Re: [PATCH 19/22] mm: always return EBUSY for invalid ranges in hmm_range_{fault,snapshot}

Kuehling, Felix
2019-07-02T21:44:02Z

On 2019-07-01 2:20 a.m., Christoph Hellwig wrote:
> We should not have two different error codes for the same condition. In
> addition this really complicates the code due to the special handling of
> EAGAIN that drops the mmap_sem due to the FAULT_FLAG_ALLOW_RETRY logic
> in the core vm.

I think the comment above hmm_range_snapshot needs an update. Also
Documentation/vm/hmm.rst shows some example code using
hmm_range_snapshot that retries on -EAGAIN. That would need to be
updated to use -EBUSY or remove the retry logic altogether.

Other than that, this patch is Reviewed-by: Felix Kuehling
<Felix.Kuehling@amd.com>

Philip, this means we should remove our retry logic again in
amdgpu_ttm_tt_get_user_pages. According to the comment above
hmm_range_fault, it can only return -EAGAIN if the block parameter is
false. I think this statement is now actually true. We set block=true,
so we can't get -EAGAIN. On -EBUSY we can let
amdgpu_amdkfd_restore_userptr_worker schedule the retry (which it does
already anyway).

Regards,
Felix

>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> mm/hmm.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/mm/hmm.c b/mm/hmm.c
> index c85ed7d4e2ce..d125df698e2b 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -974,7 +974,7 @@ long hmm_range_snapshot(struct hmm_range *range)
> do {
> /* If range is no longer valid force retry. */
> if (!range->valid)
> - return -EAGAIN;
> + return -EBUSY;
>
> vma = find_vma(hmm->mm, start);
> if (vma == NULL || (vma->vm_flags & device_vma))
> @@ -1069,10 +1069,8 @@ long hmm_range_fault(struct hmm_range *range, bool block)
>
> do {
> /* If range is no longer valid force retry. */
> - if (!range->valid) {
> - up_read(&hmm->mm->mmap_sem);
> - return -EAGAIN;
> - }
> + if (!range->valid)
> + return -EBUSY;
>
> vma = find_vma(hmm->mm, start);
> if (vma == NULL || (vma->vm_flags & device_vma))