[RFC PATCH 02/31] mm: migrate: Add THP exchange support.

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

Add support for exchange between two THPs.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/exchange.c | 47 ++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 38 insertions(+), 9 deletions(-)

diff --git a/mm/exchange.c b/mm/exchange.c
index a607348cc6f4..8cf286fc0f10 100644
--- a/mm/exchange.c
+++ b/mm/exchange.c
@@ -48,7 +48,8 @@ struct page_flags {
 	unsigned int page_swapcache:1;
 	unsigned int page_writeback:1;
 	unsigned int page_private:1;
-	unsigned int __pad:3;
+	unsigned int page_doublemap:1;
+	unsigned int __pad:2;
 };
 
 
@@ -125,20 +126,23 @@ static void exchange_huge_page(struct page *dst, struct page *src)
 static void exchange_page_flags(struct page *to_page, struct page *from_page)
 {
 	int from_cpupid, to_cpupid;
-	struct page_flags from_page_flags, to_page_flags;
+	struct page_flags from_page_flags = {0}, to_page_flags = {0};
 	struct mem_cgroup *to_memcg = page_memcg(to_page),
 					  *from_memcg = page_memcg(from_page);
 
 	from_cpupid = page_cpupid_xchg_last(from_page, -1);
 
-	from_page_flags.page_error = TestClearPageError(from_page);
+	from_page_flags.page_error = PageError(from_page);
+	if (from_page_flags.page_error)
+		ClearPageError(from_page);
 	from_page_flags.page_referenced = TestClearPageReferenced(from_page);
 	from_page_flags.page_uptodate = PageUptodate(from_page);
 	ClearPageUptodate(from_page);
 	from_page_flags.page_active = TestClearPageActive(from_page);
 	from_page_flags.page_unevictable = TestClearPageUnevictable(from_page);
 	from_page_flags.page_checked = PageChecked(from_page);
-	ClearPageChecked(from_page);
+	if (from_page_flags.page_checked)
+		ClearPageChecked(from_page);
 	from_page_flags.page_mappedtodisk = PageMappedToDisk(from_page);
 	ClearPageMappedToDisk(from_page);
 	from_page_flags.page_dirty = PageDirty(from_page);
@@ -148,18 +152,22 @@ static void exchange_page_flags(struct page *to_page, struct page *from_page)
 	clear_page_idle(from_page);
 	from_page_flags.page_swapcache = PageSwapCache(from_page);
 	from_page_flags.page_writeback = test_clear_page_writeback(from_page);
+	from_page_flags.page_doublemap = PageDoubleMap(from_page);
 
 
 	to_cpupid = page_cpupid_xchg_last(to_page, -1);
 
-	to_page_flags.page_error = TestClearPageError(to_page);
+	to_page_flags.page_error = PageError(to_page);
+	if (to_page_flags.page_error)
+		ClearPageError(to_page);
 	to_page_flags.page_referenced = TestClearPageReferenced(to_page);
 	to_page_flags.page_uptodate = PageUptodate(to_page);
 	ClearPageUptodate(to_page);
 	to_page_flags.page_active = TestClearPageActive(to_page);
 	to_page_flags.page_unevictable = TestClearPageUnevictable(to_page);
 	to_page_flags.page_checked = PageChecked(to_page);
-	ClearPageChecked(to_page);
+	if (to_page_flags.page_checked)
+		ClearPageChecked(to_page);
 	to_page_flags.page_mappedtodisk = PageMappedToDisk(to_page);
 	ClearPageMappedToDisk(to_page);
 	to_page_flags.page_dirty = PageDirty(to_page);
@@ -169,6 +177,7 @@ static void exchange_page_flags(struct page *to_page, struct page *from_page)
 	clear_page_idle(to_page);
 	to_page_flags.page_swapcache = PageSwapCache(to_page);
 	to_page_flags.page_writeback = test_clear_page_writeback(to_page);
+	to_page_flags.page_doublemap = PageDoubleMap(to_page);
 
 	/* set to_page */
 	if (from_page_flags.page_error)
@@ -195,6 +204,8 @@ static void exchange_page_flags(struct page *to_page, struct page *from_page)
 		set_page_young(to_page);
 	if (from_page_flags.page_is_idle)
 		set_page_idle(to_page);
+	if (from_page_flags.page_doublemap)
+		SetPageDoubleMap(to_page);
 
 	/* set from_page */
 	if (to_page_flags.page_error)
@@ -221,6 +232,8 @@ static void exchange_page_flags(struct page *to_page, struct page *from_page)
 		set_page_young(from_page);
 	if (to_page_flags.page_is_idle)
 		set_page_idle(from_page);
+	if (to_page_flags.page_doublemap)
+		SetPageDoubleMap(from_page);
 
 	/*
 	 * Copy NUMA information to the new page, to prevent over-eager
@@ -280,6 +293,7 @@ static int exchange_page_move_mapping(struct address_space *to_mapping,
 
 	VM_BUG_ON_PAGE(to_mapping != page_mapping(to_page), to_page);
 	VM_BUG_ON_PAGE(from_mapping != page_mapping(from_page), from_page);
+	VM_BUG_ON(PageCompound(from_page) != PageCompound(to_page));
 
 	if (!to_mapping) {
 		/* Anonymous page without mapping */
@@ -600,7 +614,6 @@ static int unmap_and_exchange(struct page *from_page,
 	to_mapping = to_page->mapping;
 	from_index = from_page->index;
 	to_index = to_page->index;
-
 	/*
 	 * Corner case handling:
 	 * 1. When a new swap-cache page is read into, it is added to the LRU
@@ -691,6 +704,23 @@ static int unmap_and_exchange(struct page *from_page,
 	return rc;
 }
 
+static bool can_be_exchanged(struct page *from, struct page *to)
+{
+	if (PageCompound(from) != PageCompound(to))
+		return false;
+
+	if (PageHuge(from) != PageHuge(to))
+		return false;
+
+	if (PageHuge(from) || PageHuge(to))
+		return false;
+
+	if (compound_order(from) != compound_order(to))
+		return false;
+
+	return true;
+}
+
 /*
  * Exchange pages in the exchange_list
  *
@@ -751,9 +781,8 @@ static int exchange_pages(struct list_head *exchange_list,
 			continue;
 		}
 
-		/* TODO: compound page not supported */
 		/* to_page can be file-backed page  */
-		if (PageCompound(from_page) ||
+		if (!can_be_exchanged(from_page, to_page) ||
 			page_mapping(from_page)
 			) {
 			++failed;
-- 
2.20.1