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SubjectRe: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial
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Hi Zhong,


On 2018/11/19 18:18, zhong jiang 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 ?
Yup, I hit the real issue. The root cause is out by reviewing 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")
I am not sure if the patch you mentioned intended to fix the problem here.
With that patch the negative page->pobjects would become a large
positive value,
it will win the compare with s->cpu_partial and go ahead to unfreeze the
partial slabs.
Though it may be not a perfect fix for this issue, it really fixes (or
workarounds) the issue here.
I'd like to skip my patch..

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;
>>
>

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