the folio containing
+ * page will be split next and should not be released,
+ * until the folio's order is new_order or stop_split
+ * is set to true by the above xas_split() failure.
+ */
+ if (release == page_folio(split_at)) {
+ folio = release;
+ if (split_order != new_order && !stop_split)
+ continue;
+ }
+ if (folio_test_anon(release)) {
+ mod_mthp_stat(folio_order(release),
+ MTHP_STAT_NR_ANON, 1);
+ }
+
+ /*
+ * Unfreeze refcount first. Additional reference from
+ * page cache.
+ */
+ folio_ref_unfreeze(release,
+ 1 + ((!folio_test_anon(origin_folio) ||
+ folio_test_swapcache(origin_folio)) ?
+ folio_nr_pages(release) : 0));
+
+ if (release == origin_folio)
+ continue;
+
+ lru_add_page_tail(origin_folio, &release->page,
+ lruvec, list);
+
+ /* Some pages can be beyond EOF: drop them from page cache */
+ if (release->index >= end) {
+ if (shmem_mapping(origin_folio->mapping))
+ nr_dropped += folio_nr_pages(release);
+ else if (folio_test_clear_dirty(release))
+ folio_account_cleaned(release,
+ inode_to_wb(origin_folio->mapping->host));
+ __filemap_remove_folio(release, NULL);
+ folio_put(release);
+ } else if (!folio_test_anon(release)) {
+ __xa_store(&origin_folio->mapping->i_pages,
+ release->index, &release->page, 0);
+ } else if (swap_cache) {
+ __xa_store(&swap_cache->i_pages,
+ swap_cache_index(release->swap),
+ &release->page, 0);
+ }
+ }
+ }
+
+ unlock_page_lruvec(lruvec);
+
+ if (folio_test_anon(origin_folio)) {
+ if (folio_test_swapcache(origin_folio))
+ xa_unlock(&swap_cache->i_pages);
+ } else
+ xa_unlock(&mapping->i_pages);
+
+ /* Caller disabled irqs, so they are still disabled here */
+ local_irq_enable();
+
+ if (nr_dropped)
+ shmem_uncharge(mapping->host, nr_dropped);
+
+ remap_page(origin_folio, 1 << order,
+ folio_test_anon(origin_folio) ?
+ RMP_USE_SHARED_ZEROPAGE : 0);
+
+ /*
+ * At this point, folio should contain the specified page.
+ * For uniform split, it is left for caller to unlock.
+ * For buddy allocator like split, the first after-split folio is left
+ * for caller to unlock.
+ */
+ for (new_folio = origin_folio, next = folio_next(origin_folio);
+ new_folio != next_folio;
+ new_folio = next, next = folio_next(next)) {
+ if (new_folio == page_folio(lock_at))
+ continue;
+
+ folio_unlock(new_folio);
+ /*
+ * Subpages may be freed if there wasn't any mapping
+ * like if add_to_swap() is running on a lru page that
+ * had its mapping zapped. And freeing these pages
+ * requires taking the lru_lock so we do the put_page
+ * of the tail pages after the split is complete.
+ */
+ free_page_and_swap_cache(&new_folio->page);
+ }
+ return ret;
+}
+
/*
* This function splits a large folio into smaller folios of order @new_order.
* @page can point to any page of the large folio to split. The split operation
--
2.47.2