Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial
From: Wengang Wang
Date: Mon Nov 26 2018 - 11:58:26 EST
On 2018/11/25 17:59, Wei Yang wrote:
On Tue, Nov 20, 2018 at 10:58 AM zhong jiang <zhongjiang@xxxxxxxxxx> wrote:
On 2018/11/17 9:33, Wengang Wang wrote:
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.
Have you hit the really issue or just review the code ?
I did hit the issue and fixed in the upstream patch unpredictably by the following patch.
e5d9998f3e09 ("slub: make ->cpu_partial unsigned int")
Zhong,
I took a look into your upstream patch, while I am confused how your patch
fix this issue?
In put_cpu_partial(), the cmpxchg compare cpu_slab->partial (a page struct)
instead of the cpu_partial (an unsigned integer). I didn't get the
point of this fix.
I think the patch can't prevent pobjects from being set as 0xdead0000
(the primary 4 bytes of LIST_POISON2).
But if pobjects is treated as unsigned integer,
2266ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pobjects = oldpage->pobjects;
2267ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pages = oldpage->pages;
2268ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (drain && pobjects > s->cpu_partial) {
2269ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned long flags;
line 2268 will be true in put_cpu_partial(), thus code goes to
unfreeze_partials(). This way the slabs in the cpu partial list can be
moved to kmem_cache_nod and then freed. So it fixes (or say workarounds)
the problem I see here (huge number of empty slabs stay in cpu partial
list).
thanks
wengang
Thanks,
zhong jiang
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;