Am 09.07.21 um 21:31 schrieb Felix Kuehling:
On 2021-07-09 2:38 a.m., Christian König wrote:
Am 08.07.21 um 21:36 schrieb Alex Deucher:
From: Felix Kuehling <Felix.Kuehling@xxxxxxx>
KFD Thunk maps invisible VRAM BOs with PROT_NONE, MAP_PRIVATE.
is_cow_mapping returns true for these mappings. Add a check for
vm_flags & VM_WRITE to avoid mmap failures on private read-only or
PROT_NONE mappings.
I'm pretty sure that this is not working as expected.
Not sure what you mean. Debugger access to the memory through the
PROT_NONE VMAs is definitely working, with both ptrace and
/proc/<pid>/mem.
Fixes: f91142c62161 ("drm/ttm: nuke VM_MIXEDMAP on BO mappings v3")
Signed-off-by: Felix Kuehling <Felix.Kuehling@xxxxxxx>
Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
---
drivers/gpu/drm/ttm/ttm_bo_vm.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index f56be5bc0861..a75e90c7d4aa 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -552,7 +552,7 @@ static const struct vm_operations_struct
ttm_bo_vm_ops = {
int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct
ttm_buffer_object *bo)
{
/* Enforce no COW since would have really strange behavior
with it. */
- if (is_cow_mapping(vma->vm_flags))
+ if (is_cow_mapping(vma->vm_flags) && (vma->vm_flags & VM_WRITE))
is_cow_mapping() already checks for VM_MAYWRITE, so this here
shouldn't be necessary.
AFAICT, VM_MAYWRITE is not based on the PROT_... bits used to create
the VMA, but based on the permissions of the file. So as long as the
render node is writable, VM_MAYWRITE is set for all VMAs that map it.
I would agree that it's probably a bad idea for the Thunk to map these
VMAs with MAP_PRIVATE. We can try changing the Thunk to use MAP_SHARED
for these PROT_NONE mappings. But that doesn't change the fact that
this kernel patch broke existing usermode.
Yeah, but see the discussion around MAP_PRIVATE and COW regarding this.
What Thunk did here was a really bad idea.
I think we have only two options here:
1. Accept the breakage of thunk.
2. Add a workaround in amdgpu/amdkfd to force MAP_PRIVATE into
MAP_SHARED in the kernel.
Regards,
Christian.
Regards,
Felix
Christian.
return -EINVAL;
ttm_bo_get(bo);
