[RFC: DMA_PMD 03/22] mm: Add split_page_compound()

[Luigi Rizzo]

Add split_page_compound(), which splits an order-@old_order page into
independently refcounted order-@new_order compound pages (or order-0
pages when @new_order == 0). This is the high-order counterpart of
split_page(), used by DMA_PMD pools to carve PMD pages into smaller
compound blocks.

All resulting pieces are returned frozen (refcount 0) so the caller can
park them in a pool and publish each piece with page_ref_unfreeze(piece, 1)
when handed out.

Also add a KUnit test suite (CONFIG_SPLIT_PAGE_COMPOUND_KUNIT_TEST) in
mm/split_page_compound_kunit.c.

Signed-off-by: Luigi Rizzo <lrizzo@xxxxxxxxxx>
---
 include/linux/mm.h             |   2 +
 mm/Kconfig.debug               |  11 ++++
 mm/Makefile                    |   1 +
 mm/page_alloc.c                |  85 +++++++++++++++++++++++++
 mm/split_page_compound_kunit.c | 113 +++++++++++++++++++++++++++++++++
 5 files changed, 212 insertions(+)
 create mode 100644 mm/split_page_compound_kunit.c

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd09c438fa23e..799a42005627f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1985,6 +1985,8 @@ static inline struct folio *virt_to_folio(const void *x)
 void __folio_put(struct folio *folio);
 
 void split_page(struct page *page, unsigned int order);
+int split_page_compound(struct page *page, unsigned int old_order,
+			unsigned int new_order);
 void folio_copy(struct folio *dst, struct folio *src);
 int folio_mc_copy(struct folio *dst, struct folio *src);
 
diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index 15dca19dd07da..e1a04671c3b0d 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -347,3 +347,14 @@ config MEM_ALLOC_PROFILING_DEBUG
 	help
 	  Adds warnings with helpful error messages for memory allocation
 	  profiling.
+
+config SPLIT_PAGE_COMPOUND_KUNIT_TEST
+	bool "KUnit test for split_page_compound() (built-in)" if !KUNIT_ALL_TESTS
+	depends on KUNIT=y
+	default KUNIT_ALL_TESTS
+	help
+	  Builds KUnit unit tests for split_page_compound() in mm/page_alloc.c,
+	  verifying compound splitting, sub-block freezing, and speculative
+	  reference handling.
+
+	  If unsure, say N.
diff --git a/mm/Makefile b/mm/Makefile
index e7245cb88c665..448c0f5f783f3 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -148,3 +148,4 @@ obj-$(CONFIG_EXECMEM) += execmem.o
 obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
 obj-$(CONFIG_LAZY_MMU_MODE_KUNIT_TEST) += tests/lazy_mmu_mode_kunit.o
 obj-$(CONFIG_MEM_ALLOC_PROFILING) += alloc_tag.o
+obj-$(CONFIG_SPLIT_PAGE_COMPOUND_KUNIT_TEST) += split_page_compound_kunit.o
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e8e9905cdea5b..a663ec81a588b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3133,6 +3133,91 @@ void split_page(struct page *page, unsigned int order)
 }
 EXPORT_SYMBOL_GPL(split_page);
 
