Re: [PATCH] RDMA/ucontext: fix building with CONFIG_MMU=n

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Sep 27, 2018 at 12:10:03PM +0200, Arnd Bergmann wrote:
> The zap_vma_ptes() is declared but not defined on NOMMU kernels,
> causing a link error for the newly added uverbs code:
> 
> drivers/infiniband/core/uverbs_main.o: In function `uverbs_user_mmap_disassociate':
> uverbs_main.c:(.text+0x114c): undefined reference to `zap_vma_ptes'
> drivers/infiniband/core/uverbs_main.o: In function `rdma_umap_open':
> uverbs_main.c:(.text+0x53c): undefined reference to `zap_vma_ptes'

I'm a bit surprised by this - the code is not new, it just got moved
out of the drivers, and I don't see any protections there against
CONFIG_MMU ??
 
> To fix this, we can either make uverbs depend on CONFIG_MMU, or try
> to build it anyway. Since this is the only compile-time dependency,
> I decided to allow building it with an extra compile-time check for
> CONFIG_MMU before calling the one function.

Hrm. So this code doesn't work at all on NOMMU, it relies on
remap_pfn_range, which always fails on those kernels, 

Due to this of this the entirety of INFINIBAND_USER_ACCESS is broken.

I think a kconfig change is the better way to go?

Jason