Message-ID: <a26caf5f-81be-4760-9724-9772d5ec339c@arm.com>
Date: Wed, 8 May 2024 14:44:36 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 08/05/2024 14:07, David Hildenbrand wrote:
> On 08.05.24 14:54, Ryan Roberts wrote:
>> 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.
>>
> 
> There are more details around that and the sparsity (memory ballooning,
> virtio-mem, free page reporting), but it might distract here :) I'll note that
> shmem+THP is known to be problematic with memory ballooning.
> 
>> Gottya, thanks. And out of curiosity, what's the benefit of using tmpfs rather
>> than private (or shared) anonymous memory for VMs?
> 
> The primary use case I know of is sharing VM memory with other processes
> (usually not child processes): DPDK/SPDK and other vhost-user variants (such as
> virtiofs) mmap() all guest memory to access it directly (some sort of
> multi-process hypervisors). They either use real-file-based shmem or memfd
> (essentially the same without a named file) for that.
> 
> Then, there is live-hypervisor upgrade, whereby you start a second hypervisor
> process that will take over. People use shmem for that, so you can minimize
> downtime by migrating guest memory simply by mmap'ing the shmem file into the
> new hypervisor.
> 
> Shared anonymous memory is basically never used (I only know of one corner case
> in QEMU).
> 
> I would assume that there are also DBs making use of rather sparse shmem? But no
> expert on that.
> 

That all makes sense - thanks for the lesson!

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