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Date:   Sun, 7 Feb 2021 14:02:36 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Zhou Wang <wangzhou1@...ilicon.com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-api@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        gregkh@...uxfoundation.org, song.bao.hua@...ilicon.com,
        jgg@...pe.ca, kevin.tian@...el.com, jean-philippe@...aro.org,
        eric.auger@...hat.com, liguozhu@...ilicon.com,
        zhangfei.gao@...aro.org, Sihang Chen <chensihang1@...ilicon.com>
Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory pin



> On Feb 7, 2021, at 12:31 AM, Zhou Wang <wangzhou1@...ilicon.com> 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?

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

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