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

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Message-ID: <BLU436-SMTP11907D46F39F24F62D7E440807C0@phx.gbl>
Date:	Fri, 14 Aug 2015 15:59:21 +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 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

Regards,
Wanpeng Li
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