[PATCH v9 4/8] mm/huge_memory: add buddy allocator like (non-uniform) folio_split()

Zi Yan
2025-02-26T21:00:56Z

folio_split() splits a large folio in the same way as buddy allocator
splits a large free page for allocation. The purpose is to minimize the
number of folios after the split. For example, if user wants to free the
3rd subpage in a order-9 folio, folio_split() will split the order-9 folio
as:

O-0, O-0, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-8 if it is anon,
since anon folio does not support order-1 yet.
-----------------------------------------------------------------
| | | | | | | | |
|O-0|O-0|O-0|O-0| O-2 |...| O-7 | O-8 |
| | | | | | | | |
-----------------------------------------------------------------

O-1, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-9 if it is pagecache
---------------------------------------------------------------
| | | | | | | |
| O-1 |O-0|O-0| O-2 |...| O-7 | O-8 |
| | | | | | | |
---------------------------------------------------------------

It generates fewer folios (i.e., 11 or 10) than existing page split
approach, which splits the order-9 to 512 order-0 folios. It also reduces
the number of new xa_node needed during a pagecache folio split from 8 to
1, potentially decreasing the folio split failure rate due to memory
constraints.

folio_split() and existing split_huge_page_to_list_to_order() share the
folio unmapping and remapping code in __folio_split() and the common
backend split code in __split_unmapped_folio() using uniform_split
variable to distinguish their operations.

uniform_split_supported() and non_uniform_split_supported() are added to
factor out check code and will be used outside __folio_split() in the
following commit.

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 | 160 ++++++++++++++++++++++++++++++++++++-----------
1 file changed, 123 insertions(+), 37 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4c79f54566d4..0e45937c0d91 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3847,12 +3847,85 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
return ret;
}

+static bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
+ bool warns)
+{
+ if (folio_test_anon(folio)) {
+ /* order-1 is not supported for anonymous THP. */
+ VM_WARN_ONCE(warns && new_order == 1,
+ "Cannot split to order-1 folio");
+ return new_order != 1;
+ } else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
+ !mapping_large_folio_support(folio->mapping)) {
+ /*
+ * No split if the file system does not support large folio.
+ * Note that we might still have THPs in such mappings due to
+ * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
+ * does not actually support large folios properly.
+ */
+ VM_WARN_ONCE(warns,
+ "Cannot split file folio to non-0 order");
+ return false;
+ }
+
+ /* Only swapping a whole PMD-mapped folio is supported */
+ if (folio_test_swapcache(folio)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split swapcache folio to non-0 order");
+ return false;
+ }
+
+ return true;
+}
+
+/* See comments in non_uniform_split_supported() */
+static bool uniform_split_supported(struct folio *folio, unsigned int new_order,
+ bool warns)
+{
+ if (folio_test_anon(folio)) {
+ VM_WARN_ONCE(warns && new_order == 1,
+ "Cannot split to order-1 folio");
+ return new_order != 1;
+ } else if (new_order) {
+ if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
+ !mapping_large_folio_support(folio->mapping)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split file folio to non-0 order");
+ return false;
+ }
+ }
+
+ if (new_order && folio_test_swapcache(folio)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split swapcache folio to non-0 order");
+ return false;
+ }
+
+ return true;
+}
+
+/*
+ * __folio_split: split a folio at @split_at to a @new_order folio
+ * @folio: folio to split
+ * @new_order: the order of the new folio
+ * @split_at: a page within the new folio
+ * @lock_at: a page within @folio to be left locked to caller
+ * @list: after-split folios will be put on it if non NULL
+ * @uniform_split: perform uniform split or not (non-uniform split)
+ *
+ * It calls __split_unmapped_folio() to perform uniform and non-uniform split.
+ * It is in charge of checking whether the split is supported or not and
+ * preparing @folio for __split_unmapped_folio().
+ *
+ * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
+ * split but not to @new_order, the caller needs to check)
+ */
static int __folio_split(struct folio *folio, unsigned int new_order,
- struct page *page, struct list_head *list)
+ struct page *split_at, struct page *lock_at,
+ struct list_head *list, bool uniform_split)
{
struct deferred_split *ds_queue = get_deferred_split_queue(folio);
- /* reset xarray order to new order after split */
- XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
+ XA_STATE(xas, &folio->mapping->i_pages, folio->index);
bool is_anon = folio_test_anon(folio);
struct address_space *mapping = NULL;
struct anon_vma *anon_vma = NULL;
@@ -3864,32 +3937,17 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);

