Am 09.07.20 um 15:54 schrieb Steven Price:
On 09/07/2020 09:48, Christian König wrote:
Am 08.07.20 um 18:19 schrieb Daniel Vetter:
On Wed, Jul 8, 2020 at 6:11 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
On Wed, Jul 8, 2020 at 5:05 PM Christian König
<christian.koenig@xxxxxxx> wrote:
Am 08.07.20 um 17:01 schrieb Daniel Vetter:
On Wed, Jul 8, 2020 at 4:37 PM Christian König
<christian.koenig@xxxxxxx> 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@xxxxxxxxxxxxxxxxxx> wrote:
Quoting lepton (2020-07-07 18:05:21)
On Tue, Jul 7, 2020 at 9:00 AM Chris Wilson
<chris@xxxxxxxxxxxxxxxxxx> 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.
Ok, I read the shmem helpers again, I think those are the only ones
which do the importer mmap -> dma_buf_mmap() forwarding, and hence
break stuff all around here.
They also remove the vma->vm_pgoff offset, which means
unmap_mapping_range wont work anyway. I guess for drivers which use
shmem helpers the hard assumption is that a) can't use p2p dma and b)
pin everything into system memory.
So not a problem. But something to keep in mind. I'll try to do a
kerneldoc patch to note this somewhere. btw on that, did the
timeline/syncobj documentation patch land by now? Just trying to make
sure that doesn't get lost for another few months or so :-/
Ok, so maybe it is a problem. Because there is a drm_gem_shmem_purge()
which uses unmap_mapping_range underneath, and that's used by
panfrost. And panfrost also uses the mmap helper. Kinda wondering
whether we broke some stuff here, or whether the reverse map is
installed before we touch vma->vm_pgoff.
I think the key problem here is that unmap_mapping_range() doesn't
blow up immediately when this is wrong.
E.g. we just have a stale CPU page table entry which allows userspace
to write to freed up memory, but we don't really notice that
immediately....
Maybe we should stop allowing to mmap() DMA-buf through the importer
file descriptor altogether and only allow mapping it through its own
fd or the exporter.
That is what I tried to do with panfrost a while ago:
583bbf46133c drm/panfrost: Use drm_gem_map_offset()
panfrost_ioctl_mmap_bo() contains a reimplementation of
drm_gem_map_offset() but with a bug - it allows mapping imported
objects (without going through the exporter). Fix this by switching to
use the newly renamed drm_gem_map_offset() function instead which has
the bonus of simplifying the code.
Sadly it was followed by:
be855382bacb Revert "drm/panfrost: Use drm_gem_map_offset()"
This reverts commit 583bbf46133c726bae277e8f4e32bfba2a528c7f.
Turns out we need mmap to work on imported BOs even if the current
code
is buggy.
Mhm, we might need to push back on the revert.
Christian.
panfrost folks, does panfrost purged buffer handling of mmap still
work correctly? Do you have some kind of igt or similar for this?
I'm not aware of any real testing of this. And I fear it probably
isn't getting much in the way of real-world testing either otherwise
someone would have grown tired of the "Purging xx bytes" message[1]
I'm a little bit lost on this thread - are you expecting this patch to
break panfrost? We shouldn't be purging imported memory. Although I'm
not sure what (if anything) stops you trying to mark imported memory
as "don't need". Or indeed what would happen.
As long as panfrost keeps the imported DMA-bufs pinned everything should
be fine as it is.
But this can open up a security hole you can push an elephant through.
So you need to keep this in the back of your mind if this is ever
implemented.
Christian.
Steve
[1] I have a patch silencing that, but recently haven't had much time
for working on Panfrost, and don't have my WFH setup quite as slick as
I did when I was in the office.
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