Message-ID: <f7efdbf6-958c-4576-a375-8f7548f58dec@redhat.com>
Date: Wed, 8 May 2024 14:45:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, ioworker0@...il.com, wangkefeng.wang@...wei.com,
 ying.huang@...el.com, 21cnbao@...il.com, shy828301@...il.com,
 ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] mm: shmem: add multi-size THP sysfs interface for
 anonymous shmem

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.

-- 
Cheers,

David / dhildenb

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