[PATCH v4 2/6] mm: remap unused subpages to shared zeropage when splitting isolated thp

[Usama Arif <usamaarif642@gmail.com>]

From: Yu Zhao <yuzhao@google.com>

Here being unused means containing only zeros and inaccessible to
userspace. When splitting an isolated thp under reclaim or migration,
the unused subpages can be mapped to the shared zeropage, hence saving
memory. This is particularly helpful when the internal
fragmentation of a thp is high, i.e. it has many untouched subpages.

This is also a prerequisite for THP low utilization shrinker which will
be introduced in later patches, where underutilized THPs are split, and
the zero-filled pages are freed saving memory.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Shuang Zhai <zhais@google.com>
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 include/linux/rmap.h |  7 ++++-
 mm/huge_memory.c     |  8 ++---
 mm/migrate.c         | 72 ++++++++++++++++++++++++++++++++++++++------
 mm/migrate_device.c  |  4 +--
 4 files changed, 75 insertions(+), 16 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 0978c64f49d8..07854d1f9ad6 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -745,7 +745,12 @@ int folio_mkclean(struct folio *);
 int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
 		      struct vm_area_struct *vma);
 
-void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked);
+enum rmp_flags {
+	RMP_LOCKED		= 1 << 0,
+	RMP_USE_SHARED_ZEROPAGE	= 1 << 1,
+};
+
+void remove_migration_ptes(struct folio *src, struct folio *dst, int flags);
 
 /*
  * rmap_walk_control: To control rmap traversing for specific needs
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 147655821f09..2d77b5d2291e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2911,7 +2911,7 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
 	return false;
 }
 
-static void remap_page(struct folio *folio, unsigned long nr)
+static void remap_page(struct folio *folio, unsigned long nr, int flags)
 {
 	int i = 0;
 
@@ -2919,7 +2919,7 @@ static void remap_page(struct folio *folio, unsigned long nr)
 	if (!folio_test_anon(folio))
 		return;
 	for (;;) {
-		remove_migration_ptes(folio, folio, true);
+		remove_migration_ptes(folio, folio, RMP_LOCKED | flags);
 		i += folio_nr_pages(folio);
 		if (i >= nr)
 			break;
@@ -3129,7 +3129,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 
 	if (nr_dropped)
 		shmem_uncharge(folio->mapping->host, nr_dropped);
-	remap_page(folio, nr);
+	remap_page(folio, nr, PageAnon(head) ? RMP_USE_SHARED_ZEROPAGE : 0);
 
 	/*
 	 * set page to its compound_head when split to non order-0 pages, so
@@ -3425,7 +3425,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		if (mapping)
 			xas_unlock(&xas);
 		local_irq_enable();
-		remap_page(folio, folio_nr_pages(folio));
+		remap_page(folio, folio_nr_pages(folio), 0);
 		ret = -EAGAIN;
 	}
 
diff --git a/mm/migrate.c b/mm/migrate.c
index 66a5f73ebfdf..2d2e65d69427 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -178,13 +178,57 @@ void putback_movable_pages(struct list_head *l)
 	}
 }
 
+static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
+					  struct folio *folio,
+					  unsigned long idx)
+{
+	struct page *page = folio_page(folio, idx);
+	bool contains_data;
+	pte_t newpte;
+	void *addr;
+
+	VM_BUG_ON_PAGE(PageCompound(page), page);
+	VM_BUG_ON_PAGE(!PageAnon(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
+
+	if (PageMlocked(page) || (pvmw->vma->vm_flags & VM_LOCKED) ||
+	    mm_forbids_zeropage(pvmw->vma->vm_mm))
+		return false;
+
+	/*
+	 * The pmd entry mapping the old thp was flushed and the pte mapping
+	 * this subpage has been non present. If the subpage is only zero-filled
+	 * then map it to the shared zeropage.
+	 */
+	addr = kmap_local_page(page);
+	contains_data = memchr_inv(addr, 0, PAGE_SIZE);
+	kunmap_local(addr);
+
+	if (contains_data)
+		return false;
+
+	newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
+					pvmw->vma->vm_page_prot));
+	set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
+
+	dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
+	return true;
+}
+
+struct rmap_walk_arg {
+	struct folio *folio;
+	bool map_unused_to_zeropage;
+};
+
 /*
  * Restore a potential migration pte to a working pte entry
  */
 static bool remove_migration_pte(struct folio *folio,
-		struct vm_area_struct *vma, unsigned long addr, void *old)
+		struct vm_area_struct *vma, unsigned long addr, void *arg)
 {
-	DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+	struct rmap_walk_arg *rmap_walk_arg = arg;
+	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
@@ -208,6 +252,9 @@ static bool remove_migration_pte(struct folio *folio,
 			continue;
 		}
 #endif
+		if (rmap_walk_arg->map_unused_to_zeropage &&
+		    try_to_map_unused_to_zeropage(&pvmw, folio, idx))
+			continue;
 
