netdev - Re: [PATCH 0/3] Add gup fast + longterm and use it in HFI1

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Date:   Mon, 11 Feb 2019 15:50:52 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Weiny, Ira" <ira.weiny@...el.com>
Cc:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Marciniszyn, Mike" <mike.marciniszyn@...el.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH 0/3] Add gup fast + longterm and use it in HFI1

On Mon, Feb 11, 2019 at 10:40:02PM +0000, Weiny, Ira wrote:

> > Many drivers do this, the 'doorbell' is a PCI -> CPU thing of some sort
> 
> My surprise is why does _userspace_ allocate this memory?

Well, userspace needs to read the memory, so either userpace allocates
it and the kernel GUP's it, or userspace mmap's a kernel page which
was DMA mapped.

The GUP version lets the doorbells have lower alignment than a PAGE,
and thes RDMA drivers hard requires GUP->DMA to function..

So why not use a umem here? It already has to work.

> > > This does not seem to be allocating memory regions.  Jason, do you
> > > want a patch to just convert these calls and consider it legacy code?
> > 
> > It needs to use umem like all the other drivers on this path.
> > Otherwise it doesn't get the page pinning logic right
>
> Not sure what you mean regarding the pinning logic?

The RLIMIT_MEMLOCK stuff and so on.

> > There is also something else rotten with these longterm callsites,
> > they seem to have very different ideas how to handle
> > RLIMIT_MEMLOCK.
> > 
> > ie vfio doesn't even touch pinned_vm.. and rdma is applying
> > RLIMIT_MEMLOCK to mm->pinned_vm, while vfio is using locked_vm.. No
> > idea which is right, but they should be the same, and this pattern should
> > probably be in core code someplace.
> 
> Neither do I.  But AFAIK pinned_vm is a subset of locked_vm.

I thought so..

> So should we be accounting both of the counters?

Someone should check :)

Since we don't increment locked_vm when we increment pinned_vm and
vfio only checke RLIMIT_MEMLOCK against locked_vm one can certainly
exceed the limit by mixing and matching RDMA and VFIO pins in the same
process. Sure seems like there is a bug somewhere here.

Jason