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Date:   Tue, 9 Feb 2021 13:01:23 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Zhou Wang <wangzhou1@hisilicon.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        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>,
        song.bao.hua@hisilicon.com, jgg@ziepe.ca, kevin.tian@intel.com,
        jean-philippe@linaro.org, eric.auger@redhat.com,
        liguozhu@hisilicon.com, zhangfei.gao@linaro.org,
        Sihang Chen <chensihang1@hisilicon.com>
Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory
 pin
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On Tue, Feb 09, 2021 at 07:58:15PM +0800, Zhou Wang wrote:
> 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@hisilicon.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?
> >>
> >> 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?

It's good to show how it will be used, don't you think?

thanks,

greg k-h