Re: [RFC] mm: khugepaged: use largest enabled hugepage order for min_free_kbytes

Usama Arif
2025-06-09T20:03:28Z

On 09/06/2025 20:49, Zi Yan wrote:
> On 9 Jun 2025, at 15:40, Lorenzo Stoakes wrote:
>
>> On Mon, Jun 09, 2025 at 11:20:04AM -0400, Zi Yan wrote:
>>> On 9 Jun 2025, at 10:50, Lorenzo Stoakes wrote:
>>>
>>>> On Mon, Jun 09, 2025 at 10:37:26AM -0400, Zi Yan wrote:
>>>>> On 9 Jun 2025, at 10:16, Lorenzo Stoakes wrote:
>>>>>
>>>>>> On Mon, Jun 09, 2025 at 03:11:27PM +0100, Usama Arif wrote:
>>>>
>>>> [snip]
>>>>
>>>>>>> So I guess the question is what should be the next step? The following has been discussed:
>>>>>>>
>>>>>>> - Changing pageblock_order at runtime: This seems unreasonable after Zi's explanation above
>>>>>>> and might have unintended consequences if done at runtime, so a no go?
>>>>>>> - Decouple only watermark calculation and defrag granularity from pageblock order (also from Zi).
>>>>>>> The decoupling can be done separately. Watermark calculation can be decoupled using the
>>>>>>> approach taken in this RFC. Although max order used by pagecache needs to be addressed.
>>>>>>>
>>>>>>
>>>>>> I need to catch up with the thread (workload crazy atm), but why isn't it
>>>>>> feasible to simply statically adjust the pageblock size?
>>>>>>
>>>>>> The whole point of 'defragmentation' is to _heuristically_ make it less
>>>>>> likely there'll be fragmentation when requesting page blocks.
>>>>>>
>>>>>> And the watermark code is explicitly about providing reserves at a
>>>>>> _pageblock granularity_.
>>>>>>
>>>>>> Why would we want to 'defragment' to 512MB physically contiguous chunks
>>>>>> that we rarely use?
>>>>>>
>>>>>> Since it's all heuristic, it seems reasonable to me to cap it at a sensible
>>>>>> level no?
>>>>>
>>>>> What is a sensible level? 2MB is a good starting point. If we cap pageblock
>>>>> at 2MB, everyone should be happy at the moment. But if one user wants to
>>>>> allocate 4MB mTHP, they will most likely fail miserably, because pageblock
>>>>> is 2MB, kernel is OK to have a 2MB MIGRATE_MOVABLE pageblock next to a 2MB
>>>>> MGIRATE_UNMOVABLE one, making defragmenting 4MB an impossible job.
>>>>>
>>>>> Defragmentation has two components: 1) pageblock, which has migratetypes
>>>>> to prevent mixing movable and unmovable pages, as a single unmovable page
>>>>> blocks large free pages from being created; 2) memory compaction granularity,
>>>>> which is the actual work to move pages around and form a large free pages.
>>>>> Currently, kernel assumes pageblock size = defragmentation granularity,
>>>>> but in reality, as long as pageblock size >= defragmentation granularity,
>>>>> memory compaction would still work, but not the other way around. So we
>>>>> need to choose pageblock size carefully to not break memory compaction.
>>>>
>>>> OK I get it - the issue is that compaction itself operations at a pageblock
>>>> granularity, and once you get so fragmented that compaction is critical to
>>>> defragmentation, you are stuck if the pageblock is not big enough.
>>>
>>> Right.
>>>
>>>>
>>>> Thing is, 512MB pageblock size for compaction seems insanely inefficient in
>>>> itself, and if we're complaining about issues with unavailable reserved
>>>> memory due to crazy PMD size, surely one will encounter the compaction
>>>> process simply failing to succeed/taking forever/causing issues with
>>>> reclaim/higher order folio allocation.
>>>
>>> Yep. Initially, we probably never thought PMD THP would be as large as
>>> 512MB.
>>
>> Of course, such is the 'organic' nature of kernel development :)
>>
>>>
>>>>
>>>> I mean, I don't really know the compaction code _at all_ (ran out of time
>>>> to cover in book ;), but is it all or-nothing? Does it grab a pageblock or
>>>> gives up?
>>>
>>> compaction works on one pageblock at a time, trying to migrate in-use pages
>>> within the pageblock away to create a free page for THP allocation.
>>> It assumes PMD THP size is equal to pageblock size. It will keep working
>>> until a PMD THP size free page is created. This is a very high level
>>> description, omitting a lot of details like how to avoid excessive compaction
