Re: mm: CMA reservations require 32MiB alignment in 16KiB page size kernels instead of 8MiB in 4KiB page size kernel.

[David Hildenbrand <david@redhat.com>]

On 22.01.25 03:24, Zi Yan wrote:
> On Tue Jan 21, 2025 at 9:08 PM EST, Juan Yescas wrote:
>> On Mon, Jan 20, 2025 at 9:59 AM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 20.01.25 16:29, Zi Yan wrote:
>>>> On Mon Jan 20, 2025 at 3:14 AM EST, David Hildenbrand wrote:
>>>>> On 20.01.25 01:39, Zi Yan wrote:
>>>>>> On Sun Jan 19, 2025 at 6:55 PM EST, Barry Song wrote:
>>>>>> <snip>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> However, with this workaround, we can't use transparent huge pages.
>>>>>>>>>>>
>>>>>>>>>>> Is the CMA_MIN_ALIGNMENT_BYTES requirement alignment only to support huge pages?
>>>>>>>> No. CMA_MIN_ALIGNMENT_BYTES is limited by CMA_MIN_ALIGNMENT_PAGES, which
>>>>>>>> is equal to pageblock size. Enabling THP just bumps the pageblock size.
>>>>>>>
>>
>> Thanks, I can see the initialization in include/linux/pageblock-flags.h
>>
>> #define pageblock_order MIN_T(unsigned int, HUGETLB_PAGE_ORDER, MAX_PAGE_ORDER)
>>
>>>>>>> Currently, THP might be mTHP, which can have a significantly smaller
>>>>>>> size than 32MB. For
>>>>>>> example, on arm64 systems with a 16KiB page size, a 2MB CONT-PTE mTHP
>>>>>>> is possible.
>>>>>>> Additionally, mTHP relies on the CONFIG_TRANSPARENT_HUGEPAGE configuration.
>>>>>>>
>>>>>>> I wonder if it's possible to enable CONFIG_TRANSPARENT_HUGEPAGE
>>>>>>> without necessarily
>>>>>>> using 32MiB THP. If we use other sizes, such as 64KiB, perhaps a large
>>>>>>> pageblock size wouldn't
>>>>>>> be necessary?
>>
>> Do you mean with mTHP? We haven't explored that option.
> 
> Yes. Unless your applications have special demands for PMD THPs. 2MB
> mTHP should work.
> 
>>
>>>>>>
>>>>>> I think this should work by reducing MAX_PAGE_ORDER like Juan did for
>>>>>> the experiment. But MAX_PAGE_ORDER is a macro right now, Kconfig needs
>>>>>> to be changed and kernel needs to be recompiled. Not sure if it is OK
>>>>>> for Juan's use case.
>>>>>
>>
>> The main goal is to reserve only the necessary CMA memory for the
>> drivers, which is
>> usually the same for 4kb and 16kb page size kernels.
> 
> Got it. Based on your experiment, you changed MAX_PAGE_ORDER to get the
> minimal CMA alignment size. Can you deploy that kernel to production?
> If yes, you can use mTHP instead of PMD THP and still get the CMA
> alignemnt you want.
> 
>>
>>>>>
>>>>> IIRC, we set pageblock size == THP size because this is the granularity
>>>>> we want to optimize defragmentation for. ("try keep pageblock
>>>>> granularity of the same memory type: movable vs. unmovable")
>>>>
>>>> Right. In past, it is optimized for PMD THP. Now we have mTHP. If user
>>>> does not care about PMD THP (32MB in ARM64 16KB base page case) and mTHP
>>>> (2MB mTHP here) is good enough, reducing pageblock size works.
>>>>
>>>>>
>>>>> However, the buddy already supports having different pagetypes for large
>>>>> allocations.
>>>>
>>>> Right. To be clear, only MIGRATE_UNMOVABLE, MIGRATE_RECLAIMABLE, and
>>>> MIGRATE_MOVABLE can be merged.
>>>
>>> Yes! An a THP cannot span partial MIGRATE_CMA, which would be fine.
>>>
>>>>
>>>>>
>>>>> So we could leave MAX_ORDER alone and try adjusting the pageblock size
>>>>> in these setups. pageblock size is already variable on some
>>>>> architectures IIRC.
>>>>
>>
>> Which values would work for the CMA_MIN_ALIGNMENT_BYTES macro? In the
>> 16KiB page size kernel,
>> I tried these 2 configurations:
>>
>> #define CMA_MIN_ALIGNMENT_BYTES (2048 * CMA_MIN_ALIGNMENT_PAGES)
>>
>> and
>>
>> #define CMA_MIN_ALIGNMENT_BYTES (4096 * CMA_MIN_ALIGNMENT_PAGES)
>>
>> with both of them, the kernel failed to boot.
> 
> CMA_MIN_ALIGNMENT_BYTES needs to be PAGE_SIZE * CMA_MIN_ALIGNMENT_PAGES.
> So you need to adjust CMA_MIN_ALIGNMENT_PAGES, which is set by pageblock
> size. pageblock size is determined by pageblock order, which is
> affected by MAX_PAGE_ORDER.

