Re: [PATCH 4.9 12/16] net/rds: dma_map_sg is entitled to merge entries

[Gerd Rausch]

Hi Pavel,

On 02/09/2021 23.50, Pavel Machek wrote:
>> [ Upstream commit fb4b1373dcab086d0619c29310f0466a0b2ceb8a ]
>>
>> Function "dma_map_sg" is entitled to merge adjacent entries
>> and return a value smaller than what was passed as "nents".
>>
>> Subsequently "ib_map_mr_sg" needs to work with this value ("sg_dma_len")
>> rather than the original "nents" parameter ("sg_len").
>>
>> This old RDS bug was exposed and reliably causes kernel panics
>> (using RDMA operations "rds-stress -D") on x86_64 starting with:
>> commit c588072bba6b ("iommu/vt-d: Convert intel iommu driver to the iommu ops")
>>
>> Simply put: Linux 5.11 and later.
> 
> I see this queued for 4.19 and 5.10 where "iommu/vt-d: Convert intel
> iommu driver to the iommu ops" is not present. It may be okay for
> older kernels, too, but I wanted to double-check.
> 

It should be okay for older kernels as well.

The bug has always been there, but only started to cause panics
in cases where "dma_map_sg" actually did merge adjacent entries.

We bisected the crash down to the commit mentioned above (c588072bba6b),
on platforms that use the intel iommu.

That intel-iommu commit wasn't there on Linux-5.10 and older.
But the RDS bug was.

Hope this helps,

  Gerd