Re: [PATCH v3 04/20] PCI/P2PDMA: introduce helpers for dma_map_sg implementations

[Jason Gunthorpe]

On Thu, Sep 16, 2021 at 05:40:44PM -0600, Logan Gunthorpe wrote:
> Add pci_p2pdma_map_segment() as a helper for simple dma_map_sg()
> implementations. It takes an scatterlist segment that must point to a
> pci_p2pdma struct page and will map it if the mapping requires a bus
> address.
> 
> The return value indicates whether the mapping required a bus address
> or whether the caller still needs to map the segment normally. If the
> segment should not be mapped, -EREMOTEIO is returned.
> 
> This helper uses a state structure to track the changes to the
> pgmap across calls and avoid needing to lookup into the xarray for
> every page.
> 
> Also add pci_p2pdma_map_bus_segment() which is useful for IOMMU
> dma_map_sg() implementations where the sg segment containing the page
> differs from the sg segment containing the DMA address.
> 
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>  drivers/pci/p2pdma.c       | 59 ++++++++++++++++++++++++++++++++++++++
>  include/linux/pci-p2pdma.h | 21 ++++++++++++++
>  2 files changed, 80 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index b656d8c801a7..58c34f1f1473 100644
> +++ b/drivers/pci/p2pdma.c
> @@ -943,6 +943,65 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>  }
>  EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>  
> +/**
> + * pci_p2pdma_map_segment - map an sg segment determining the mapping type
> + * @state: State structure that should be declared outside of the for_each_sg()
> + *	loop and initialized to zero.
> + * @dev: DMA device that's doing the mapping operation
> + * @sg: scatterlist segment to map
> + *
> + * This is a helper to be used by non-iommu dma_map_sg() implementations where
> + * the sg segment is the same for the page_link and the dma_address.
> + *
> + * Attempt to map a single segment in an SGL with the PCI bus address.
> + * The segment must point to a PCI P2PDMA page and thus must be
> + * wrapped in a is_pci_p2pdma_page(sg_page(sg)) check.
> + *
> + * Returns the type of mapping used and maps the page if the type is
> + * PCI_P2PDMA_MAP_BUS_ADDR.
> + */
> +enum pci_p2pdma_map_type
> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev,
> +		       struct scatterlist *sg)
> +{
> +	if (state->pgmap != sg_page(sg)->pgmap) {
> +		state->pgmap = sg_page(sg)->pgmap;
> +		state->map = pci_p2pdma_map_type(state->pgmap, dev);
> +		state->bus_off = to_p2p_pgmap(state->pgmap)->bus_offset;
> +	}

Is this safe? I was just talking with Joao about this,

 https://lore.kernel.org/r/20210928180150.GI3544071@xxxxxxxx

API wise I absolutely think this should be safe as written, but is it
really?

Does pgmap ensure that a positive refcount struct page always has a
valid pgmap pointer (and thus the mess in gup can be deleted) or does
this need to get the pgmap as well to keep it alive?

Jason