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Message-ID: <87eei84ijx.fsf@manicouagan.localdomain>
Date: Mon, 25 Jan 2021 22:02:10 -0300
From: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To: Konrad Rzeszutek Wilk <konrad@...nok.org>
Cc: Ram Pai <linuxram@...ibm.com>, linuxppc-dev@...ts.ozlabs.org,
linux-kernel@...r.kernel.org,
Michael Ellerman <mpe@...erman.id.au>,
Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>
Subject: Re: [PATCH] powerpc/mm: Limit allocation of SWIOTLB on server machines
Konrad Rzeszutek Wilk <konrad@...nok.org> writes:
> On Fri, Jan 08, 2021 at 09:27:01PM -0300, Thiago Jung Bauermann wrote:
>>
>> Ram Pai <linuxram@...ibm.com> writes:
>>
>> > On Wed, Dec 23, 2020 at 09:06:01PM -0300, Thiago Jung Bauermann wrote:
>> >>
>> >> Hi Ram,
>> >>
>> >> Thanks for reviewing this patch.
>> >>
>> >> Ram Pai <linuxram@...ibm.com> writes:
>> >>
>> >> > On Fri, Dec 18, 2020 at 03:21:03AM -0300, Thiago Jung Bauermann wrote:
>> >> >> On server-class POWER machines, we don't need the SWIOTLB unless we're a
>> >> >> secure VM. Nevertheless, if CONFIG_SWIOTLB is enabled we unconditionally
>> >> >> allocate it.
>> >> >>
>> >> >> In most cases this is harmless, but on a few machine configurations (e.g.,
>> >> >> POWER9 powernv systems with 4 GB area reserved for crashdump kernel) it can
>> >> >> happen that memblock can't find a 64 MB chunk of memory for the SWIOTLB and
>> >> >> fails with a scary-looking WARN_ONCE:
>> >> >>
>> >> >> ------------[ cut here ]------------
>> >> >> memblock: bottom-up allocation failed, memory hotremove may be affected
>> >> >> WARNING: CPU: 0 PID: 0 at mm/memblock.c:332 memblock_find_in_range_node+0x328/0x340
>> >> >> Modules linked in:
>> >> >> CPU: 0 PID: 0 Comm: swapper Not tainted 5.10.0-rc2-orig+ #6
>> >> >> NIP: c000000000442f38 LR: c000000000442f34 CTR: c0000000001e0080
>> >> >> REGS: c000000001def900 TRAP: 0700 Not tainted (5.10.0-rc2-orig+)
>> >> >> MSR: 9000000002021033 <SF,HV,VEC,ME,IR,DR,RI,LE> CR: 28022222 XER: 20040000
>> >> >> CFAR: c00000000014b7b4 IRQMASK: 1
>> >> >> GPR00: c000000000442f34 c000000001defba0 c000000001deff00 0000000000000047
>> >> >> GPR04: 00000000ffff7fff c000000001def828 c000000001def820 0000000000000000
>> >> >> GPR08: 0000001ffc3e0000 c000000001b75478 c000000001b75478 0000000000000001
>> >> >> GPR12: 0000000000002000 c000000002030000 0000000000000000 0000000000000000
>> >> >> GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000002030000
>> >> >> GPR20: 0000000000000000 0000000000010000 0000000000010000 c000000001defc10
>> >> >> GPR24: c000000001defc08 c000000001c91868 c000000001defc18 c000000001c91890
>> >> >> GPR28: 0000000000000000 ffffffffffffffff 0000000004000000 00000000ffffffff
>> >> >> NIP [c000000000442f38] memblock_find_in_range_node+0x328/0x340
>> >> >> LR [c000000000442f34] memblock_find_in_range_node+0x324/0x340
>> >> >> Call Trace:
>> >> >> [c000000001defba0] [c000000000442f34] memblock_find_in_range_node+0x324/0x340 (unreliable)
>> >> >> [c000000001defc90] [c0000000015ac088] memblock_alloc_range_nid+0xec/0x1b0
>> >> >> [c000000001defd40] [c0000000015ac1f8] memblock_alloc_internal+0xac/0x110
>> >> >> [c000000001defda0] [c0000000015ac4d0] memblock_alloc_try_nid+0x94/0xcc
>> >> >> [c000000001defe30] [c00000000159c3c8] swiotlb_init+0x78/0x104
>> >> >> [c000000001defea0] [c00000000158378c] mem_init+0x4c/0x98
>> >> >> [c000000001defec0] [c00000000157457c] start_kernel+0x714/0xac8
>> >> >> [c000000001deff90] [c00000000000d244] start_here_common+0x1c/0x58
>> >> >> Instruction dump:
>> >> >> 2c230000 4182ffd4 ea610088 ea810090 4bfffe84 39200001 3d42fff4 3c62ff60
>> >> >> 3863c560 992a8bfc 4bd0881d 60000000 <0fe00000> ea610088 4bfffd94 60000000
>> >> >> random: get_random_bytes called from __warn+0x128/0x184 with crng_init=0
>> >> >> ---[ end trace 0000000000000000 ]---
>> >> >> software IO TLB: Cannot allocate buffer
>> >> >>
>> >> >> Unless this is a secure VM the message can actually be ignored, because the
>> >> >> SWIOTLB isn't needed. Therefore, let's avoid the SWIOTLB in those cases.
>> >> >
>> >> > The above warn_on is conveying a genuine warning. Should it be silenced?
>> >>
>> >> Not sure I understand your point. This patch doesn't silence the
>> >> warning, it avoids the problem it is warning about.
>> >
>> > Sorry, I should have explained it better. My point is...
>> >
>> > If CONFIG_SWIOTLB is enabled, it means that the kernel is
>> > promising the bounce buffering capability. I know, currently we
>> > do not have any kernel subsystems that use bounce buffers on
>> > non-secure-pseries-kernel or powernv-kernel. But that does not
>> > mean, there wont be any. In case there is such a third-party
>> > module needing bounce buffering, it wont be able to operate,
>> > because of the proposed change in your patch.
>> >
>> > Is that a good thing or a bad thing, I do not know. I will let
>> > the experts opine.
>>
>> Ping? Does anyone else has an opinion on this? The other option I can
>> think of is changing the crashkernel code to not reserve so much memory
>> below 4 GB. Other people are considering this option, but it's not
>> planned for the near future.
>
> That seems a more suitable solution regardless, but there is always
> the danger of not being enough or being too big.
>
> There was some autocrashkernel allocation patches going around
> for x86 and ARM that perhaps could be re-used?
Do you mean this?
https://lore.kernel.org/lkml/20201118232431.21832-1-saeed.mirzamohammadi@oracle.com/
I agree it would be great to have crashkernel=auto upstream.
>> Also, there's a patch currently in linux-next which removes the scary
>> warning because of unrelated reasons:
>>
>> https://lore.kernel.org/lkml/20201217201214.3414100-2-guro@fb.com
>>
>> So assuming that the patch above goes in and keeping the assumption that
>> the swiotlb won't be needed in the powernv machines where I've seen the
>> warning happen, we can just leave things as they are now.
>
> If that solves the problem, then that is OK.
Awesome! Today Mike Rapoport mentioned that the patch above uncovers a
few issues with late memory reservations on on x86 that he is fixing:
https://lore.kernel.org/lkml/20210115083255.12744-1-rppt@kernel.org/
So that would be the last piece of the puzzle.
--
Thiago Jung Bauermann
IBM Linux Technology Center
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