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

[Ryan Roberts]

On 31/10/2023 18:29, Yang Shi wrote:
> On Tue, Oct 31, 2023 at 4:55 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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 enabled="never" is for anonymous VMAs, DAX VMAs are typically file VMAs.

That's not quite true; enabled="never" is honoured for non-DAX/non-shmem file
VMAs (for collapse via CONFIG_READ_ONLY_THP_FOR_FS and more recently for
anything that implements huge_fault() - see
7a81751fcdeb833acc858e59082688e3020bfe12).