Re: [PATCH v4 01/14] drm/ttm: Remap all page faults to per process dummy page.

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Hey Daniel, just a ping.

Andrey

On 1/25/21 10:28 AM, Andrey Grodzovsky wrote:

On 1/19/21 8:56 AM, Daniel Vetter wrote:
On Mon, Jan 18, 2021 at 04:01:10PM -0500, Andrey Grodzovsky wrote:
On device removal reroute all CPU mappings to dummy page.

v3:
Remove loop to find DRM file and instead access it
by vma->vm_file->private_data. Move dummy page installation
into a separate function.

v4:
Map the entire BOs VA space into on demand allocated dummy page
on the first fault for that BO.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
---
  drivers/gpu/drm/ttm/ttm_bo_vm.c | 82 ++++++++++++++++++++++++++++++++++++++++-
  include/drm/ttm/ttm_bo_api.h    |  2 +
  2 files changed, 83 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 6dc96cf..ed89da3 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -34,6 +34,8 @@
  #include <drm/ttm/ttm_bo_driver.h>
  #include <drm/ttm/ttm_placement.h>
  #include <drm/drm_vma_manager.h>
+#include <drm/drm_drv.h>
+#include <drm/drm_managed.h>
  #include <linux/mm.h>
  #include <linux/pfn_t.h>
  #include <linux/rbtree.h>
@@ -380,25 +382,103 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
  }
  EXPORT_SYMBOL(ttm_bo_vm_fault_reserved);
  +static void ttm_bo_release_dummy_page(struct drm_device *dev, void *res)
+{
+    struct page *dummy_page = (struct page *)res;
+
+    __free_page(dummy_page);
+}
+
+vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot)
+{
+    struct vm_area_struct *vma = vmf->vma;
+    struct ttm_buffer_object *bo = vma->vm_private_data;
+    struct ttm_bo_device *bdev = bo->bdev;
+    struct drm_device *ddev = bo->base.dev;
+    vm_fault_t ret = VM_FAULT_NOPAGE;
+    unsigned long address = vma->vm_start;
+    unsigned long num_prefault = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+    unsigned long pfn;
+    struct page *page;
+    int i;
+
+    /*
+     * Wait for buffer data in transit, due to a pipelined
+     * move.
+     */
+    ret = ttm_bo_vm_fault_idle(bo, vmf);
+    if (unlikely(ret != 0))
+        return ret;
+
+    /* Allocate new dummy page to map all the VA range in this VMA to it*/
+    page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+    if (!page)
+        return VM_FAULT_OOM;
+
+    pfn = page_to_pfn(page);
+
+    /*
+     * Prefault the entire VMA range right away to avoid further faults
+     */
+    for (i = 0; i < num_prefault; ++i) {
+
+        if (unlikely(address >= vma->vm_end))
+            break;
+
+        if (vma->vm_flags & VM_MIXEDMAP)
+            ret = vmf_insert_mixed_prot(vma, address,
+                            __pfn_to_pfn_t(pfn, PFN_DEV),
+                            prot);
+        else
+            ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
+
+        /* Never error on prefaulted PTEs */
+        if (unlikely((ret & VM_FAULT_ERROR))) {
+            if (i == 0)
+                return VM_FAULT_NOPAGE;
+            else
+                break;
+        }
+
+        address += PAGE_SIZE;
+    }
+
+    /* Set the page to be freed using drmm release action */
+    if (drmm_add_action_or_reset(ddev, ttm_bo_release_dummy_page, page))
+        return VM_FAULT_OOM;
+
+    return ret;
+}
+EXPORT_SYMBOL(ttm_bo_vm_dummy_page);
I think we can lift this entire thing (once the ttm_bo_vm_fault_idle is
gone) to the drm level, since nothing ttm specific in here. Probably stuff
it into drm_gem.c (but really it's not even gem specific, it's fully
generic "replace this vma with dummy pages pls" function.


Once I started with this I noticed that drmm_add_action_or_reset depends
on struct drm_device *ddev = bo->base.dev  and bo is the private data
we embed at the TTM level when setting up the mapping and so this forces
to move drmm_add_action_or_reset out of this function to every client who uses
this function, and then you separate the logic of page allocation from it's release.
So I suggest we keep it as is.

Andrey



Aside from this nit I think the overall approach you have here is starting
to look good. Lots of work&polish, but imo we're getting there and can
start landing stuff soon.
-Daniel

+
  vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
  {
      struct vm_area_struct *vma = vmf->vma;
      pgprot_t prot;
      struct ttm_buffer_object *bo = vma->vm_private_data;
+    struct drm_device *ddev = bo->base.dev;
      vm_fault_t ret;
+    int idx;
        ret = ttm_bo_vm_reserve(bo, vmf);
      if (ret)
          return ret;
        prot = vma->vm_page_prot;
-    ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT, 1);
+    if (drm_dev_enter(ddev, &idx)) {
+        ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT, 1);
+        drm_dev_exit(idx);
+    } else {
+        ret = ttm_bo_vm_dummy_page(vmf, prot);
+    }
      if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
          return ret;
        dma_resv_unlock(bo->base.resv);
        return ret;
+
+    return ret;
  }
  EXPORT_SYMBOL(ttm_bo_vm_fault);
  diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
index e17be32..12fb240 100644
--- a/include/drm/ttm/ttm_bo_api.h
+++ b/include/drm/ttm/ttm_bo_api.h
@@ -643,4 +643,6 @@ void ttm_bo_vm_close(struct vm_area_struct *vma);
  int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
               void *buf, int len, int write);
  +vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot);
+
  #endif
--
2.7.4

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