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

[Zhou Wang]

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.

> 
> Userfaultfd is an fd because one program controls another.  Is mempinfd like this?

We use mempinfd like: (see patch 2/2)

fd = mempinfd();
va = malloc(size);
struct mem_pin_address addr;
addr.va = va;
addr.size = size;
ioctl(fd, MEM_CMD_PIN, &addr);
ioctl(fd, MEM_CMD_UNPIN, &addr);
close(fd);

Best,
Zhou

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