Possible memory leak in the kernel (contigmalloc)

Bennett, Ciunas ciunas.bennett at intel.com
Tue Oct 30 09:47:12 UTC 2018


I was debugging the issue by viewing the free ques "sysctl vm.phys_free" and also using "show page" in ddb.
The inactive memory is never being released back into the free que. 
I thought that when inactive memory reaches a certain threshold that the kernel will start reclaiming and move it to the free list?
In my program this is not happening, the program uses free memory (contigmalloc), and then it is put into the inactive que (contiigfree) when the program frees it.
This inactive memory is never released by the kernel, and the inactive que grows until all the memory is in this que.
I  have attached a xml sheet that shows the memory usage in the system.


-----Original Message-----
From: Konstantin Belousov [mailto:kostikbel at gmail.com] 
Sent: Friday, October 26, 2018 9:13 PM
To: Bennett, Ciunas <ciunas.bennett at intel.com>
Cc: freebsd-stable at freebsd.org
Subject: Re: Possible memory leak in the kernel (contigmalloc)

On Wed, Oct 24, 2018 at 04:27:52PM +0000, Bennett, Ciunas wrote:
> Hello,
> I have encountered an issue with a kernel application that I have 
> written, the issue might be caused by a memory leak in the kernel.
> The application allocates and deallocates contiguous memory using
> contigmalloc() and contigfree(). The application will fail after a 
> period of time because there is not enough free contiguous memory 
> left. There could be an issue with the freeing of memory when using 
> the contigfree() function.

It is unlikely that there is an issue with a leak, but I would be not surprised if your allocation/free pattern would cause fragmentation on free lists that results in contigmalloc(9) failures after.

Look at the vmstat -z/vmstat -m output to see uma and malloc stats.
More interesting for your case can be the output from
	sysctl vm.phys_free
which provides information about the free queues and order of free pages on them.
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