>>> work, how to reduce compaction latency.
>>
>> Yeah this matches my assumptions.
>>
>>>
>>>>
>>>> Because it strikes me that a crazy pageblock size would cause really
>>>> serious system issues on that basis alone if that's the case.
>>>>
>>>> And again this leads me back to thinking it should just be the page block
>>>> size _as a whole_ that should be adjusted.
>>>>
>>>> Keep in mind a user can literally reduce the page block size already via
>>>> CONFIG_PAGE_BLOCK_MAX_ORDER.
>>>>
>>>> To me it seems that we should cap it at the highest _reasonable_ mTHP size
>>>> you can get on a 64KB (i.e. maximum right? RIGHT? :P) base page size
>>>> system.
>>>>
>>>> That way, people _can still get_ super huge PMD sized huge folios up to the
>>>> point of fragmentation.
>>>>
>>>> If we do reduce things this way we should give a config option to allow
>>>> users who truly want collosal PMD sizes with associated
>>>> watermarks/compaction to be able to still have it.
>>>>
>>>> CONFIG_PAGE_BLOCK_HARD_LIMIT_MB or something?
>>>
>>> I agree with capping pageblock size at a highest reasonable mTHP size.
>>> In case there is some user relying on this huge PMD THP, making
>>> pageblock a boot time variable might be a little better, since
>>> they do not need to recompile the kernel for their need, assuming
>>> distros will pick something like 2MB as the default pageblock size.
>>
>> Right, this seems sensible, as long as we set a _default_ that limits to
>> whatever it would be, 2MB or such.
>>
>> I don't think it's unreasonable to make that change since this 512 MB thing
>> is so entirely unexpected and unusual.
>>
>> I think Usama said it would be a pain it working this way if it had to be
>> explicitly set as a boot time variable without defaulting like this.
>>
>>>
>>>>
>>>> I also question this de-coupling in general (I may be missing somethig
>>>> however!) - the watermark code _very explicitly_ refers to providing
>>>> _pageblocks_ in order to ensure _defragmentation_ right?
>>>
>>> Yes. Since without enough free memory (bigger than a PMD THP),
>>> memory compaction will just do useless work.
>>
>> Yeah right, so this is a key thing and why we need to rework the current
>> state of the patch.
>>
>>>
>>>>
>>>> We would need to absolutely justify why it's suddenly ok to not provide
>>>> page blocks here.
>>>>
>>>> This is very very delicate code we have to be SO careful about.
>>>>
>>>> This is why I am being cautious here :)
>>>
>>> Understood. In theory, we can associate watermarks with THP allowed orders
>>> the other way around too, meaning if user lowers vm.min_free_kbytes,
>>> all THP/mTHP sizes bigger than the watermark threshold are disabled
>>> automatically. This could fix the memory compaction issues, but
>>> that might also drive user crazy as they cannot use the THP sizes
>>> they want.
>>
>> Yeah that's interesting but I think that's just far too subtle and people will
>> have no idea what's going on.
>>
>> I really think a hard cap, expressed in KB/MB, on pageblock size is the way to
>> go (but overrideable for people crazy enough to truly want 512 MB pages - and
>> who cannot then complain about watermarks).
>
> I agree. Basically, I am thinking:
> 1) use something like 2MB as default pageblock size for all arch (the value can
> be set differently if some arch wants a different pageblock size due to other reasons), this can be done by modifying PAGE_BLOCK_MAX_ORDER’s default
> value;
>
> 2) make pageblock_order a boot time parameter, so that user who wants
> 512MB pages can still get it by changing pageblock order at boot time.
>
> WDYT?
>

I was really hoping we would come up with a dynamic way of doing this,
especially one that doesn't require any more input from the user apart
from just setting the mTHP size via sysfs..

1) in a way is already done. We can set it to 2M by setting
ARCH_FORCE_MAX_ORDER to 5:

In arch/arm64/Kconfig we already have:

config ARCH_FORCE_MAX_ORDER
int
default "13" if ARM64_64K_PAGES
default "11" if ARM64_16K_PAGES
default "10"

Doing 2) would require reboot and doing this just for changing mTHP size
will probably be a nightmare for workload orchestration.