Re: [PATCH] hv: hv_balloon: avoid memory leak on alloc_error of 2MB memory block

[Jason Wang <jasowang@redhat.com>]

On 11/24/2014 03:54 PM, Dexuan Cui wrote:
>> -----Original Message-----
>> From: Jason Wang [mailto:jasowang@redhat.com]
>> Sent: Monday, November 24, 2014 15:28 PM
>> To: Dexuan Cui; gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
>> driverdev-devel@linuxdriverproject.org; olaf@aepfle.de;
>> apw@canonical.com; KY Srinivasan
>> Cc: Haiyang Zhang
>> Subject: Re: [PATCH] hv: hv_balloon: avoid memory leak on alloc_error of
>> 2MB memory block
>>
>> On 11/24/2014 02:08 PM, Dexuan Cui wrote:
>>>> -----Original Message-----
>>>>> From: Jason Wang [mailto:jasowang@redhat.com]
>>>>> Sent: Monday, November 24, 2014 13:18 PM
>>>>> To: Dexuan Cui; gregkh@linuxfoundation.org; linux-
>> kernel@vger.kernel.org;
>>>>> driverdev-devel@linuxdriverproject.org; olaf@aepfle.de;
>>>>> apw@canonical.com; KY Srinivasan
>>>>> Cc: Haiyang Zhang
>>>>> Subject: Re: [PATCH] hv: hv_balloon: avoid memory leak on
>> alloc_error of
>>>>> 2MB memory block
>>>>>
>>>>> On 11/24/2014 01:56 PM, Dexuan Cui wrote:
>>>>>>> If num_ballooned is not 0, we shouldn't neglect the already-
>> allocated
>>>>> 2MB
>>>>>>> memory block(s).
>>>>>>>
>>>>>>> Cc: K. Y. Srinivasan <kys@microsoft.com>
>>>>>>> Cc: <stable@vger.kernel.org>
>>>>>>> Signed-off-by: Dexuan Cui <decui@microsoft.com>
>>>>>>> ---
>>>>>>>  drivers/hv/hv_balloon.c | 4 +++-
>>>>>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
>>>>>>> index 5e90c5d..cba2d3b 100644
>>>>>>> --- a/drivers/hv/hv_balloon.c
>>>>>>> +++ b/drivers/hv/hv_balloon.c
>>>>>>> @@ -1091,6 +1091,8 @@ static void balloon_up(struct
>> work_struct
>>>>> *dummy)
>>>>>>>  	bool done = false;
>>>>>>>  	int i;
>>>>>>>
>>>>>>> +	/* The host does balloon_up in 2MB. */
>>>>>>> +	WARN_ON(num_pages % PAGES_IN_2M != 0);
>>>>>>>
>>>>>>>  	/*
>>>>>>>  	 * We will attempt 2M allocations. However, if we fail to
>>>>>>> @@ -1111,7 +1113,7 @@ static void balloon_up(struct
>> work_struct
>>>>> *dummy)
>>>>>>>  						bl_resp, alloc_unit,
>>>>>>>  						 &alloc_error);
>>>>>>>
>>>>>>> -		if ((alloc_error) && (alloc_unit != 1)) {
>>>>>>> +		if (alloc_error && (alloc_unit != 1) &&
>> num_ballooned == 0)
>>>>> {
>>>>>>>  			alloc_unit = 1;
>>>>>>>  			continue;
>>>>>>>  		}
>>>>> Before the change, we may retry the 4K allocation when part or all 2M
>>>>> allocations were failed. This makes sense when memory is fragmented.
>> But
>>> Yes, but all the partially-allocated 2MB memory blocks are lost(mem leak).
>>>
>>>>> after the change, if part of 2M allocation were failed, we won't retry
>>>>> 4K allocation. Is this expected?
>>> Hi Jason,
>>> The patch doesn't break the "try 2MB first; then try 4K" logic:
>>>
>>> With the change, we'll retry the 2MB allocation in the next iteration of the
>>> same while (!done) loop -- we expect this retry will cause
>>> "alloc_error && (alloc_unit != 1) && num_ballooned == 0" to be true,
>>> so we'll later try 4K allocation, as we did before.
>> If I read the code correctly, if part of 2M allocation fails.
>> alloc_balloon_pages() will have a non zero return value with alloc_error
>> set. Then it will match the following check:
>>
>>                 if ((alloc_error) || (num_ballooned == num_pages))
>> {
>>
>> which will set done to true. So there's no second iteration of while
>> (!done) loop?
> Oh... I see the issue in my patch.
> Thanks for pointing this out, Jason!
>
>> Probably you need something like:
>>
>> if ((alloc_unit != 1) && (num_ballooned == 0)) {
>>     alloc_unit = 1;
>>     continue;
>> }
>>
>> if ((alloc_unit == 1) || (num_ballooned == num_pages)) {
>>     ...
>> }
> Your code is good, except for one thing:
> alloc_balloon_pages() can return due to:
>
> if (bl_resp->hdr.size + sizeof(union dm_mem_page_range) >
>                          PAGE_SIZE)
>                         return i * alloc_unit;
>
> In this case, "alloc_unit == 1" is true, but we shouldn't run "done = true". 
>
> How do you like this? I made a few changes based on your code.
>
> @@ -1086,16 +1088,18 @@ static void balloon_up(struct work_struct *dummy)
>                 num_pages -= num_ballooned;
> +               alloc_error = false;
>                 num_ballooned = alloc_balloon_pages(&dm_device, num_pages,
>                                                 bl_resp, alloc_unit,
>                                                  &alloc_error);
>
> -               if ((alloc_error) && (alloc_unit != 1)) {
> +               if (alloc_unit != 1 && num_ballooned == 0) {
>                         alloc_unit = 1;
>                         continue;
>                 }
>
> -               if ((alloc_error) || (num_ballooned == num_pages)) {
> +               if ((alloc_unit == 1 && alloc_error) ||
> +                       (num_ballooned == num_pages)) {
>                         bl_resp->more_pages = 0;
>                         done = true;
>                         dm_device.state = DM_INITIALIZED;
>
>
> If you're Ok with this, I'll send out a v2 patch.
>
> Thanks,
> -- Dexuan

Looks good.

Thanks