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

[Zhou Wang]

On 2021/2/9 17:37, Greg KH wrote:
> On Tue, Feb 09, 2021 at 05:17:46PM +0800, Zhou Wang wrote:
>> On 2021/2/8 6:02, Andy Lutomirski wrote:
>>>
>>>
>>>> On Feb 7, 2021, at 12:31 AM, Zhou Wang <wangzhou1@xxxxxxxxxxxxx> wrote:
>>>>
>>>> SVA(share virtual address) offers a way for device to share process virtual
>>>> address space safely, which makes more convenient for user space device
>>>> driver coding. However, IO page faults may happen when doing DMA
>>>> operations. As the latency of IO page fault is relatively big, DMA
>>>> performance will be affected severely when there are IO page faults.
>>>> From a long term view, DMA performance will be not stable.
>>>>
>>>> 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.
>>>>
>>>> Various drivers working under traditional non-SVA mode are using
>>>> their own specific ioctl to do pin. Such ioctl can be seen in v4l2,
>>>> gpu, infiniband, media, vfio, etc. Drivers are usually doing dma
>>>> mapping while doing pin.
>>>>
>>>> But, in SVA mode, pin could be a common need which isn't necessarily
>>>> bound with any drivers, and neither is dma mapping needed by drivers
>>>> since devices are using the virtual address of CPU. Thus, It is better
>>>> to introduce a new common syscall for it.
>>>>
>>>> This patch leverages the design of userfaultfd and adds mempinfd for pin
>>>> to avoid messing up mm_struct. A fd will be got by mempinfd, then user
>>>> space can do pin/unpin pages by ioctls of this fd, all pinned pages under
>>>> one file will be unpinned in file release process. Like pin page cases in
>>>> other places, can_do_mlock is used to check permission and input
>>>> parameters.
>>>
>>>
>>> Can you document what the syscall does?
>>
>> Will add related document in Documentation/vm.
> 
> A manpage is always good, and will be required eventually :)

manpage is maintained in another repo. Do you mean add a manpage
patch in this series?

Best,
Zhou

> 
> thanks,
> 
> greg k-h
> 
> .
>