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 13:10, David Hildenbrand wrote:
> On 08.05.24 14:02, David Hildenbrand wrote:
>> On 08.05.24 11:02, 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).
>>
>> Note that with shmem you're inherently facing the same memory waste
>> issues etc as you would with anonymous memory. (sometimes even worse, if
>> you're running shmem that's configured to be unswappable!).
> 
> Also noting that memory waste is not really a problem when a write to a shmem
> file allocates a large folio that stays within boundaries of that write; issues
> only pop up if you end up over-allocating, especially, during page faults where
> you have not that much clue about what to do (single address, no real range
> provided).
> 
> There is the other issue that wasting large chunks of contiguous memory on stuff
> that barely benefits from it. With memory that maybe never gets evicted, there
> is no automatic "handing back" of that memory to the system to be used by
> something else. With ordinary files, that's a bit different. But I did not look
> closer into that issue yet, it's one of the reasons MADV_HUGEPAGE was added IIRC.

OK understood. Although, with tmpfs you're not going to mmap it then randomly
extend the file through page faults - mmap doesn't permit that, I don't think?
So presumably the user must explicitly set the size of the file first? Are you
suggesting there are a lot of use cases where a large tmpfs file is created,
mmaped then only accessed sparsely?