Re: [PATCH v3 3/7] dma-buf: Document RDMA non-ODP invalidate_mapping() special case

["Christian König" <christian.koenig@amd.com>]

On 1/21/26 10:14, Leon Romanovsky wrote:
> On Wed, Jan 21, 2026 at 09:59:59AM +0100, Christian König wrote:
>> On 1/20/26 15:07, Leon Romanovsky wrote:
>>> From: Leon Romanovsky <leonro@nvidia.com>
>>>
>>> The .invalidate_mapping() callback is documented as optional, yet it
>>> effectively became mandatory whenever importer_ops were provided. This
>>> led to cases where RDMA non-ODP code had to supply an empty stub just to
>>> provide allow_peer2peer.
>>>
>>> Document this behavior by creating a dedicated export for the
>>> dma_buf_unsupported_invalidate_mappings() function. This function is
>>> intended solely for the RDMA non-ODP case and must not be used by any
>>> other dma-buf importer.
>>>
>>> This makes it possible to rely on a valid .invalidate_mappings()
>>> callback to determine whether an importer supports revocation.
>>>
>>> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
>>> ---
>>>  drivers/dma-buf/dma-buf.c             | 14 ++++++++++++++
>>>  drivers/infiniband/core/umem_dmabuf.c | 11 +----------
>>>  include/linux/dma-buf.h               |  4 +++-
>>>  3 files changed, 18 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index cd3b60ce4863..c4fa35034b92 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -1238,6 +1238,20 @@ void dma_buf_unmap_attachment_unlocked(struct dma_buf_attachment *attach,
>>>  }
>>>  EXPORT_SYMBOL_NS_GPL(dma_buf_unmap_attachment_unlocked, "DMA_BUF");
>>>  
>>> +/*
>>> + * This function shouldn't be used by anyone except RDMA non-ODP case.
>>> + * The reason to it is UAPI mistake where dma-buf was exported to the
>>> + * userspace without knowing that .invalidate_mappings() can be called
>>> + * for pinned memory too.
>>> + *
>>> + * This warning shouldn't be seen in real production scenario.
>>> + */
>>> +void dma_buf_unsupported_invalidate_mappings(struct dma_buf_attachment *attach)
>>> +{
>>> +	pr_warn("Invalidate callback should not be called when memory is pinned\n");
>>> +}
>>> +EXPORT_SYMBOL_FOR_MODULES(dma_buf_unsupported_invalidate_mappings, "ib_uverbs");
>>> +
>>
>> Well that is exactly the opposite of what I had in mind.
>>
>> The RDMA non-ODP case should explicitly not provide an invalidate_mappings callback, but only the dma_buf_attach_ops with allow_peer2peer set to true.
>>
>> This is done to explicitly note that RDMA non-ODP can't do invalidation's.
> 
> We want to achieve two goals:
> 1. Provide a meaningful warning to developers, rather than failing later
>    because dma_buf_move_notify() was called on this problematic imported dma-buf.
> 2. Require all users to supply a valid .invalidate_mapping().

Nope, that is something I would reject. invalidate_mappings must stay optional.

> 
> If I allow empty .invalidate_mapping(), this check will go too:

Correct, that is the whole idea.

>    932 struct dma_buf_attachment *
>    933 dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
>    934                        const struct dma_buf_attach_ops *importer_ops,
>    935                        void *importer_priv)
> ...
>    943         if (WARN_ON(importer_ops && !importer_ops->invalidate_mappings))
>    944                 return ERR_PTR(-EINVAL);
> 
> And it is important part of dma-buf.

No, as far as I can see that is what we try to avoid.

The whole idea is to make invalidate_mappings truly optional.

Regards,
Christian.

> 
> Thanks