Re: [PATCH 0/3] mm/page_alloc: Fix pageblock_order with HUGETLB_PAGE_SIZE_VARIABLE

[David Hildenbrand]

On 11.02.21 07:22, Anshuman Khandual wrote:
> The following warning gets triggered while trying to boot a 64K page size
> without THP config kernel on arm64 platform.
>
> WARNING: CPU: 5 PID: 124 at mm/vmstat.c:1080 __fragmentation_index+0xa4/0xc0
> Modules linked in:
> CPU: 5 PID: 124 Comm: kswapd0 Not tainted 5.11.0-rc6-00004-ga0ea7d62002 #159
> Hardware name: linux,dummy-virt (DT)
> [    8.810673] pstate: 20400005 (nzCv daif +PAN -UAO -TCO BTYPE=--)
> [    8.811732] pc : __fragmentation_index+0xa4/0xc0
> [    8.812555] lr : fragmentation_index+0xf8/0x138
> [    8.813360] sp : ffff0000864079b0
> [    8.813958] x29: ffff0000864079b0 x28: 0000000000000372
> [    8.814901] x27: 0000000000007682 x26: ffff8000135b3948
> [    8.815847] x25: 1fffe00010c80f48 x24: 0000000000000000
> [    8.816805] x23: 0000000000000000 x22: 000000000000000d
> [    8.817764] x21: 0000000000000030 x20: ffff0005ffcb4d58
> [    8.818712] x19: 000000000000000b x18: 0000000000000000
> [    8.819656] x17: 0000000000000000 x16: 0000000000000000
> [    8.820613] x15: 0000000000000000 x14: ffff8000114c6258
> [    8.821560] x13: ffff6000bff969ba x12: 1fffe000bff969b9
> [    8.822514] x11: 1fffe000bff969b9 x10: ffff6000bff969b9
> [    8.823461] x9 : dfff800000000000 x8 : ffff0005ffcb4dcf
> [    8.824415] x7 : 0000000000000001 x6 : 0000000041b58ab3
> [    8.825359] x5 : ffff600010c80f48 x4 : dfff800000000000
> [    8.826313] x3 : ffff8000102be670 x2 : 0000000000000007
> [    8.827259] x1 : ffff000086407a60 x0 : 000000000000000d
> [    8.828218] Call trace:
> [    8.828667]  __fragmentation_index+0xa4/0xc0
> [    8.829436]  fragmentation_index+0xf8/0x138
> [    8.830194]  compaction_suitable+0x98/0xb8
> [    8.830934]  wakeup_kcompactd+0xdc/0x128
> [    8.831640]  balance_pgdat+0x71c/0x7a0
> [    8.832327]  kswapd+0x31c/0x520
> [    8.832902]  kthread+0x224/0x230
> [    8.833491]  ret_from_fork+0x10/0x30
> [    8.834150] ---[ end trace 472836f79c15516b ]---
>
> This warning comes from __fragmentation_index() when the requested order
> is greater than MAX_ORDER.
>
> static int __fragmentation_index(unsigned int order,
> 				 struct contig_page_info *info)
> {
>          unsigned long requested = 1UL << order;
>
>          if (WARN_ON_ONCE(order >= MAX_ORDER)) <===== Triggered here
>                  return 0;
>
> Digging it further reveals that pageblock_order has been assigned a value
> which is greater than MAX_ORDER failing the above check. But why this
> happened ? Because HUGETLB_PAGE_ORDER for the given config on arm64 is
> greater than MAX_ORDER.
>
> The solution involves enabling HUGETLB_PAGE_SIZE_VARIABLE which would make
> pageblock_order a variable instead of constant HUGETLB_PAGE_ORDER. But that
> change alone also did not really work as pageblock_order still got assigned
> as HUGETLB_PAGE_ORDER in set_pageblock_order(). HUGETLB_PAGE_ORDER needs to
> be less than MAX_ORDER for its appropriateness as pageblock_order otherwise
> just fallback to MAX_ORDER - 1 as before. While here it also fixes a build
> problem via type casting MAX_ORDER in rmem_cma_setup().

I'm wondering, is there any real value in allowing FORCE_MAX_ZONEORDER 
to be "11" with ARM64_64K_PAGES/ARM64_16K_PAGES?

Meaning: are there any real use cases that actually build a kernel 
without TRANSPARENT_HUGEPAGE and with ARM64_64K_PAGES/ARM64_16K_PAGES?

As builds are essentially broken, I assume this is not that relevant? Or 
how long has it been broken?

It might be easier to just drop the "TRANSPARENT_HUGEPAGE" part from the 
FORCE_MAX_ZONEORDER config.

--
Thanks,

David / dhildenb