On Thu, Feb 18, 2021 at 4:26 PM Steven Price steven.price@arm.com wrote:
On 17/02/2021 16:59, Neil Roberts wrote:
When mmapping the shmem, it would previously adjust the pgoff in the vm_area_struct to remove the fake offset that is added to be able to identify the buffer. This patch removes the adjustment and makes the fault handler use the vm_fault address to calculate the page offset instead. Although using this address is apparently discouraged, several DRM drivers seem to be doing it anyway.
The problem with removing the pgoff is that it prevents drm_vma_node_unmap from working because that searches the mapping tree by address. That doesn't work because all of the mappings are at offset 0. drm_vma_node_unmap is being used by the shmem helpers when purging the buffer.
It looks like panfrost is using drm_gem_shmem_purge so this might fix a potential bug there.
Signed-off-by: Neil Roberts nroberts@igalia.com
As the test robot points out pgoff_t is unsigned, so the <0 test makes no sense. But apart from that it looks good to me.
I think this is worth a "Fixes:" line too - as you point out drm_vma_node_unmap() won't be working correctly - which means we're potentially leaving user space with pages pointing to freed memory - not good! 17acb9f35ed7 is my best guess at the commit that introduced this.
Yeah plus Cc: stable for backporting and I think an igt or similar for panfrost to check this works correctly would be pretty good too. Since if it took us over 1 year to notice this bug it's pretty clear that normal testing doesn't catch this. So very likely we'll break this again.
btw for testing shrinkers recommended way is to have a debugfs file that just force-shrinks everything. That way you avoid all the trouble that tend to happen when you drive a system close to OOM on linux, and it's also much faster. -Daniel
Steve
drivers/gpu/drm/drm_gem_shmem_helper.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c index 9825c378dfa6..4b14157f1962 100644 --- a/drivers/gpu/drm/drm_gem_shmem_helper.c +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c @@ -526,11 +526,16 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf) struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj); loff_t num_pages = obj->size >> PAGE_SHIFT; struct page *page;
pgoff_t page_offset;
if (vmf->pgoff >= num_pages || WARN_ON_ONCE(!shmem->pages))
/* We don't use vmf->pgoff since that has the fake offset */page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;if (page_offset < 0 || page_offset >= num_pages ||WARN_ON_ONCE(!shmem->pages)) return VM_FAULT_SIGBUS;
page = shmem->pages[vmf->pgoff];
page = shmem->pages[page_offset]; return vmf_insert_page(vma, vmf->address, page);}
@@ -581,9 +586,6 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma) struct drm_gem_shmem_object *shmem; int ret;
/* Remove the fake offset */vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);if (obj->import_attach) { /* Drop the reference drm_gem_mmap_obj() acquired.*/ drm_gem_object_put(obj);
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