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

[Leon Romanovsky <leon@kernel.org>]

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().

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

Thanks