RDMA memory registration (was: [openib-general] Re:[PATCH][RFC][0/4] InfiniBand userspace verbs implementation)

From: Roland Dreier
Date: Fri Apr 29 2005 - 11:54:03 EST

Is there anything wrong with the following plan?

1) For memory registration, use get_user_pages() in the kernel. Use
locked_vm and RLIMIT_MEMLOCK to limit the amount of memory pinned
by a given process. One disadvantage of this is that the
accounting will overestimate the amount of pinned memory if a
process pins the same page twice, but this doesn't seem that bad to
me -- it errs on the side of safety.

2) For fork() support:

a) Extend mprotect() with PROT_DONTCOPY so processes can avoid
copy-on-write problems.

b) (maybe someday?) Add a VM_ALWAYSCOPY flag and extend mprotect()
with PROT_ALWAYSCOPY so processes can mark pages to be
pre-copied into child processes, to handle the case where only
half a page is registered.

I believe this puts the code that must be trusted into the kernel and
gives userspace primitives that let apps handle the rest.

- R.
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