Re: [Bug 201085] New: Kernel allows mlock() on pages in CMA without migrating pages out of CMA first

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 9/14/18 8:39 AM, Aneesh Kumar K.V wrote:
> On 9/13/18 3:49 AM, Mike Kravetz wrote:
>> On 09/12/2018 12:47 PM, Andrew Morton wrote:
>>>
>>> (switched to email.A  Please respond via emailed reply-to-all, not via 
>>> the
>>> bugzilla web interface).
>>>
>>> On Tue, 11 Sep 2018 03:59:11 +0000 
>>> bugzilla-daemon@bugzilla.kernel.org wrote:
>>>
>>>> https://bugzilla.kernel.org/show_bug.cgi?id=201085
>>>>
>>>> A A A A A A A A A A A A  Bug ID: 201085
>>>> A A A A A A A A A A A  Summary: Kernel allows mlock() on pages in CMA without
>>>> A A A A A A A A A A A A A A A A A A A A  migrating pages out of CMA first
>>>> A A A A A A A A A A A  Product: Memory Management
>>>> A A A A A A A A A A A  Version: 2.5
>>>> A A A A  Kernel Version: 4.18
>>>> A A A A A A A A A A  Hardware: All
>>>> A A A A A A A A A A A A A A A A  OS: Linux
>>>> A A A A A A A A A A A A A A  Tree: Mainline
>>>> A A A A A A A A A A A A  Status: NEW
>>>> A A A A A A A A A A  Severity: normal
>>>> A A A A A A A A A A  Priority: P1
>>>> A A A A A A A A A  Component: Page Allocator
>>>> A A A A A A A A A A  Assignee: akpm@linux-foundation.org
>>>> A A A A A A A A A A  Reporter: tpearson@raptorengineering.com
>>>> A A A A A A A A  Regression: No
>>>>
>>>> Pages allocated in CMA are not migrated out of CMA when non-CMA 
>>>> memory is
>>>> available and locking is attempted via mlock().A  This can result in 
>>>> rapid
>>>> exhaustion of the CMA pool if memory locking is used by an 
>>>> application with
>>>> large memory requirements such as QEMU.
>>>>
>>>> To reproduce, on a dual-CPU (NUMA) POWER9 host try to launch a VM 
>>>> with mlock=on
>>>> and 1/2 or more of physical memory allocated to the guest.A  Observe 
>>>> full CMA
>>>> pool depletion occurs despite plenty of normal free RAM available.
>>>>
>>>> -- 
>>>> You are receiving this mail because:
>>>> You are the assignee for the bug.
>>
>> IIRC, Aneesh is working on some powerpc IOMMU patches for a similar issue
>> (long term pinning of cma pages).A  Added him on Cc:
>> https://lkml.kernel.org/r/20180906054342.25094-2-aneesh.kumar@linux.ibm.com 
>>
>>
>> This report seems to be suggesting a more general solution/change.  
>> Wondering
>> if there is any overlap with this and Aneesh's work.
>>
> 
> This is a related issue. I am looking at doing something similar to what 
> I did with IOMMU patches. That is migrate pages out of CMA region bfore 
> mlock.
> 
> The problem mentioned is similar to vfio. With VFIO we do pin the guest 
> pages and that is similar with -realtime mlock=on option of Qemu.
> 
> We can endup backing guest RAM with pages from CMA area and these are 
> different qemu options that do pin these guest pages for the lifetime of 
> the guest.
> 

Another option is to look at the possibility of having something similar 
to prctl that will avoid allocation from CMA region?

-aneesh