Re: [PATCH] dma: mark unmapped DMA scatter/gather invalid

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jan 11, 2021 at 07:43:35AM -0800, Marc Orr wrote:
> This patch updates dma_direct_unmap_sg() to mark each scatter/gather
> entry invalid, after it's unmapped. This fixes two issues:
> 
> 1. It makes the unmapping code able to tolerate a double unmap.
> 2. It prevents the NVMe driver from erroneously treating an unmapped DMA
> address as mapped.
> 
> The bug that motivated this patch was the following sequence, which
> occurred within the NVMe driver, with the kernel flag `swiotlb=force`.
> 
> * NVMe driver calls dma_direct_map_sg()
> * dma_direct_map_sg() fails part way through the scatter gather/list
> * dma_direct_map_sg() calls dma_direct_unmap_sg() to unmap any entries
>   succeeded.
> * NVMe driver calls dma_direct_unmap_sg(), redundantly, leading to a
>   double unmap, which is a bug.
> 
> With this patch, a hadoop workload running on a cluster of three AMD
> SEV VMs, is able to succeed. Without the patch, the hadoop workload
> suffers application-level and even VM-level failures.
> 
> Tested-by: Jianxiong Gao <jxgao@google.com>
> Tested-by: Marc Orr <marcorr@google.com>
> Reviewed-by: Jianxiong Gao <jxgao@google.com>
> Signed-off-by: Marc Orr <marcorr@google.com>
> ---
>  kernel/dma/direct.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>