On 2/11/21 2:07 PM, David Hildenbrand wrote:
> 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?
MAX_ORDER should be as high as would be required for the current config.
Unless THP is enabled, there is no need for it to be any higher than 11.
But I might be missing historical reasons around this as well. Probably
others from arm64 could help here.
>
> Meaning: are there any real use cases that actually build a kernel without
> TRANSPARENT_HUGEPAGE and with ARM64_64K_PAGES/ARM64_16K_PAGES?
THP is always optional. Besides kernel builds without THP should always
be supported. Assuming that all builds will have THP enabled, might not
be accurate.
>
> As builds are essentially broken, I assume this is not that relevant? Or how
> long has it been broken?
Git blame shows that it's been there for some time now. But how does
that make this irrelevant ? A problem should be fixed nonetheless.
>
> It might be easier to just drop the "TRANSPARENT_HUGEPAGE" part from the
> FORCE_MAX_ZONEORDER config.
>
Not sure if it would be a good idea to unnecessarily have larger MAX_ORDER
value for a given config. But I might be missing other contexts here.
