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 13:45, David Hildenbrand wrote:
> On 08.05.24 14:43, Ryan Roberts wrote:
>> 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?
> 
> I don't know about "a lot of use cases", but for VMs that's certainly how it's
> used.

Gottya, thanks. And out of curiosity, what's the benefit of using tmpfs rather
than private (or shared) anonymous memory for VMs?