linux-kernel - Re: [dm-devel] [PATCH] md/dm-ioctl.c: optimize memory allocation in copy

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Date:	Thu, 24 Jul 2014 13:53:58 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Mikulas Patocka <mpatocka@...hat.com>,
	Alasdair G Kergon <agk@...hat.com>
CC:	"xinhui.pan" <xinhuix.pan@...el.com>, linux-kernel@...r.kernel.org,
	snitzer@...hat.com, dm-devel@...hat.com,
	"Liu, ShuoX" <shuox.liu@...el.com>
Subject: Re: [dm-devel] [PATCH] md/dm-ioctl.c: optimize memory allocation
 in copy_params

On 2014/7/24 1:14, Mikulas Patocka wrote:
>
> On Wed, 23 Jul 2014, Alasdair G Kergon wrote:
>
>> On Wed, Jul 23, 2014 at 08:16:58AM -0400, Mikulas Patocka wrote:
>>> So, it means that you do not use device mapper at all. So, why are you
>>> trying to change memory allocation in device mapper?
>>   
>> So the *test* they run is asking device-mapper to briefly reserve a 64KB
>> buffer when there is no data to report:  The answer is not to run that
>> pointless test:)
>>
>> And if a single 64KB allocation really is a big deal, then patch 'vold'
>> in userspace so it doesn't ask for 64KB when it clearly doesn't need it!
>>
>> + int Devmapper::dumpState(SocketClient *c) {
>> +    char *buffer = (char *) malloc(1024 * 64);
>>
>> The code has just
>> #define DEVMAPPER_BUFFER_SIZE 4096
>> for all the other dm ioctls it issues.
>>
>> Only use a larger value when it is needed i.e. if DM_BUFFER_FULL_FLAG gets set.
>>
>> Alasdair
> Device mapper shouldn't depend on allocation on any contiguous memory - it
> will fall back to vmalloc. I still can't believe that their suggested
> patch makes any difference.
>
> This pattern is being repeated over and over in the kernel - for example:
>
>          if (PIDLIST_TOO_LARGE(count))
>                  return vmalloc(count * sizeof(pid_t));
>          else
>                  return kmalloc(count * sizeof(pid_t), GFP_KERNEL);
>
>
>          if (is_vmalloc_addr(p))
>                  vfree(p);
>          else
>                  kfree(p);
>
> - I think we should make two functions that do this operation (for example
> kvalloc and kvfree) and convert device mapper and other users to these
> functions. Then, other kernel subsystems can start to use them to fix
> memory fragmentation issues.

Thank Mikulas and Alasdair. Before sending out the patch, we know the result. :)
It's hard to balance between performance and stability.

Anyway, we would try to change seq_read.

Yanmin

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