[PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial

[Wengang Wang]

The this_cpu_cmpxchg makes the do-while loop pass as long as the
s->cpu_slab->partial as the same value. It doesn't care what happened to
that slab. Interrupt is not disabled, and new alloc/free can happen in the
interrupt handlers. Theoretically, after we have a reference to the it,
stored in _oldpage_, the first slab on the partial list on this CPU can be
moved to kmem_cache_node and then moved to different kmem_cache_cpu and
then somehow can be added back as head to partial list of current
kmem_cache_cpu, though that is a very rare case. If that rare case really
happened, the reading of oldpage->pobjects may get a 0xdead0000
unexpectedly, stored in _pobjects_, if the reading happens just after
another CPU removed the slab from kmem_cache_node, setting lru.prev to
LIST_POISON2 (0xdead000000000200). The wrong _pobjects_(negative) then
prevents slabs from being moved to kmem_cache_node and being finally freed.

We see in a vmcore, there are 375210 slabs kept in the partial list of one
kmem_cache_cpu, but only 305 in-use objects in the same list for
kmalloc-2048 cache. We see negative values for page.pobjects, the last page
with negative _pobjects_ has the value of 0xdead0004, the next page looks
good (_pobjects is 1).

For the fix, I wanted to call this_cpu_cmpxchg_double with
oldpage->pobjects, but failed due to size difference between
oldpage->pobjects and cpu_slab->partial. So I changed to call
this_cpu_cmpxchg_double with _tid_. I don't really want no alloc/free
happen in between, but just want to make sure the first slab did expereince
a remove and re-add. This patch is more to call for ideas.

Signed-off-by: Wengang Wang <wen.gang.wang@xxxxxxxxxx>
---
 mm/slub.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index e3629cd..26539e6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2248,6 +2248,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 {
 #ifdef CONFIG_SLUB_CPU_PARTIAL
 	struct page *oldpage;
+	unsigned long tid;
 	int pages;
 	int pobjects;
 
@@ -2255,8 +2256,12 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 	do {
 		pages = 0;
 		pobjects = 0;
-		oldpage = this_cpu_read(s->cpu_slab->partial);
 
+		tid = this_cpu_read(s->cpu_slab->tid);
+		/* read tid before reading oldpage */
+		barrier();
+
+		oldpage = this_cpu_read(s->cpu_slab->partial);
 		if (oldpage) {
 			pobjects = oldpage->pobjects;
 			pages = oldpage->pages;
@@ -2283,8 +2288,17 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 		page->pobjects = pobjects;
 		page->next = oldpage;
 
-	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
-								!= oldpage);
+		/* we dont' change tid, but want to make sure it didn't change
+		 * in between. We don't really hope alloc/free not happen on
+		 * this CPU, but don't want the first slab be removed from and
+		 * then re-added as head to this partial list. If that case
+		 * happened, pobjects may read 0xdead0000 when this slab is just
+		 * removed from kmem_cache_node by other CPU setting lru.prev
+		 * to LIST_POISON2.
+		 */
+	} while (this_cpu_cmpxchg_double(s->cpu_slab->partial, s->cpu_slab->tid,
+					 oldpage, tid, page, tid) == 0);
+
 	if (unlikely(!s->cpu_partial)) {
 		unsigned long flags;
 
-- 
2.9.5