On 27-Oct-17 3:28 PM, Jonas Pfefferle1 wrote:
"Burakov, Anatoly" <anatoly.bura...@intel.com> wrote on 10/27/2017
04:06:44 PM:
> From: "Burakov, Anatoly" <anatoly.bura...@intel.com>
> To: Jonas Pfefferle1 <j...@zurich.ibm.com>, dev@dpdk.org
> Cc: chao...@linux.vnet.ibm.com, bruce.richard...@intel.com
> Date: 10/27/2017 04:06 PM
> Subject: Re: [dpdk-dev] Huge mapping secondary process linux
>
> On 27-Oct-17 1:43 PM, Jonas Pfefferle1 wrote:
> >
> >
> > 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 <chao...@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 (
> > https://urldefense.proofpoint.com/v2/url?
>
u=http-3A__elixir.free-2Delectrons.com_linux_latest_source_arch_powerpc_mm_mmap.c-23L103&d=DwICaQ&c=jf_iaSHvJObTbx-
> siA1ZOg&r=rOdXhRsgn8Iur7bDE0vgwvo6TC8OpoDN-
> pXjigIjRW0&m=cttQcHlAYixhsYS3lz-
> BAdEeg4dpbwGdPnj2R3I8Do0&s=Gp0TIjUtIed05Jgb7XnlocpCYZdFXZXiH0LqIWiNMhA&e=
> > ). 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
> >
> hi Jonas,
>
> MAP_FIXED is not used because it's dangerous, it unmaps anything that is
> already mapped into that space. We would rather know that we can't map
> something than unwittingly unmap something that was mapped before.
Ok, I see. Maybe we can add a check to the primary process's memory
mappings whether the hint has been respected or not? At least warn if it
hasn't.
Hi Jonas,
I'm unfamiliar with POWER platform, so i'm afraid you'd have to explain
a bit more what you mean by "hint has been respected" :)
--
Thanks,
Anatoly