Yes, most importantly we must not exceed MAX_PAGE_ORDER. Going smaller 
is the common case.

> 
>>
>>>> Making pageblock size a boot time variable? We might want to warn
>>>> sysadmin/user that >pageblock_order THP/mTHP creation will suffer.
>>>
>>> Yes, some way to configure it.
>>>
>>>>
>>>>>
>>>>> We'd only have to check if all of the THP logic can deal with pageblock
>>>>> size < THP size.
>>>>
>>
>> The reason that THP was disabled in my experiment is because this
>> assertion failed
>>
>> mm/huge_memory.c
>> /*
>> * hugepages can't be allocated by the buddy allocator
>> */
>> MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER > MAX_PAGE_ORDER);
>>
>> when
>>
>>      config ARCH_FORCE_MAX_ORDER
>>          int
>>          .....
>>          default "8" if ARM64_16K_PAGES
>>
> 
> You can remove that BUILD_BUG_ON and turn on mTHP and see if mTHP works.
> 
>>
>>>> Probably yes, pageblock should be independent of THP logic, although
>>>> compaction (used to create THPs) logic is based on pageblock.
>>>
>>> Right. As raised in the past, we need a higher level mechanism that
>>> tries to group pageblocks together during comapction/conversion to limit
>>> fragmentation on a higher level.
>>>
>>> I assume that many use cases would be fine with not using 32MB/512MB
>>> THPs at all for now -- and instead using 2 MB ones. Of course, for very
>>> large installations it might be different.
>>>
>>>>>
>>>>> This issue is even more severe on arm64 with 64k (pageblock = 512MiB).
>>>>
>>
>> I agree, and if ARCH_FORCE_MAX_ORDER is configured to the max value we get:
>>
>> PAGE_SIZE  | max MAX_PAGE_ORDER | CMA_MIN_ALIGNMENT_BYTES
>> 4KiB              |                      15                   |  4KiB
>> * 32KiB = 128MiB
>> 16KiB            |                      13                   |  16KiB
>> * 8KiB = 128MiB
>> 64KiB            |                      13                   |  64KiB
>> * 8KiB = 512MiB
>>
>>>> This is also good for virtio-mem, since the offline memory block size
>>>> can also be reduced. I remember you complained about it before.
>>>
>>> Yes, yes, yes! :)
>>>
> 
> David's proposal should work in general, but will might take non-trivial
> amount of work:
> 
> 1. keep pageblock size always at 4MB for all arch.

My proposal was to leave it unchanged for most archs, but allow for 
overriding it on aarch64 as a first step.

s390x is happy with 1MiB, x86 with 2MiB. It's aarch64 that does 
questionable things :)

CONFIG_HUGETLB_PAGE_SIZE_VARIABLE already allows for variable 
pageblock_order. That whole code likely needs some love, but most of it 
should already be there.


In the future, I could imagine just going for a smaller pageblock size 
on aarch64, and handling fragmentation avoidance for larger THPs (512 
MiB really is close to 1 GiB on x86) differently, not using pageblocks.

-- 
Cheers,

David / dhildenb