Re: [PATCH 4/5] nouveau: unlock mmap_sem on all errors from nouveau_range_fault

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On 7/3/19 11:45 AM, Christoph Hellwig wrote:
Currently nouveau_svm_fault expects nouveau_range_fault to never unlock
mmap_sem, but the latter unlocks it for a random selection of error
codes. Fix this up by always unlocking mmap_sem for non-zero return
values in nouveau_range_fault, and only unlocking it in the caller
for successful returns.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

---
  drivers/gpu/drm/nouveau/nouveau_svm.c | 15 ++++++++-------
  1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
index e831f4184a17..c0cf7aeaefb3 100644
--- a/drivers/gpu/drm/nouveau/nouveau_svm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
@@ -500,8 +500,10 @@ nouveau_range_fault(struct hmm_mirror *mirror, struct hmm_range *range,

You can delete the comment "With the old API the driver must ..."
(not visible in the patch here).
I suggest moving the two assignments:
	range->default_flags = 0;
	range->pfn_flags_mask = -1UL;
to just above the "again:" where the other range.xxx fields are
initialized in nouveau_svm_fault().

  	ret = hmm_range_register(range, mirror,
  				 range->start, range->end,
  				 PAGE_SHIFT);
-	if (ret)
+	if (ret) {
+		up_read(&range->vma->vm_mm->mmap_sem; >   		return (int)ret;
+	}
if (!hmm_range_wait_until_valid(range, NOUVEAU_RANGE_FAULT_TIMEOUT)) {
  		/*

You can delete this comment (only the first line is visible here)
since it is about the "old API".
Also, it should return -EBUSY not -EAGAIN since it means there was a
range invalidation collision (similar to hmm_range_fault() if
!range->valid).

@@ -515,15 +517,14 @@ nouveau_range_fault(struct hmm_mirror *mirror, struct hmm_range *range,
ret = hmm_range_fault(range, block);

nouveau_range_fault() is only called with "block = true" so
could eliminate the block parameter and pass true here.

  	if (ret <= 0) {
-		if (ret == -EBUSY || !ret) {
-			/* Same as above, drop mmap_sem to match old API. */
-			up_read(&range->vma->vm_mm->mmap_sem);
-			ret = -EBUSY;
-		} else if (ret == -EAGAIN)
+		if (ret == 0)
  			ret = -EBUSY;
+		if (ret != -EAGAIN)
+			up_read(&range->vma->vm_mm->mmap_sem);

Can ret == -EAGAIN happen if "block = true"?
Generally, I prefer the read_down()/read_up() in the same function
(i.e., nouveau_svm_fault()) but I can see why it should be here
if hmm_range_fault() can return with mmap_sem unlocked.

  		hmm_range_unregister(range);
  		return ret;
  	}
+
  	return 0;
  }
@@ -718,8 +719,8 @@ nouveau_svm_fault(struct nvif_notify *notify)
  						NULL);
  			svmm->vmm->vmm.object.client->super = false;
  			mutex_unlock(&svmm->mutex);
+			up_read(&svmm->mm->mmap_sem);
  		}
-		up_read(&svmm->mm->mmap_sem);

The "else" case should check for -EBUSY and goto again.

  		/* Cancel any faults in the window whose pages didn't manage
  		 * to keep their valid bit, or stay writeable when required.

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