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SubjectRe: [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@lst.de>

Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

> ---
> 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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