Messages in this thread | | | Subject | Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial | From | Wengang Wang <> | Date | Mon, 26 Nov 2018 17:42:28 -0800 |
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On 2018/11/26 16:36, Wei Yang wrote: > On Mon, Nov 26, 2018 at 08:57:54AM -0800, Wengang Wang wrote: >> >> 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; >> > Ehh..., you mean (0xdead0000 > 0x02) ? Yes.
> This is really a bad thing, if it wordarounds the problem like this. It does. > I strongly don't agree this is a *fix*. This is too tricky. It's tricky. I don't know for what purpose the patch went in. The commit message was just saying _pobjects_ should be "unsigned"...
thanks, wengang
>> 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
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