Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory pin

[David Hildenbrand <david@redhat.com>]

On 08.02.21 03:27, Song Bao Hua (Barry Song) wrote:
> 
> 
>> -----Original Message-----
>> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On Behalf Of
>> Matthew Wilcox
>> Sent: Monday, February 8, 2021 2:31 PM
>> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
>> Cc: Wangzhou (B) <wangzhou1@hisilicon.com>; linux-kernel@vger.kernel.org;
>> iommu@lists.linux-foundation.org; linux-mm@kvack.org;
>> linux-arm-kernel@lists.infradead.org; linux-api@vger.kernel.org; Andrew
>> Morton <akpm@linux-foundation.org>; Alexander Viro <viro@zeniv.linux.org.uk>;
>> gregkh@linuxfoundation.org; jgg@ziepe.ca; kevin.tian@intel.com;
>> jean-philippe@linaro.org; eric.auger@redhat.com; Liguozhu (Kenneth)
>> <liguozhu@hisilicon.com>; zhangfei.gao@linaro.org; chensihang (A)
>> <chensihang1@hisilicon.com>
>> Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory
>> pin
>>
>> On Sun, Feb 07, 2021 at 10:24:28PM +0000, Song Bao Hua (Barry Song) wrote:
>>>>> In high-performance I/O cases, accelerators might want to perform
>>>>> I/O on a memory without IO page faults which can result in dramatically
>>>>> increased latency. Current memory related APIs could not achieve this
>>>>> requirement, e.g. mlock can only avoid memory to swap to backup device,
>>>>> page migration can still trigger IO page fault.
>>>>
>>>> Well ... we have two requirements.  The application wants to not take
>>>> page faults.  The system wants to move the application to a different
>>>> NUMA node in order to optimise overall performance.  Why should the
>>>> application's desires take precedence over the kernel's desires?  And why
>>>> should it be done this way rather than by the sysadmin using numactl to
>>>> lock the application to a particular node?
>>>
>>> NUMA balancer is just one of many reasons for page migration. Even one
>>> simple alloc_pages() can cause memory migration in just single NUMA
>>> node or UMA system.
>>>
>>> The other reasons for page migration include but are not limited to:
>>> * memory move due to CMA
>>> * memory move due to huge pages creation
>>>
>>> Hardly we can ask users to disable the COMPACTION, CMA and Huge Page
>>> in the whole system.
>>
>> You're dodging the question.  Should the CMA allocation fail because
>> another application is using SVA?
>>
>> I would say no.
> 
> I would say no as well.
> 
> While IOMMU is enabled, CMA almost has one user only: IOMMU driver
> as other drivers will depend on iommu to use non-contiguous memory
> though they are still calling dma_alloc_coherent().
> 
> In iommu driver, dma_alloc_coherent is called during initialization
> and there is no new allocation afterwards. So it wouldn't cause
> runtime impact on SVA performance. Even there is new allocations,
> CMA will fall back to general alloc_pages() and iommu drivers are
> almost allocating small memory for command queues.
> 
> So I would say general compound pages, huge pages, especially
> transparent huge pages, would be bigger concerns than CMA for
> internal page migration within one NUMA.
> 
> Not like CMA, general alloc_pages() can get memory by moving
> pages other than those pinned.
> 
> And there is no guarantee we can always bind the memory of
> SVA applications to single one NUMA, so NUMA balancing is
> still a concern.
> 
> But I agree we need a way to make CMA success while the userspace
> pages are pinned. Since pin has been viral in many drivers, I
> assume there is a way to handle this. Otherwise, APIs like
> V4L2_MEMORY_USERPTR[1] will possibly make CMA fail as there
> is no guarantee that usersspace will allocate unmovable memory
> and there is no guarantee the fallback path- alloc_pages() can
> succeed while allocating big memory.
> 

Long term pinnings cannot go onto CMA-reserved memory, and there is 
similar work to also fix ZONE_MOVABLE in that regard.

https://lkml.kernel.org/r/20210125194751.1275316-1-pasha.tatashin@soleen.com

One of the reasons I detest using long term pinning of pages where it 
could be avoided. Take VFIO and RDMA as an example: these things 
currently can't work without them.

What I read here: "DMA performance will be affected severely". That does 
not sound like a compelling argument to me for long term pinnings. 
Please find another way to achieve the same goal without long term 
pinnings controlled by user space - e.g., controlling when migration 
actually happens.

For example, CMA/alloc_contig_range()/memory unplug are corner cases 
that happen rarely, you shouldn't have to worry about them messing with 
your DMA performance.

-- 
Thanks,

David / dhildenb

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