Re: [PATCH hmm 2/5] mm/hmm: make hmm_range_fault return 0 or -1

Christoph Hellwig
2020-04-22T05:52:32Z

On Tue, Apr 21, 2020 at 09:21:43PM -0300, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
>
> hmm_vma_walk->last is supposed to be updated after every write to the
> pfns, so that it can be returned by hmm_range_fault(). However, this is
> not done consistently. Fortunately nothing checks the return code of
> hmm_range_fault() for anything other than error.
>
> More importantly last must be set before returning -EBUSY as it is used to
> prevent reading an output pfn as an input flags when the loop restarts.
>
> For clarity and simplicity make hmm_range_fault() return 0 or -ERRNO. Only
> set last when returning -EBUSY.
>
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> ---
> Documentation/vm/hmm.rst | 2 +-
> drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 4 ++--
> drivers/gpu/drm/nouveau/nouveau_svm.c | 6 +++---
> include/linux/hmm.h | 2 +-
> mm/hmm.c | 25 +++++++++----------------
> 5 files changed, 16 insertions(+), 23 deletions(-)
>
> diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
> index 4e3e9362afeb10..9924f2caa0184c 100644
> --- a/Documentation/vm/hmm.rst
> +++ b/Documentation/vm/hmm.rst
> @@ -161,7 +161,7 @@ device must complete the update before the driver callback returns.
> When the device driver wants to populate a range of virtual addresses, it can
> use::
>
> - long hmm_range_fault(struct hmm_range *range);
> + int hmm_range_fault(struct hmm_range *range);
>
> It will trigger a page fault on missing or read-only entries if write access is
> requested (see below). Page faults use the generic mm page fault code path just
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> index 6309ff72bd7876..efc1329a019127 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> @@ -852,12 +852,12 @@ int amdgpu_ttm_tt_get_user_pages(struct amdgpu_bo *bo, struct page **pages)
> down_read(&mm->mmap_sem);
> r = hmm_range_fault(range);
> up_read(&mm->mmap_sem);
> - if (unlikely(r <= 0)) {
> + if (unlikely(r)) {
> /*
> * FIXME: This timeout should encompass the retry from
> * mmu_interval_read_retry() as well.
> */
> - if ((r == 0 || r == -EBUSY) && !time_after(jiffies, timeout))
> + if ((r == -EBUSY) && !time_after(jiffies, timeout))

Please also kill the superflous inner braces here.

> + * Return: 0 or -ERRNO with one of the following status codes:

Maybe say something like:

* Returns 0 on success or one of the following error codes:

Otherwise this looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>