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Date:   Fri, 08 Jan 2021 21:27:01 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     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


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.

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.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center

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