Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial

[Wengang Wang <wen.gang.wang@oracle.com>]

On 2018/11/25 17:59, Wei Yang wrote:
> On Tue, Nov 20, 2018 at 10:58 AM zhong jiang <zhongjiang@huawei.com> 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,

2266                         pobjects = oldpage->pobjects;
2267                         pages = oldpage->pages;
2268                         if (drain && pobjects > s->cpu_partial) {
2269                                 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@oracle.com>
>>> ---
>>>   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;
>>>
>>