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 12:45:46 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...lanox.com>, Christoph Hellwig <hch@....de>
Cc:     Jerome Glisse <jglisse@...hat.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>,
        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 2019-01-30 12:06 p.m., Jason Gunthorpe wrote:
>> Way less problems than not having struct page for doing anything
>> non-trivial.  If you map the BAR to userspace with remap_pfn_range
>> and friends the mapping is indeed very simple.  But any operation
>> that expects a page structure, which is at least everything using
>> get_user_pages won't work.
> 
> GUP doesn't work anyhow today, and won't work with BAR struct pages in
> the forseeable future (Logan has sent attempts on this before).

I don't recall ever attempting that... But patching GUP for special
pages or VMAS; or working around by not calling it in some cases seems
like the thing that's going to need to be done one way or another.

> Jerome made the HMM mirror API use this flow, so afer his patch to
> switch the ODP MR to use HMM, and to switch GPU drivers, it will work
> for those cases. Which is more than the zero cases than we have today
> :)

But we're getting the same bait and switch here... If you are using HMM
you are using struct pages, but we're told we need this special VMA hack
for cases that don't use HMM and thus don't have struct pages...

Logan