linux-kernel - Re: [PATCH] mm: fix use-after free of page

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Date:   Tue, 19 Jul 2022 17:54:06 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>,
        Charan Teja Kalla <quic_charante@...cinc.com>
Cc:     akpm@...ux-foundation.org, pasha.tatashin@...een.com,
        sjpark@...zon.de, sieberf@...zon.com, shakeelb@...gle.com,
        dhowells@...hat.com, willy@...radead.org, vbabka@...e.cz,
        minchan@...nel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        "iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
        Pavan Kondeti <quic_pkondeti@...cinc.com>
Subject: Re: [PATCH] mm: fix use-after free of page_ext after race with
 memory-offline

On 19.07.22 17:43, Michal Hocko wrote:
> On Tue 19-07-22 20:42:42, Charan Teja Kalla wrote:
>> Thanks Michal!!
>>
>> On 7/18/2022 8:24 PM, Michal Hocko wrote:
> [...]
>>>>>> diff --git a/mm/page_ext.c b/mm/page_ext.c
>>>>>> index 3dc715d..5ccd3ee 100644
>>>>>> --- a/mm/page_ext.c
>>>>>> +++ b/mm/page_ext.c
>>>>>> @@ -299,8 +299,9 @@ static void __free_page_ext(unsigned long pfn)
>>>>>>  	if (!ms || !ms->page_ext)
>>>>>>  		return;
>>>>>>  	base = get_entry(ms->page_ext, pfn);
>>>>>> -	free_page_ext(base);
>>>>>>  	ms->page_ext = NULL;
>>>>>> +	synchronize_rcu();
>>>>>> +	free_page_ext(base);
>>>>>>  }
>>>>> So you are imposing the RCU grace period for each page_ext! This can get
>>>>> really expensive. Have you tried to measure the effect?
>>> I was wrong here! This is for each memory section which is not as
>>> terrible as every single page_ext. This can be still quite a lot memory
>>> sections in a single memory block (e.g. on ppc memory sections are
>>> ridiculously small).
>>>
>>
>> On the ARM64, I see that the minimum a section size will go is 128MB. I
>> think 16MB is the section size on ppc. Any inputs on how frequently
>> offline/online operation is being done on this ppc arch?
> 
> I have seen several reports where 16MB sections were used on PPC LPARs
> with a non trivial size. My usual answer to that is tha this is mostly a
> self inflicted injury but I am told that for some reasons I cannot
> udnerstand this is not easy to change. So reasonable or not this is not
> all that uncommon in PPC land.
> 
> We definitely shouldn't optimize for those setups but we shouldn't make
> them suffer even more as well. Besides that it seems that a single
> rcu_synchronize per offline operation should be doable.

IIRC, any reasonable PPC64 installation uses LMB >= 256 MiB, which maps
to the logical memory block size, and we only online/offline complete
memory blocks, not individual memory sections.

So one these installations, you'll see memory getting onlined/offlined
in at least 256MiB granularity.

-- 
Thanks,

David / dhildenb