linux-kernel - Re: [PATCH] mm/hwpoison: fix race between soft_offline_page and unpoison

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Date:	Fri, 14 Aug 2015 17:01:34 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/hwpoison: fix race between soft_offline_page and
 unpoison_memory

On 8/14/15 4:38 PM, Naoya Horiguchi wrote:
> On Fri, Aug 14, 2015 at 03:59:21PM +0800, Wanpeng Li wrote:
>> On 8/14/15 3:54 PM, Wanpeng Li wrote:
>>> [...]
>>>> OK, then I rethink of handling the race in unpoison_memory().
>>>>
>>>> Currently properly contained/hwpoisoned pages should have page refcount 1
>>>> (when the memory error hits LRU pages or hugetlb pages) or refcount 0
>>>> (when the memory error hits the buddy page.) And current unpoison_memory()
>>>> implicitly assumes this because otherwise the unpoisoned page has no place
>>>> to go and it's just leaked.
>>>> So to avoid the kernel panic, adding prechecks of refcount and mapcount
>>>> to limit the page to unpoison for only unpoisonable pages looks OK to me.
>>>> The page under soft offlining always has refcount >=2 and/or mapcount > 0,
>>>> so such pages should be filtered out.
>>>>
>>>> Here's a patch. In my testing (run soft offline stress testing then repeat
>>>> unpoisoning in background,) the reported (or similar) bug doesn't happen.
>>>> Can I have your comments?
>>> As page_action() prints out page maybe still referenced by some users,
>>> however, PageHWPoison has already set. So you will leak many poison pages.
>>>
>> Anyway, the bug is still there.
>>
>> [  944.387559] BUG: Bad page state in process expr  pfn:591e3
>> [  944.393053] page:ffffea00016478c0 count:-1 mapcount:0 mapping:
>> (null) index:0x2
>> [  944.401147] flags: 0x1fffff80000000()
>> [  944.404819] page dumped because: nonzero _count
> Hmm, no luck :(
>
> To investigate more, I'd like to test the exactly same kernel as yours, so
> could you share the kernel info (.config and base kernel and what patches
> you applied)? or pushing your tree somewhere like github?
> # if you like, sending to me privately is fine.
>
> I think that I tested v4.2-rc6 + <your recent 7 hwpoison patches> +
> "mm/hwpoison: fix race between soft_offline_page and unpoison_memory",
> but I experienced some conflict in applying your patches for some reason,
> so it might happen that we are testing on different kernels.

I don't have special config and tree, the latest mmotm has already
merged my recent 8 hwpoison patches, you can test based on it.

Regards,
Wanpeng Li

>
> Mine is here:
>   https://github.com/Naoya-Horiguchi/linux v4.2-rc6/fix_race_soft_offline_unpoison
>
> Thanks,
> Naoya Horiguchi

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