Re: [PATCH 1/2] drm/vgem: Do not allocate backing shmemfs file for an import dmabuf object

["Christian König" <christian.koenig@amd.com>]

Am 08.07.20 um 17:01 schrieb Daniel Vetter:
> On Wed, Jul 8, 2020 at 4:37 PM Christian König <christian.koenig@amd.com> wrote:
>> Am 08.07.20 um 11:54 schrieb Daniel Vetter:
>>> On Wed, Jul 08, 2020 at 11:22:00AM +0200, Christian König wrote:
>>>> Am 07.07.20 um 20:35 schrieb Chris Wilson:
>>>>> Quoting lepton (2020-07-07 19:17:51)
>>>>>> On Tue, Jul 7, 2020 at 10:20 AM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>>>>>>> Quoting lepton (2020-07-07 18:05:21)
>>>>>>>> On Tue, Jul 7, 2020 at 9:00 AM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>>>>>>>>> If we assign obj->filp, we believe that the create vgem bo is native and
>>>>>>>>> allow direct operations like mmap() assuming it behaves as backed by a
>>>>>>>>> shmemfs inode. When imported from a dmabuf, the obj->pages are
>>>>>>>>> not always meaningful and the shmemfs backing store misleading.
>>>>>>>>>
>>>>>>>>> Note, that regular mmap access to a vgem bo is via the dumb buffer API,
>>>>>>>>> and that rejects attempts to mmap an imported dmabuf,
>>>>>>>> What do you mean by "regular mmap access" here?  It looks like vgem is
>>>>>>>> using vgem_gem_dumb_map as .dumb_map_offset callback then it doesn't call
>>>>>>>> drm_gem_dumb_map_offset
>>>>>>> As I too found out, and so had to correct my story telling.
>>>>>>>
>>>>>>> By regular mmap() access I mean mmap on the vgem bo [via the dumb buffer
>>>>>>> API] as opposed to mmap() via an exported dma-buf fd. I had to look at
>>>>>>> igt to see how it was being used.
>>>>>> Now it seems your fix is to disable "regular mmap" on imported dma buf
>>>>>> for vgem. I am not really a graphic guy, but then the api looks like:
>>>>>> for a gem handle, user space has to guess to find out the way to mmap
>>>>>> it. If user space guess wrong, then it will fail to mmap. Is this the
>>>>>> expected way
>>>>>> for people to handle gpu buffer?
>>>>> You either have a dumb buffer handle, or a dma-buf fd. If you have the
>>>>> handle, you have to use the dumb buffer API, there's no other way to
>>>>> mmap it. If you have the dma-buf fd, you should mmap it directly. Those
>>>>> two are clear.
>>>>>
>>>>> It's when you import the dma-buf into vgem and create a handle out of
>>>>> it, that's when the handle is no longer first class and certain uAPI
>>>>> [the dumb buffer API in particular] fail.
>>>>>
>>>>> It's not brilliant, as you say, it requires the user to remember the
>>>>> difference between the handles, but at the same time it does prevent
>>>>> them falling into coherency traps by forcing them to use the right
>>>>> driver to handle the object, and have to consider the additional ioctls
>>>>> that go along with that access.
>>>> Yes, Chris is right. Mapping DMA-buf through the mmap() APIs of an importer
>>>> is illegal.
>>>>
>>>> What we could maybe try to do is to redirect this mmap() API call on the
>>>> importer to the exporter, but I'm pretty sure that the fs layer wouldn't
>>>> like that without changes.
>>> We already do that, there's a full helper-ified path from I think shmem
>>> helpers through prime helpers to forward this all. Including handling
>>> buffer offsets and all the other lolz back&forth.
>> Oh, that most likely won't work correctly with unpinned DMA-bufs and
>> needs to be avoided.
>>
>> Each file descriptor is associated with an struct address_space. And
>> when you mmap() through the importer by redirecting the system call to
>> the exporter you end up with the wrong struct address_space in your VMA.
>>
>> That in turn can go up easily in flames when the exporter tries to
>> invalidate the CPU mappings for its DMA-buf while moving it.
>>
>> Where are we doing this? My last status was that this is forbidden.
> Hm I thought we're doing all that already, but looking at the code
> again we're only doing this when opening a new drm fd or dma-buf fd.
> So the right file->f_mapping is always set at file creation time.
>
> And we indeed don't frob this more when going another indirection ...
> Maybe we screwed up something somewhere :-/
>
> Also I thought the mapping is only taken after the vma is instatiated,
> otherwise the tricks we're playing with dma-buf already wouldn't work:
> dma-buf has the buffer always at offset 0, whereas gem drm_fd mmap has
> it somewhere else. We already adjust vma->vm_pgoff, so I'm wondering
> whether we could adjust vm_file too. Or is that the thing that's
> forbidden?

Yes, exactly. Modifying vm_pgoff is harmless, tons of code does that.

But changing vma->vm_file, that's something I haven't seen before and 
most likely could blow up badly.

Christian.

> -Daniel
>
>> Christian.
>>
>>> Of course there's still the problem that many drivers don't forward the
>>> cache coherency calls for begin/end cpu access, so in a bunch of cases
>>> you'll cache cacheline dirt soup. But that's kinda standard procedure for
>>> dma-buf :-P
>>>
>>> But yeah trying to handle the mmap as an importer, bypassing the export:
>>> nope. The one exception is if you have some kind of fancy gart with
>>> cpu-visible pci bar (like at least integrated intel gpus have). But in
>>> that case the mmap very much looks&acts like device access in every way.
>>>
>>> Cheers, Daniel
>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>
>>>>> -Chris
>

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