linux-kernel - Re: shmem_getpage_gfp VM_BUG

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Date:	Fri, 26 Oct 2012 09:48:17 +0800
From:	Ni zhan Chen <nizhan.chen@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Dave Jones <davej@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

On 10/26/2012 05:27 AM, Hugh Dickins wrote:
> On Thu, 25 Oct 2012, Ni zhan Chen wrote:
>> On 10/25/2012 02:59 PM, Hugh Dickins wrote:
>>> On Thu, 25 Oct 2012, Ni zhan Chen wrote:
>>>> I think it maybe caused by your commit [d189922862e03ce: shmem: fix
>>>> negative
>>>> rss in memcg memory.stat], one question:
>>> Well, yes, I added the VM_BUG_ON in that commit.
>>>
>>>> if function shmem_confirm_swap confirm the entry has already brought back
>>>> from swap by a racing thread,
>>> The reverse: true confirms that the swap entry has not been brought back
>>> from swap by a racing thread; false indicates that there has been a race.
>>>
>>>> then why call shmem_add_to_page_cache to add
>>>> page from swapcache to pagecache again?
>>> Adding it to pagecache again, after such a race, would set error to
>>> -EEXIST (originating from radix_tree_insert); but we don't do that,
>>> we add it to pagecache when it has not already been added.
>>>
>>> Or that's the intention: but Dave seems to have found an unexpected
>>> exception, despite us holding the page lock across all this.
>>>
>>> (But if it weren't for the memcg and replace_page issues, I'd much
>>> prefer to let shmem_add_to_page_cache discover the race as before.)
>>>
>>> Hugh
>> Hi Hugh
>>
>> Thanks for your response. You mean the -EEXIST originating from
>> radix_tree_insert, in radix_tree_insert:
>> if (slot != NULL)
>>      return -EEXIST;
>> But why slot should be NULL? if no race, the pagecache related radix tree
>> entry should be RADIX_TREE_EXCEPTIONAL_ENTRY+swap_entry_t.val, where I miss?
> I was describing what would happen in a case that should not exist,
> that you had thought the common case.  In actuality, the entry should
> not be NULL, it should be as you say there.

Thanks for your patience. So in the common case, the entry should be the 
value I mentioned, then why has this check?
if (slot != NULL)
     return -EEXIST;

the common case will return -EEXIST.

>
> Hugh
>

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