On 2/12/21 3:09 PM, David Hildenbrand wrote:
> On 12.02.21 08:02, Anshuman Khandual wrote:
>>
>> 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.
>
> Theoretically yes, practically no. If nobody cares about a configuration, no
> need to make the code more complicated for that configuration.
>
>>
>>>
>>> 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.
>
> When exactly did I say not to fix it? I'm saying if nobody uses it, we might
> be able to simplify.
I might have misunderstood the relevance of a problematic configuration
wrt the need for it to be fixed. My bad, never mind :)
>
>>
>>>
>>> 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.
>
> My point is: keep it simple if there is no need to make it complicated. If
> these arm64 variants are the only cases where we run into that issue and
> nobody uses them ("hat it's been there for some time now"), why make stuff
> complicated?
>
> The current code seems to assume that HUGETLB_PAGE_ORDER <= MAX_ORDER.
> Instead of changing that for optimizing an unused use case (it is broken),
> just simplify the arm64 conditions. I'd even say add a
Will try and change MAX_ORDER on arm64.
>
> /*
> * Some code assumes that HUGETLB_PAGE_ORDER <= MAX_ORDER. For now, only
> * !TRANSPARENT_HUGEPAGE could lead in archs creating such setups.
> * Forbid it for now; anybody that has a valid use case has to sanitize
> * the code.
> */
> BUILD_BUG_ON(HUGETLB_PAGE_ORDER <= MAX_ORDER);
Also add another, might have to find a proper place to put these.
BUILD_BUG_ON(HPAGE_PMD_ORDER <= MAX_ORDER)
>
>
> But again, if there are valid use cases then sure, let's make the code fully
> compatible with HUGETLB_PAGE_ORDER > MAX_ORDER.
Given that gigantic HugeTLB allocation can fallback on alloc_contig_pages()
or CMA if/when available, is there a real need for HUGETLB_PAGE_ORDER to be
upto MAX_ORDER, used as pageblock_order or as COMPACTION_HPAGE_ORDER ? With
gigantic HugeTLB pages being available, HUGETLB_PAGE_ORDER seems to be just
detached from the buddy allocator. But I am not sure, will keep looking.
>
>
> BTW: can we be sure that you caught all issues? For example:
>
> #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER
>
> I'm not sure if it will work as expected if HUGETLB_PAGE_ORDER > MAX_ORDER
> ... fill_contig_page_info() will never find any suitable free blocks ...
>
Right, there is an assumption that COMPACTION_HPAGE_ORDER < MAX_ORDER in
fill_contig_page_info() and will never find any suitable free blocks. This
along with pageblock_order needs to change when HUGETLB_PAGE_ORDER can be
greater than equal to MAX_ORDER.
- Anshuman
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu
