[RFC/PATCH 4/5] mm: get_user_pages: migrate out CMA pages whenFOLL_DURABLE flag is set

[Marek Szyprowski]

When __get_user_pages() is called with FOLL_DURABLE flag, ensure that no
page in CMA pageblocks gets locked. This workarounds the permanent
migration failures caused by locking the pages by get_user_pages() call for
a long period of time.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
---
 mm/internal.h |   12 ++++++++++++
 mm/memory.c   |   43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/mm/internal.h b/mm/internal.h
index 8562de0..a290d04 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -105,6 +105,18 @@ extern void prep_compound_page(struct page *page, unsigned long order);
 extern bool is_free_buddy_page(struct page *page);
 #endif
 
+#ifdef CONFIG_CMA
+static inline int is_cma_page(struct page *page)
+{
+	unsigned mt = get_pageblock_migratetype(page);
+	if (mt == MIGRATE_ISOLATE || mt == MIGRATE_CMA)
+		return true;
+	return false;
+}
+#else
+#define is_cma_page(page) 0
+#endif
+
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
 
 /*
diff --git a/mm/memory.c b/mm/memory.c
index 2b9c2dd..f81b273 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1650,6 +1650,45 @@ static inline int stack_guard_page(struct vm_area_struct *vma, unsigned long add
 }
 
 /**
+ * replace_cma_page() - migrate page out of CMA page blocks
+ * @page:	source page to be migrated
+ *
+ * Returns either the old page (if migration was not possible) or the pointer
+ * to the newly allocated page (with additional reference taken).
+ *
+ * get_user_pages() might take a reference to a page for a long period of time,
+ * what prevent such page from migration. This is fatal to the preffered usage
+ * pattern of CMA pageblocks. This function replaces the given user page with
+ * a new one allocated from NON-MOVABLE pageblock, so locking CMA page can be
+ * avoided.
+ */
+static inline struct page *migrate_replace_cma_page(struct page *page)
+{
+	struct page *newpage = alloc_page(GFP_HIGHUSER);
+
+	if (!newpage)
+		goto out;
+
+	/*
+	 * Take additional reference to the new page to ensure it won't get
+	 * freed after migration procedure end.
+	 */
+	get_page_foll(newpage);
+
+	if (migrate_replace_page(page, newpage) == 0)
+		return newpage;
+
+	put_page(newpage);
+	__free_page(newpage);
+out:
+	/*
+	 * Migration errors in case of get_user_pages() might not
+	 * be fatal to CMA itself, so better don't fail here.
+	 */
+	return page;
+}
+
+/**
  * __get_user_pages() - pin user pages in memory
  * @tsk:	task_struct of target task
  * @mm:		mm_struct of target mm
@@ -1884,6 +1923,10 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 			}
 			if (IS_ERR(page))
 				return i ? i : PTR_ERR(page);
+
+			if ((gup_flags & FOLL_DURABLE) && is_cma_page(page))
+				page = migrate_replace_cma_page(page);
+
 			if (pages) {
 				pages[i] = page;
 
-- 
1.7.9.5

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