Am 15.12.20 um 21:18 schrieb Andrey Grodzovsky:
[SNIP]
While we can't control user application accesses to the mapped
buffers explicitly and hence we use page fault rerouting
I am thinking that in this case we may be able to sprinkle
drm_dev_enter/exit in any such sensitive place were we might
CPU access a DMA buffer from the kernel ?
Yes, I fear we are going to need that.
Things like CPU page table updates, ring buffer accesses and FW
memcpy ? Is there other places ?
Puh, good question. I have no idea.
Another point is that at this point the driver shouldn't access any
such buffers as we are at the process finishing the device.
AFAIK there is no page fault mechanism for kernel mappings so I
don't think there is anything else to do ?
Well there is a page fault handler for kernel mappings, but that one
just prints the stack trace into the system log and calls BUG(); :)
Long story short we need to avoid any access to released pages after
unplug. No matter if it's from the kernel or userspace.
I was just about to start guarding with drm_dev_enter/exit CPU
accesses from kernel to GTT ot VRAM buffers but then i looked more in
the code
and seems like ttm_tt_unpopulate just deletes DMA mappings (for the
sake of device to main memory access). Kernel page table is not touched
until last bo refcount is dropped and the bo is released
(ttm_bo_release->destroy->amdgpu_bo_destroy->amdgpu_bo_kunmap). This
is both
for GTT BOs maped to kernel by kmap (or vmap) and for VRAM BOs mapped
by ioremap. So as i see it, nothing will bad will happen after we
unpopulate a BO while we still try to use a kernel mapping for it,
system memory pages backing GTT BOs are still mapped and not freed and
for
VRAM BOs same is for the IO physical ranges mapped into the kernel
page table since iounmap wasn't called yet.
The problem is the system pages would be freed and if we kernel driver
still happily write to them we are pretty much busted because we write
to freed up memory.
Christian.
I loaded the driver with vm_update_mode=3
meaning all VM updates done using CPU and hasn't seen any OOPs after
removing the device. I guess i can test it more by allocating GTT and
VRAM BOs
and trying to read/write to them after device is removed.
Andrey
Regards,
Christian.
Andrey
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