linux-kernel - Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

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Date:   Wed, 30 Jan 2019 17:47:05 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian Koenig <christian.koenig@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma

On Wed, Jan 30, 2019 at 10:33:04PM +0000, Jason Gunthorpe wrote:
> On Wed, Jan 30, 2019 at 05:30:27PM -0500, Jerome Glisse wrote:
> 
> > > What is the problem in the HMM mirror that it needs this restriction?
> > 
> > No restriction at all here. I think i just wasn't understood.
> 
> Are you are talking about from the exporting side - where the thing
> creating the VMA can really only put one distinct object into it?

The message i was trying to get accross is that HMM mirror will
always succeed for everything* except for special vma ie mmap of
device file. For those it can only succeed if a p2p_map() call
succeed.

So any user of HMM mirror might to know why the mirroring fail ie
was it because something exceptional is happening ? Or is it because
i was trying to map a special vma which can be forbiden.

Hence why i assume that you might want to know about such p2p_map
failure at the time you create the umem odp object as it might be
some failure you might want to report differently and handle
differently. If you do not care about differentiating OOM or
exceptional failure from p2p_map failure than you have nothing to
worry about you will get the same error from HMM for both.

Cheers,
Jérôme

* Everything except when they are exceptional condition like OOM or
  poisonous memory.