Re: [PATCH 5/8] mm: shmem: add multi-size THP sysfs interface for anonymous shmem

[Ryan Roberts <ryan.roberts@arm.com>]

On 08/05/2024 10:56, Baolin Wang wrote:
> 
> 
> On 2024/5/8 17:02, Ryan Roberts wrote:
>> On 08/05/2024 08:12, David Hildenbrand wrote:
>>> On 08.05.24 09:08, David Hildenbrand wrote:
>>>> On 08.05.24 06:45, Baolin Wang wrote:
>>>>>
>>>>>
>>>>> On 2024/5/7 18:52, Ryan Roberts wrote:
>>>>>> On 06/05/2024 09:46, Baolin Wang wrote:
>>>>>>> To support the use of mTHP with anonymous shmem, add a new sysfs interface
>>>>>>> 'shmem_enabled' in the '/sys/kernel/mm/transparent_hugepage/hugepages-kB/'
>>>>>>> directory for each mTHP to control whether shmem is enabled for that mTHP,
>>>>>>> with a value similar to the top level 'shmem_enabled', which can be set to:
>>>>>>> "always", "inherit (to inherit the top level setting)", "within_size",
>>>>>>> "advise",
>>>>>>> "never", "deny", "force". These values follow the same semantics as the top
>>>>>>> level, except the 'deny' is equivalent to 'never', and 'force' is equivalent
>>>>>>> to 'always' to keep compatibility.
>>>>>>
>>>>>> We decided at [1] to not allow 'force' for non-PMD-sizes.
>>>>>>
>>>>>> [1]
>>>>>> https://lore.kernel.org/linux-mm/533f37e9-81bf-4fa2-9b72-12cdcb1edb3f@redhat.com/
>>>>>>
>>>>>> However, thinking about this a bit more, I wonder if the decision we made to
>>>>>> allow all hugepages-xxkB/enabled controls to take "inherit" was the wrong
>>>>>> one.
>>>>>> Perhaps we should have only allowed the PMD-sized enable=inherit (this is
>>>>>> just
>>>>>> for legacy back compat after all, I don't think there is any use case where
>>>>>> changing multiple mTHP size controls atomically is actually useful). Applying
>>>>>
>>>>> Agree. This is also our usage of 'inherit'.
>>>
>>> Missed that one: there might be use cases in the future once we would start
>>> defaulting to "inherit" for all knobs (a distro might default to that) and
>>> default-enable THP in the global knob. Then, it would be easy to disable any THP
>>> by disabling the global knob. (I think that's the future we're heading to, where
>>> we'd have an "auto" mode that can be set on the global toggle).
>>>
>>> But I am just making up use cases ;) I think it will be valuable and just doing
>>> it consistently now might be cleaner.
>>
>> I agree that consistency between enabled and shmem_enabled is top priority. And
>> yes, I had forgotten about the glorious "auto" future. So probably continuing
>> all sizes to select "inherit" is best.
>>
>> But for shmem_enabled, that means we need the following error checking:
>>
>>   - It is an error to set "force" for any size except PMD-size
>>
>>   - It is an error to set "force" for the global control if any size except PMD-
>>     size is set to "inherit"
>>
>>   - It is an error to set "inherit" for any size except PMD-size if the global
>>     control is set to "force".
>>
>> Certainly not too difficult to code and prove to be correct, but not the nicest
>> UX from the user's point of view when they start seeing errors.
>>
>> I think we previously said this would likely be temporary, and if/when tmpfs
>> gets mTHP support, we could simplify and allow all sizes to be set to "force".
>> But I wonder if tmpfs would ever need explicit mTHP control? Maybe it would be
>> more suited to the approach the page cache takes to transparently ramp up the
>> folio size as it faults more in. (Just saying there is a chance that this error
>> checking becomes permanent).
> 
> The strategy for tmpfs supporting mTHP will require more discussions and
> evaluations in the future. However, regardless of the strategy (explicit mTHP
> control or page cache control), I think it would be possible to use 'force' to
> override previous strategies for some testing purposes. This appears to be
> permissible according to the explanation in the current documentation: "force
> the huge option on for all - very useful for testing". So it seems not permanent?

Yeah ok, makes sense to me.