On Fri, Jan 08, 2021 at 09:27:01PM -0300, Thiago Jung Bauermann wrote:

Ram Pai [off-list ref] writes:

quoted

On Wed, Dec 23, 2020 at 09:06:01PM -0300, Thiago Jung Bauermann wrote:

quoted

Hi Ram,

Thanks for reviewing this patch.

Ram Pai [off-list ref] writes:

quoted

On Fri, Dec 18, 2020 at 03:21:03AM -0300, Thiago Jung Bauermann wrote:

quoted

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?

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 (local)

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.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center