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

[Linhaifeng]

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, address@hidden wrote:
>>>>>> From: linhaifeng <address@hidden>
>>>>>>
>>>>>> 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 <address@hidden>
>>>>>
>>>>> 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.
>>
>> 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:
>> 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?

>>
>> 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