Re: [PATCH 1/5] mm: return valid info from hmm_range_unregister

[Jason Gunthorpe]

On Wed, Jul 03, 2019 at 11:44:58AM -0700, Christoph Hellwig wrote:
> Checking range->valid is trivial and has no meaningful cost, but
> nicely simplifies the fastpath in typical callers.  

It should not be the typical caller..

> hmm_vma_range_done function, which now is a trivial wrapper around
> hmm_range_unregister.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
>  drivers/gpu/drm/nouveau/nouveau_svm.c |  2 +-
>  include/linux/hmm.h                   | 11 +----------
>  mm/hmm.c                              |  7 ++++++-
>  3 files changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
> index 8c92374afcf2..9d40114d7949 100644
> +++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
> @@ -652,7 +652,7 @@ nouveau_svm_fault(struct nvif_notify *notify)
>  		ret = hmm_vma_fault(&svmm->mirror, &range, true);
>  		if (ret == 0) {
>  			mutex_lock(&svmm->mutex);
> -			if (!hmm_vma_range_done(&range)) {
> +			if (!hmm_range_unregister(&range)) {
>  				mutex_unlock(&svmm->mutex);
>  				goto again;
>  			}

In this case if we take the 'goto again' then we are pointlessly
removing and re-adding the range.

The pattern is supposed to be:

    hmm_range_register()
again:
    .. read page tables ..
    lock
    if (!hmm_range_valid())
        unlock
        goto again
    .. setup device ..
    unlock
    hmm_range_unregister()

I don't think the API should be encouraging some shortcut here..

We can't do the above pattern because the old hmm_vma API didn't allow
it, which is presumably a reason why it is obsolete.

I'd rather see drivers move to a consistent pattern so we can then
easily hoist the seqcount lock scheme into some common mmu notifier
code, as discussed.

Jason