Re: [dpdk-dev] Huge mapping secondary process linux

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

"dev" <dev-bounces@dpdk.org> wrote on 10/27/2017 04:58:01 PM:

> From: "Jonas Pfefferle1" <JPF@zurich.ibm.com>
> To: "Burakov, Anatoly" <anatoly.burakov@intel.com>
> Cc: bruce.richardson@intel.com, chaozhu@linux.vnet.ibm.com, dev@dpdk.org
> Date: 10/27/2017 04:58 PM
> Subject: Re: [dpdk-dev] Huge mapping secondary process linux
> Sent by: "dev" <dev-bounces@dpdk.org>
>
>
> "Burakov, Anatoly" <anatoly.burakov@intel.com> wrote on 10/27/2017
04:44:52
> PM:
>
> > From: "Burakov, Anatoly" <anatoly.burakov@intel.com>
> > To: Jonas Pfefferle1 <JPF@zurich.ibm.com>
> > Cc: bruce.richardson@intel.com, chaozhu@linux.vnet.ibm.com,
dev@dpdk.org
> > Date: 10/27/2017 04:45 PM
> > Subject: Re: [dpdk-dev] Huge mapping secondary process linux
> >
> > On 27-Oct-17 3:28 PM, Jonas Pfefferle1 wrote:
> > > "Burakov, Anatoly" <anatoly.burakov@intel.com> wrote on 10/27/2017
> > > 04:06:44 PM:
> > >
> > >  > From: "Burakov, Anatoly" <anatoly.burakov@intel.com>
> > >  > To: Jonas Pfefferle1 <JPF@zurich.ibm.com>, dev@dpdk.org
> > >  > Cc: chaozhu@linux.vnet.ibm.com, bruce.richardson@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 <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 (
> > >  > > 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" :)
>
> Hi Anatoly,
>
> What I meant was the mmap address hint:
>
> "If addr is not NULL, then the kernel takes it as a hint
>  about where to place the mapping; on Linux, the mapping will be
>  created at a nearby page boundary."
>
> This is actually not true on POWER. It can happen that the address hint
is
> ignored and you get any address back that fits your mapping.
>
> Thanks,
> Jonas

Actually looking through the kernel code this is also not guaranteed on
x86.
(
http://elixir.free-electrons.com/linux/latest/source/arch/x86/kernel/sys_x86_64.c#L165
)

So in any case the address hint can be ignored by the kernel and you get
any address that fits your mapping.
My suggestion is to check when we do the initial mapping in
get_virtual_area if the hint was respected or not, i.e. if the returned
address == PAGE_ALIGN(address_hint).

Thanks,
Jonas

>
> >
> >
> > --
> > Thanks,
> > Anatoly
> >
>