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 09:08:51 +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 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'.
> 
>> that pattern here, it means the top level can always take "force" without any
>> weird error checking. And we would allow "force" on the PMD-sized control but
>> not on the others - again this is easy to error check.
>>
>> Does this pattern make more sense? If so, is it too late to change
>> hugepages-xxkB/enabled interface?
> 
> IMO, this sounds reasonable to me. Let's see what others think, David?

Likely too late and we should try not to diverge too much from "enabled" 
for "shmem_enabled".

Having that said, to me it's much cleaner to just have "inherit" for all 
sizes and disallow invalid configurations as discussed.

Error checking cannot be too hard unless I am missing something important :)

-- 
Cheers,

David / dhildenb