Re: [Qemu-devel] [PATCH] fix the memory leak for share hugepage

[Linhaifeng <haifeng.lin@huawei.com>]

Hi,all

Maybe this is unix domain socket's bug.I found that qemu send the fd to vapp the fd's f_count inc twice in kernel.

1.kernel calls when we call send.
unix_stream_sendmsg -> unix_scm_to_skb -> unix_attach_fds -> scm_fp_dup -> get_file -> atomic_long_inc(&f->f_count)

Maybe should't inc the f_count when call send.


2.kernel calls when we call recv
unix_stream_recvmsg -> scm_fp_dup -> get_file -> atomic_long_inc(&f->f_count)



On 2014/10/20 14:26, Wen Congyang wrote:
> On 10/20/2014 02:17 PM, Linhaifeng wrote:
>>
>>
>> On 2014/10/20 13:32, Wen Congyang wrote:
>>> On 10/20/2014 12:48 PM, Linhaifeng wrote:
>>>>
>>>>
>>>> On 2014/10/20 10:12, Wen Congyang wrote:
>>>>> On 10/18/2014 11:20 AM, Linhaifeng wrote:
>>>>>>
>>>>>>
>>>>>> On 2014/10/17 21:26, Daniel P. Berrange wrote:
>>>>>>> On Fri, Oct 17, 2014 at 04:57:27PM +0800, Linhaifeng wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2014/10/17 16:33, Daniel P. Berrange wrote:
>>>>>>>>> On Fri, Oct 17, 2014 at 04:27:17PM +0800, haifeng.lin@huawei.com wrote:
>>>>>>>>>> From: linhaifeng <haifeng.lin@huawei.com>
>>>>>>>>>>
>>>>>>>>>> The VM start with share hugepage should close the hugefile fd
>>>>>>>>>> when exit.Because the hugepage fd may be send to other process
>>>>>>>>>> e.g vhost-user If qemu not close the fd the other process can
>>>>>>>>>> not free the hugepage otherwise exit process,this is ugly,so
>>>>>>>>>> qemu should close all shared fd when exit.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: linhaifeng <haifeng.lin@huawei.com>
>>>>>>>>>
>>>>>>>>> Err, all file descriptors are closed automatically when a process
>>>>>>>>> exits. So manually calling close(fd) before exit can't have any
>>>>>>>>> functional effect on a resource leak.
>>>>>>>>>
>>>>>>>>> If QEMU has sent the FD to another process, that process has a
>>>>>>>>> completely separate copy of the FD. Closing the FD in QEMU will
>>>>>>>>> not close the FD in the other process. You need the other process
>>>>>>>>> to exit for the copy to be closed.
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>> Daniel
>>>>>>>>>
>>>>>>>> Hi,daniel
>>>>>>>>
>>>>>>>> QEMU send the fd by unix domain socket.unix domain socket just install the fd to
>>>>>>>> other process and inc the f_count,if qemu not close the fd the f_count is not dec.
>>>>>>>> Then the other process even close the fd the hugepage would not freed whise the other process exit.
>>>>>>>
>>>>>>> The kernel always closes all FDs when a process exits. So if this FD is
>>>>>>> not being correctly closed then it is a kernel bug. There should never
>>>>>>> be any reason for an application to do close(fd) before exiting.
>>>>>>>
>>>>>>> Regards,
>>>>>>> Daniel
>>>>>>>
>>>>>> Hi,daniel
>>>>>>
>>>>>> I don't think this is kernel's bug.May be this a problem about usage.
>>>>>> If you open a file you should close it too.
>>>>>
>>>>> If you don't close it, the kernel will help you when the program exits.
>>>>>
>>>> Yes,when the hugepage is only used for qemu,the kernel will free the file object.If the hugepage shared for other process,when qemu exit the kernel will not free the file.
>>>
>>> Even if the hugepage is shared with the other process, the kernel will auto close the fd when qemu
>>> exits. If the kernel doesn't do it, it is a kernel bug.
>>>
>> Kernel supply close to fix this bug.If you call open you must call close.
>> If not, the result is unpredictability.
> 
> No, if the program exists, the kernel must close all fd used by the program.
> So, there is no need to close fd before program exists.
> 
> Thanks
> Wen Congyang
> 
>>>>>>
>>>>>> This is <<linux man page>>about how to free resource of file.
>>>>>> http://linux.die.net/man/2/close
>>>>>>
>>>>>>
>>>>>> I'm trying to describe my problem.
>>>>>>
>>>>>> For example, there are 2 VMs run with hugepage and the hugepage only for QEMU to use.
>>>>>>
>>>>>> Before run VM the meminfo is :
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:     4096
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>>
>>>>>> Run the two VMs.QEMU deal with hugepage as follow steps:
>>>>>> 1.open
>>>>>> 2.unlink
>>>>>> 3.mmap
>>>>>> 4.use memory of hugepage.After this step the meminfo is :
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:        0
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>> 5.shutdown VM with signal 15 without close(fd).After this step the meminfo is :
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:     4096
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>>
>>>>>> Yes,it works well,like you said the kernel recycle all resources.
>>>>>>
>>>>>> For another example,there are 2 VMs run with hugepage and share the hugepage with vapp(a vhost-user application).
>>>>>
>>>>> The vapp is your internal application?
>>>>>
>>>> Yes vapp is a application to share the QEMU's hugepage.So threr are two process use the hugepage.
>>>>
>>>>>>
>>>>>> Before run VM the meminfo is :
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:     4096
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>>
>>>>>> Run the first VM.QEMU deal with hugepage as follow steps:
>>>>>> 1.open
>>>>>> 2.unlink
>>>>>> 3.mmap
>>>>>> 4.use memory of hugepage and send the fd to vapp with unix domain socket.After this step the meminfo is:
>>>
>>> Do you modify qemu?
>>>
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:     2048
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>>
>>>>>> Run the second VM.After this step the meminfo is:
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:        0
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>>
>>>>>> Then I want to close the first VM and run another VM.After close the first VM and close the fd in vapp the meminfo is :
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:        0
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>
>>>>> Does the qemu still run after you close the first VM? If the qemu exits, the fd will be closed by the kernel, so this
>>>>> bug is very strange.
>>>>>
>>>> qemu is not run when close the first VM.If other process used the file will be closed by kernel too?
>>>
>>> If qeum doesn't run after the first vm is closed, the fd should be closed even if another process uses the file.
>>>
>>>>
>>>>>>
>>>>>> So failed to run the third VM because the first VM have not free the hugepage.After apply this patch the meminfo is:
>>>>>> HugePages_Total:    4096
>>>>>> HugePages_Free:     2048
>>>>>> HugePages_Rsvd:        0
>>>>>> HugePages_Surp:        0
>>>>>> Hugepagesize:       2048 kB
>>>>>> So i can run the third VM success.
>>>>>>
>>>>>
>>>>>
>>>>> .
>>>>>
>>>>
>>>
>>>
>>> .
>>>
>>
> 
> 
> .
> 

-- 
Regards,
Haifeng