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

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Date: Wed, 8 May 2024 09:12:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....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 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.

-- 
Cheers,

David / dhildenb