Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

[Ni zhan Chen <nizhan.chen@gmail.com>]

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On 10/25/2012 02:59 PM, Hugh Dickins wrote:
> On Thu, 25 Oct 2012, Ni zhan Chen wrote:
>> On 10/25/2012 12:36 PM, Hugh Dickins wrote:
>>> On Wed, 24 Oct 2012, Dave Jones wrote:
>>>
>>>> Machine under significant load (4gb memory used, swap usage fluctuating)
>>>> triggered this...
>>>>
>>>> WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70()
>>>> Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49
>>>>
>>>> 1148                         error = shmem_add_to_page_cache(page,
>>>> mapping, index,
>>>> 1149                                                 gfp,
>>>> swp_to_radix_entry(swap));
>>>> 1150                         /* We already confirmed swap, and make no
>>>> allocation */
>>>> 1151                         VM_BUG_ON(error);
>>>> 1152                 }
>>> That's very surprising.  Easy enough to handle an error there, but
>>> of course I made it a VM_BUG_ON because it violates my assumptions:
>>> I rather need to understand how this can be, and I've no idea.
>>>
>>> Clutching at straws, I expect this is entirely irrelevant, but:
>>> there isn't a warning on line 1151 of mm/shmem.c in 3.7.0-rc2 nor
>>> in current linux.git; rather, there's a VM_BUG_ON on line 1149.
>>>
>>> So you've inserted a couple of lines for some reason (more useful
>>> trinity behaviour, perhaps)?  And have some config option I'm
>>> unfamiliar with, that mutates a BUG_ON or VM_BUG_ON into a warning?
>> Hi Hugh,
>>
>> 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?

Regards,
Chen

>
>> otherwise, will goto unlock and then go to repeat? where I miss?
>>
>> Regards,
>> Chen


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