[dpdk-dev] Huge mapping secondary process linux

["Jonas Pfefferle1" <JPF@zurich.ibm.com>]

Hi @all,

I'm trying to make sense of the hugepage memory mappings in
librte_eal/linuxapp/eal/eal_memory.c:
* In rte_eal_hugepage_attach (line 1347) when we try to do a private
mapping on /dev/zero (line 1393) why do we not use MAP_FIXED if we need the
addresses to be identical with the primary process?
* On POWER we have this weird business going on where we use MAP_HUGETLB
because according to this commit:

commit 284ae3e9ff9a92575c28c858efd2c85c8de6d440
Author: Chao Zhu <chaozhu@linux.vnet.ibm.com>
Date:   Thu Apr 6 15:36:09 2017 +0530

    eal/ppc: fix mmap for memory initialization

    On IBM POWER platform, when mapping /dev/zero file to hugepage memory
    space, mmap will not respect the requested address hint. This will
cause
    the memory initialization for the second process fails. This patch adds
    the required mmap flags to make it work. Beside this, users need to set
    the nr_overcommit_hugepages to expand the VA range. When
    doing the initialization, users need to set both nr_hugepages and
    nr_overcommit_hugepages to the same value, like 64, 128, etc.

mmap address hints are not respected. Looking at the mmap code in the
kernel this is not true entirely however under some circumstances the hint
can be ignored (
http://elixir.free-electrons.com/linux/latest/source/arch/powerpc/mm/mmap.c#L103
). However I believe we can remove the extra case for PPC if we use
MAP_FIXED when doing the secondary process mappings because we need them to
be identical anyway. We could also use MAP_FIXED when doing the primary
process mappings resp. get_virtual_area if we want to have any guarantees
when specifying a base address. Any thoughts?

Thanks,
Jonas