+/*
+ * split_page_compound() - split an order-@old_order page into compound pages
+ *			   of order @new_order
+ * @page: the page to split
+ * @old_order: the current order of @page
+ * @new_order: the order of the resulting pages
+ *
+ * Unlike split_page(), which produces order-0 pages, this produces
+ * 1 << (@old_order - @new_order) compound pages, each with its own head.
+ * @page must not be compound, and the caller must hold the only reference to
+ * it: the block is frozen while the new heads are built.
+ *
+ * All pieces (including @page at index 0) are returned frozen, with a zero
+ * refcount. Publish each piece with
+ * page_ref_unfreeze(piece, 1) before handing it out: its release store orders
+ * the compound layout built here against anyone who then observes the
+ * refcount. A relaxed set_page_count() would not, and on a weakly ordered
+ * machine a PFN walker could see a live refcount on a head whose layout is
+ * not visible yet.
+ *
+ * Memcg-charged pages are rejected: the accounting helpers assume a split
+ * produces order-0 pages and would mis-attribute the new compound heads.
+ *
+ * Return: 0 on success, -EINVAL if @page cannot be split or if @new_order is
+ * larger than @old_order, -EBUSY if anyone but the caller holds a reference
+ * to it.
+ */
+int split_page_compound(struct page *page, unsigned int old_order,
+			unsigned int new_order)
+{
+	unsigned int i, step = 1U << new_order, nr = 1U << old_order;
+
+	if (WARN_ON_ONCE(PageCompound(page) || !page_count(page)))
+		return -EINVAL;
+
+	if (WARN_ON_ONCE(new_order > old_order))
+		return -EINVAL;
+
+	if (WARN_ON_ONCE(memcg_kmem_online() && PageMemcgKmem(page)))
+		return -EINVAL;
+
+	/*
+	 * Shape the new compound pages while the block is frozen, which is
+	 * the order __alloc_pages_noprof() itself uses: prep_new_page()
+	 * builds the page with a zero refcount and the set_page_refcounted()
+	 * that follows is what makes it visible.
+	 *
+	 * Freezing stops a speculative PFN walker from resolving a compound
+	 * page that is only half built: get_page_unless_zero() fails for as
+	 * long as the layout below is being written. It does not order
+	 * against memory_failure(), which sets PG_hwpoison before taking any
+	 * reference, so the non-atomic __SetPageHead() below can still lose a
+	 * concurrent poison bit - exactly as it can for every other
+	 * prep_compound_page() caller, the page allocator included.
+	 *
+	 * A failed freeze is not a bug, it means someone holds a speculative
+	 * reference. Every PFN walker in the tree gates
+	 * get_page_unless_zero() on PageLRU, which a freshly allocated block
+	 * is not, so in practice only the hwpoison machinery gets here. Let
+	 * the caller fall back rather than warn: a machine running
+	 * panic_on_warn should not die of a condition the caller handles.
+	 */
+	if (!page_ref_freeze(page, 1))
+		return -EBUSY;
+
+	if (!new_order) {
+		/*
+		 * The subpages of a non-compound block are already
+		 * independent frozen pages, so only the bookkeeping applies.
+		 */
+		split_page_owner(page, old_order, 0);
+		pgalloc_tag_split(page_folio(page), old_order, 0);
+		split_page_memcg(page, old_order);
+		return 0;
+	}
+
+	split_page_owner(page, old_order, new_order);
+	pgalloc_tag_split(page_folio(page), old_order, new_order);
+
+	for (i = 0; i < nr; i += step)
+		prep_compound_page(page + i, new_order);
+
+	return 0;
+}
+
 int __isolate_free_page(struct page *page, unsigned int order)
 {
 	struct zone *zone = page_zone(page);
diff --git a/mm/split_page_compound_kunit.c b/mm/split_page_compound_kunit.c
new file mode 100644
index 0000000000000..bb0ac908bbc1e
--- /dev/null
+++ b/mm/split_page_compound_kunit.c
@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+/*
+ * KUnit tests for split_page_compound().
+ */
+#include <kunit/test.h>
+#include <linux/gfp.h>
+#include <linux/mm.h>
+
+static void test_split_compound_pieces(struct kunit *test)
+{
+	const unsigned int old_order = 5, new_order = 2;
+	const unsigned int step = 1U << new_order;
+	const unsigned int nr = 1U << old_order;
+	struct page *page;
+	unsigned int i, j;
+	int ret;
+
+	page = alloc_pages(GFP_KERNEL, old_order);
+	KUNIT_ASSERT_NOT_NULL(test, page);
+	KUNIT_EXPECT_FALSE(test, PageCompound(page));
+
+	ret = split_page_compound(page, old_order, new_order);
+	if (ret)
+		__free_pages(page, old_order);
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
+	for (i = 0; i < nr; i += step) {
+		struct page *sub = page + i;
+
+		/* All heads 0..N-1 are returned frozen (refcount 0). */
+		KUNIT_EXPECT_EQ(test, page_count(sub), 0);
+		KUNIT_EXPECT_TRUE(test, PageHead(sub));
+		KUNIT_EXPECT_EQ(test, compound_order(sub), new_order);
+
+		for (j = 1; j < step; j++) {
+			KUNIT_EXPECT_TRUE(test, PageTail(sub + j));
+			KUNIT_EXPECT_PTR_EQ(test, compound_head(sub + j), sub);
+		}
+
+		page_ref_unfreeze(sub, 1);
+
+		/* Tail get_page()/put_page() must operate on this piece's head. */
+		get_page(sub + 1);
+		KUNIT_EXPECT_EQ(test, page_count(sub), 2);
+		put_page(sub + 1);
+		KUNIT_EXPECT_EQ(test, page_count(sub), 1);
+	}
+
+	/* Each compound piece frees independently at new_order. */
+	for (i = 0; i < nr; i += step)
+		__free_pages(page + i, new_order);
+}
+
+static void test_split_order0_pieces(struct kunit *test)
+{
+	const unsigned int old_order = 3, nr = 1U << old_order;
+	struct page *page;
+	unsigned int i;
+	int ret;
+
+	page = alloc_pages(GFP_KERNEL, old_order);
+	KUNIT_ASSERT_NOT_NULL(test, page);
+
+	ret = split_page_compound(page, old_order, 0);
+	if (ret)
+		__free_pages(page, old_order);
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
+	for (i = 0; i < nr; i++) {
+		struct page *sub = page + i;
+
+		KUNIT_EXPECT_FALSE(test, PageCompound(sub));
+		KUNIT_EXPECT_EQ(test, page_count(sub), 0);
+		page_ref_unfreeze(sub, 1);
+		__free_pages(sub, 0);
+	}
+}
+
+static void test_split_ebusy_extra_ref(struct kunit *test)
+{
+	const unsigned int old_order = 4;
+	const unsigned int new_order = 2;
+	struct page *page;
+	int ret;
+
+	page = alloc_pages(GFP_KERNEL, old_order);
+	KUNIT_ASSERT_NOT_NULL(test, page);
+
+	/* Simulate a concurrent speculative reference. */
+	get_page(page);
+	ret = split_page_compound(page, old_order, new_order);
+	KUNIT_EXPECT_EQ(test, ret, -EBUSY);
+	KUNIT_EXPECT_FALSE(test, PageCompound(page));
+
+	put_page(page);
+	__free_pages(page, old_order);
+}
+
+static struct kunit_case split_page_compound_test_cases[] = {
+	KUNIT_CASE(test_split_compound_pieces),
+	KUNIT_CASE(test_split_order0_pieces),
+	KUNIT_CASE(test_split_ebusy_extra_ref),
+	{}
+};
+
+static struct kunit_suite split_page_compound_test_suite = {
+	.name = "split_page_compound",
+	.test_cases = split_page_compound_test_cases,
+};
+
+kunit_test_suite(split_page_compound_test_suite);
+MODULE_DESCRIPTION("KUnit tests for split_page_compound()");
+MODULE_LICENSE("Dual BSD/GPL");
-- 
2.56.0.rc1.315.gc6ed9934b7-goog