+ if (folio != page_folio(split_at) || folio != page_folio(lock_at))
+ return -EINVAL;
+
if (new_order >= folio_order(folio))
return -EINVAL;

- if (is_anon) {
- /* order-1 is not supported for anonymous THP. */
- if (new_order == 1) {
- VM_WARN_ONCE(1, "Cannot split to order-1 folio");
- return -EINVAL;
- }
- } else if (new_order) {
- /*
- * No split if the file system does not support large folio.
- * Note that we might still have THPs in such mappings due to
- * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
- * does not actually support large folios properly.
- */
- if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
- !mapping_large_folio_support(folio->mapping)) {
- VM_WARN_ONCE(1,
- "Cannot split file folio to non-0 order");
- return -EINVAL;
- }
- }
+ if (uniform_split && !uniform_split_supported(folio, new_order, true))
+ return -EINVAL;

- /* Only swapping a whole PMD-mapped folio is supported */
- if (folio_test_swapcache(folio) && new_order)
+ if (!uniform_split &&
+ !non_uniform_split_supported(folio, new_order, true))
return -EINVAL;

is_hzp = is_huge_zero_folio(folio);
@@ -3946,10 +4004,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
goto out;
}

- xas_split_alloc(&xas, folio, folio_order(folio), gfp);
- if (xas_error(&xas)) {
- ret = xas_error(&xas);
- goto out;
+ if (uniform_split) {
+ xas_set_order(&xas, folio->index, new_order);
+ xas_split_alloc(&xas, folio, folio_order(folio), gfp);
+ if (xas_error(&xas)) {
+ ret = xas_error(&xas);
+ goto out;
+ }
}

anon_vma = NULL;
@@ -4014,7 +4075,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
if (mapping) {
int nr = folio_nr_pages(folio);

- xas_split(&xas, folio, folio_order(folio));
if (folio_test_pmd_mappable(folio) &&
new_order < HPAGE_PMD_ORDER) {
if (folio_test_swapbacked(folio)) {
@@ -4028,12 +4088,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
}
}

- if (is_anon) {
- mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
- mod_mthp_stat(new_order, MTHP_STAT_NR_ANON, 1 << (order - new_order));
- }
- __split_huge_page(page, list, end, new_order);
- ret = 0;
+ ret = __split_unmapped_folio(folio, new_order,
+ split_at, lock_at, list, end, &xas, mapping,
+ uniform_split);
} else {
spin_unlock(&ds_queue->split_queue_lock);
fail:
@@ -4111,7 +4168,36 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
{
struct folio *folio = page_folio(page);

- return __folio_split(folio, new_order, page, list);
+ return __folio_split(folio, new_order, &folio->page, page, list, true);
+}
+
+/*
+ * folio_split: split a folio at @split_at to a @new_order folio
+ * @folio: folio to split
+ * @new_order: the order of the new folio
+ * @split_at: a page within the new folio
+ *
+ * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
+ * split but not to @new_order, the caller needs to check)
+ *
+ * It has the same prerequisites and returns as
+ * split_huge_page_to_list_to_order().
+ *
+ * Split a folio at @split_at to a new_order folio, leave the
+ * remaining subpages of the original folio as large as possible. For example,
+ * in the case of splitting an order-9 folio at its third order-3 subpages to
+ * an order-3 folio, there are 2^(9-3)=64 order-3 subpages in the order-9 folio.
+ * After the split, there will be a group of folios with different orders and
+ * the new folio containing @split_at is marked in bracket:
+ * [order-4, {order-3}, order-3, order-5, order-6, order-7, order-8].
+ *
+ * After split, folio is left locked for caller.
+ */
+static int folio_split(struct folio *folio, unsigned int new_order,
+ struct page *split_at, struct list_head *list)
+{
+ return __folio_split(folio, new_order, split_at, &folio->page, list,
+ false);
}

int min_order_for_split(struct folio *folio)
--
2.47.2