 		folio_get(folio);
 		pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
@@ -286,14 +333,21 @@ static bool remove_migration_pte(struct folio *folio,
  * Get rid of all migration entries and replace them by
  * references to the indicated page.
  */
-void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked)
+void remove_migration_ptes(struct folio *src, struct folio *dst, int flags)
 {
+	struct rmap_walk_arg rmap_walk_arg = {
+		.folio = src,
+		.map_unused_to_zeropage = flags & RMP_USE_SHARED_ZEROPAGE,
+	};
+
 	struct rmap_walk_control rwc = {
 		.rmap_one = remove_migration_pte,
-		.arg = src,
+		.arg = &rmap_walk_arg,
 	};
 
-	if (locked)
+	VM_BUG_ON_FOLIO((flags & RMP_USE_SHARED_ZEROPAGE) && (src != dst), src);
+
+	if (flags & RMP_LOCKED)
 		rmap_walk_locked(dst, &rwc);
 	else
 		rmap_walk(dst, &rwc);
@@ -903,7 +957,7 @@ static int writeout(struct address_space *mapping, struct folio *folio)
 	 * At this point we know that the migration attempt cannot
 	 * be successful.
 	 */
-	remove_migration_ptes(folio, folio, false);
+	remove_migration_ptes(folio, folio, 0);
 
 	rc = mapping->a_ops->writepage(&folio->page, &wbc);
 
@@ -1067,7 +1121,7 @@ static void migrate_folio_undo_src(struct folio *src,
 				   struct list_head *ret)
 {
 	if (page_was_mapped)
-		remove_migration_ptes(src, src, false);
+		remove_migration_ptes(src, src, 0);
 	/* Drop an anon_vma reference if we took one */
 	if (anon_vma)
 		put_anon_vma(anon_vma);
@@ -1305,7 +1359,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 		lru_add_drain();
 
 	if (old_page_state & PAGE_WAS_MAPPED)
-		remove_migration_ptes(src, dst, false);
+		remove_migration_ptes(src, dst, 0);
 
 out_unlock_both:
 	folio_unlock(dst);
@@ -1443,7 +1497,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 
 	if (page_was_mapped)
 		remove_migration_ptes(src,
-			rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
+			rc == MIGRATEPAGE_SUCCESS ? dst : src, 0);
 
 unlock_put_anon:
 	folio_unlock(dst);
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 6d66dc1c6ffa..8f875636b35b 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -424,7 +424,7 @@ static unsigned long migrate_device_unmap(unsigned long *src_pfns,
 			continue;
 
 		folio = page_folio(page);
-		remove_migration_ptes(folio, folio, false);
+		remove_migration_ptes(folio, folio, 0);
 
 		src_pfns[i] = 0;
 		folio_unlock(folio);
@@ -837,7 +837,7 @@ void migrate_device_finalize(unsigned long *src_pfns,
 
 		src = page_folio(page);
 		dst = page_folio(newpage);
-		remove_migration_ptes(src, dst, false);
+		remove_migration_ptes(src, dst, 0);
 		folio_unlock(src);
 
 		if (is_zone_device_page(page))
-- 
2.43.5