Re: [PATCH v6 0/9] variable-order, large folios for anonymous memory

[David Hildenbrand <david@redhat.com>]

On 31.10.23 12:55, Ryan Roberts wrote:
> On 31/10/2023 11:50, Ryan Roberts wrote:
>> On 06/10/2023 21:06, David Hildenbrand wrote:
>> [...]
>>>
>>> Change 2: sysfs interface.
>>>
>>> If we call it THP, it shall go under "/sys/kernel/mm/transparent_hugepage/", I
>>> agree.
>>>
>>> What we expose there and how, is TBD. Again, not a friend of "orders" and
>>> bitmaps at all. We can do better if we want to go down that path.
>>>
>>> Maybe we should take a look at hugetlb, and how they added support for multiple
>>> sizes. What *might* make sense could be (depending on which values we actually
>>> support!)
>>>
>>>
>>> /sys/kernel/mm/transparent_hugepage/hugepages-64kB/
>>> /sys/kernel/mm/transparent_hugepage/hugepages-128kB/
>>> /sys/kernel/mm/transparent_hugepage/hugepages-256kB/
>>> /sys/kernel/mm/transparent_hugepage/hugepages-512kB/
>>> /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/
>>> /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/
>>>
>>> Each one would contain an "enabled" and "defrag" file. We want something minimal
>>> first? Start with the "enabled" option.
>>>
>>>
>>> enabled: always [global] madvise never
>>>
>>> Initially, we would set it for PMD-sized THP to "global" and for everything else
>>> to "never".
>>
>> Hi David,
>>
>> I've just started coding this, and it occurs to me that I might need a small
>> clarification here; the existing global "enabled" control is used to drive
>> decisions for both anonymous memory and (non-shmem) file-backed memory. But the
>> proposed new per-size "enabled" is implicitly only controlling anon memory (for
>> now).
>>
>> 1) Is this potentially confusing for the user? Should we rename the per-size
>> controls to "anon_enabled"? Or is it preferable to jsut keep it vague for now so
>> we can reuse the same control for file-backed memory in future?
>>
>> 2) The global control will continue to drive the file-backed memory decision
>> (for now), even when hugepages-2048kB/enabled != "global"; agreed?
>>
>> Thanks,
>> Ryan
>>
> 
> Also, an implementation question:
> 
> hugepage_vma_check() doesn't currently care whether enabled="never" for DAX VMAs
> (although it does honour MADV_NOHUGEPAGE and the prctl); It will return true
> regardless. Is that by design? It couldn't fathom any reasoning from the commit log:

The whole DAX "hugepage" and THP mixup is just plain confusing. We're 
simply using PUD/PMD mappings of DAX memory, and PMD/PTE- remap when 
required (VMA split I assume, COW).

It doesn't result in any memory waste, so who really cares how it's 
mapped? Apparently we want individual processes to just disable PMD/PUD 
mappings of DAX using the prctl and madvise. Maybe there are good reasons.

Looks like a design decision, probably some legacy leftovers.

-- 
Cheers,

David / dhildenb