On Wed, Jun 02, 2021 at 10:30:13AM +0200, Christian König wrote:
We discussed if that is really the right approach for quite a while now, but digging deeper into a bug report on arm turned out that this is actually horrible broken right now.
The reason for this is that vmf_insert_mixed_prot() always tries to grab a reference to the underlaying page on architectures without ARCH_HAS_PTE_SPECIAL and as far as I can see also enabled GUP.
So nuke using VM_MIXEDMAP here and use VM_PFNMAP instead.
Also set VM_SHARED, not 100% sure if that is needed with VM_PFNMAP, but better save than sorry.
Signed-off-by: Christian König christian.koenig@amd.com Bugs: https://gitlab.freedesktop.org/drm/amd/-/issues/1606#note_936174
I thought we still have the same issue open for ttm_bo_vm_insert_huge()? Or at least a potentially pretty big bug, because our current huge entries don't stop gup (because there's no pmd_mkspecial right now in the kernel).
So I think if you want to close this for good then we also need to (temporarily at least) disable the huge entry code? -Daniel
drivers/gpu/drm/ttm/ttm_bo_vm.c | 29 +++++++---------------------- 1 file changed, 7 insertions(+), 22 deletions(-)
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c index 9bd15cb39145..bf86ae849340 100644 --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c @@ -359,12 +359,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf, * at arbitrary times while the data is mmap'ed. * See vmf_insert_mixed_prot() for a discussion. */
if (vma->vm_flags & VM_MIXEDMAP)ret = vmf_insert_mixed_prot(vma, address,__pfn_to_pfn_t(pfn, PFN_DEV),prot);elseret = vmf_insert_pfn_prot(vma, address, pfn, prot);
ret = vmf_insert_pfn_prot(vma, address, pfn, prot);/* Never error on prefaulted PTEs */ if (unlikely((ret & VM_FAULT_ERROR))) {
@@ -411,15 +406,9 @@ vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot) pfn = page_to_pfn(page);
/* Prefault the entire VMA range right away to avoid further faults */
- for (address = vma->vm_start; address < vma->vm_end; address += PAGE_SIZE) {
if (vma->vm_flags & VM_MIXEDMAP)ret = vmf_insert_mixed_prot(vma, address,__pfn_to_pfn_t(pfn, PFN_DEV),prot);elseret = vmf_insert_pfn_prot(vma, address, pfn, prot);- }
for (address = vma->vm_start; address < vma->vm_end;
address += PAGE_SIZE)ret = vmf_insert_pfn_prot(vma, address, pfn, prot);return ret;
} @@ -576,14 +565,10 @@ static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_s
vma->vm_private_data = bo;
- /*
* We'd like to use VM_PFNMAP on shared mappings, where* (vma->vm_flags & VM_SHARED) != 0, for performance reasons,* but for some reason VM_PFNMAP + x86 PAT + write-combine is very* bad for performance. Until that has been sorted out, use* VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
- /* Enforce VM_SHARED here since no driver backend actually supports COW
* on TTM buffer object mappings.
- vma->vm_flags |= VM_MIXEDMAP;
- vma->vm_flags |= VM_PFNMAP | VM_SHARED; vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
}
-- 2.25.1