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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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?