[PATCH RFC 12/29] mm/zsmalloc: stop using __ClearPageMovable()

[David Hildenbrand]

Instead, let's check in the callbacks if the page was already destroyed,
which can be checked by looking at zpdesc->zspage (see reset_zpdesc()).

If we detect that the page was destroyed:

(1) Fail isolation, just like the migration core would

(2) Fake migration success just like the migration core would

In the putback case there is nothing to do, as we don't do anything just
like the migration core would do.

In the future, we should look into not letting these pages get destroyed
while they are isolated -- and instead delaying that to the
putback/migration call. Add a TODO for that.

Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/zsmalloc.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index f98747aed4330..72c2b7562c511 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -876,7 +876,6 @@ static void reset_zpdesc(struct zpdesc *zpdesc)
 {
 	struct page *page = zpdesc_page(zpdesc);
 
-	__ClearPageMovable(page);
 	ClearPagePrivate(page);
 	zpdesc->zspage = NULL;
 	zpdesc->next = NULL;
@@ -1715,10 +1714,11 @@ static void replace_sub_page(struct size_class *class, struct zspage *zspage,
 static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
 {
 	/*
-	 * Page is locked so zspage couldn't be destroyed. For detail, look at
-	 * lock_zspage in free_zspage.
+	 * Page is locked so zspage can't be destroyed concurrently
+	 * (see free_zspage()). But if the page was already destroyed
+	 * (see reset_zpdesc()), refuse isolation here.
 	 */
-	return true;
+	return page_zpdesc(page)->zspage;
 }
 
 static int zs_page_migrate(struct page *newpage, struct page *page,
@@ -1736,6 +1736,13 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 	unsigned long old_obj, new_obj;
 	unsigned int obj_idx;
 
+	/*
+	 * TODO: nothing prevents a zspage from getting destroyed while
+	 * isolated: we should disallow that and defer it.
+	 */
+	if (!zpdesc->zspage)
+		return MIGRATEPAGE_SUCCESS;
+
 	/* The page is locked, so this pointer must remain valid */
 	zspage = get_zspage(zpdesc);
 	pool = zspage->pool;
-- 
2.49.0