Re: [PATCH] dma-direct: Fix missing sg_dma_len assignment in P2PDMA bus mappings

[Leon Romanovsky <leon@kernel.org>]

On Wed, Nov 26, 2025 at 11:41:12AM +0000, Pranjal Shrivastava wrote:
> Prior to commit a25e7962db0d7 ("PCI/P2PDMA: Refactor the p2pdma mapping
> helpers"), P2P segments were mapped using the pci_p2pdma_map_segment()
> helper. This helper was responsible for populating sg->dma_address,
> marking the bus address, and also setting sg_dma_len(sg).
> 
> The refactor[1] removed this helper and moved the mapping logic directly
> into the callers. While iommu_dma_map_sg() was correctly updated to set
> the length in the new flow, it was missed in dma_direct_map_sg().
> 
> Thus, in dma_direct_map_sg(), the PCI_P2PDMA_MAP_BUS_ADDR case sets the
> dma_address and marks the segment, but immediately executes 'continue',
> which causes the loop to skip the standard assignment logic at the end:
> 
>     sg_dma_len(sg) = sg->length;
> 
> As a result, when CONFIG_NEED_SG_DMA_LENGTH is enabled, the dma_length
> field remains uninitialized (zero) for P2P bus address mappings. This
> breaks upper-layer drivers (for e.g. RDMA/IB) that rely on sg_dma_len()
> to determine the transfer size.
> 
> Fix this by explicitly setting the DMA length in the
> PCI_P2PDMA_MAP_BUS_ADDR case before continuing to the next scatterlist
> entry.
> 
> Fixes: a25e7962db0d7 ("PCI/P2PDMA: Refactor the p2pdma mapping helpers")
> Reported-by: Jacob Moroni <jmoroni@google.com>
> Signed-off-by: Pranjal Shrivastava <praan@google.com>
> 
> [1]
> https://lore.kernel.org/all/ac14a0e94355bf898de65d023ccf8a2ad22a3ece.1746424934.git.leon@kernel.org/
> ---
>  kernel/dma/direct.c | 1 +
>  1 file changed, 1 insertion(+)
> 

Thanks,
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>