linux-kernel - Re: [PATCH 1/3] mm: fix the format of the kernel parameter ``thp

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Message-ID: <9126583d-5865-4dcb-afe5-e846cccbea4c@igalia.com>
Date: Sun, 27 Oct 2024 17:36:19 -0300
From: Maíra Canal <mcanal@...lia.com>
To: Barry Song <baohua@...nel.org>
Cc: Jonathan Corbet <corbet@....net>,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
 David Hildenbrand <david@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Lance Yang
 <ioworker0@...il.com>, linux-mm@...ck.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-dev@...lia.com
Subject: Re: [PATCH 1/3] mm: fix the format of the kernel parameter
 ``thp_anon=``

Hi Barry,

On 27/10/24 16:52, Barry Song wrote:
> On Mon, Oct 28, 2024 at 1:58 AM Maíra Canal <mcanal@...lia.com> wrote:
>>
>> If we add ``thp_anon=32,64KB:always`` to the kernel command line, we
>> will see the following error:
>>
>> [    0.000000] huge_memory: thp_anon=32,64K:always: error parsing string, ignoring setting
>>
>> This happens because the correct format isn't ``thp_anon=<size>,<size>[KMG]:<state>```,
>> as [KMG] must follow each number to especify its unit. So, the correct
>> format is ``thp_anon=<size>[KMG],<size>[KMG]:<state>```.
> 
> what if 32768,64K: always?

``32768,64K:always`` works. From the kernel parameters documentation, I
see that:

"Finally, the [KMG] suffix is commonly described after a number of
kernel parameter values. These ‘K’, ‘M’, and ‘G’ letters represent the
_binary_ multipliers ‘Kilo’, ‘Mega’, and ‘Giga’, equaling 2^10, 2^20,
and 2^30 bytes respectively. Such letter suffixes can also be entirely
omitted"

AFAIU this means that [KMG] can be omitted if we use bytes. But if we
don't use bytes, it cannot be omitted.

Best Regards,
- Maíra

> 
>>
>> Therefore, adjust the documentation to reflect the correct format of the
>> parameter ``thp_anon=``.
>>
>> Fixes: dd4d30d1cdbe ("mm: override mTHP "enabled" defaults at kernel cmdline")
>> Signed-off-by: Maíra Canal <mcanal@...lia.com>
>> ---
>>   Documentation/admin-guide/kernel-parameters.txt | 2 +-
>>   Documentation/admin-guide/mm/transhuge.rst      | 2 +-
>>   2 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 1518343bbe22..1666576acc0e 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -6688,7 +6688,7 @@
>>                          0: no polling (default)
>>
>>          thp_anon=       [KNL]
>> -                       Format: <size>,<size>[KMG]:<state>;<size>-<size>[KMG]:<state>
>> +                       Format: <size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>
>>                          state is one of "always", "madvise", "never" or "inherit".
>>                          Control the default behavior of the system with respect
>>                          to anonymous transparent hugepages.
>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>> index 203ba7aaf5fc..745055c3dc09 100644
>> --- a/Documentation/admin-guide/mm/transhuge.rst
>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>> @@ -303,7 +303,7 @@ control by passing the parameter ``transparent_hugepage=always`` or
>>   kernel command line.
>>
>>   Alternatively, each supported anonymous THP size can be controlled by
>> -passing ``thp_anon=<size>,<size>[KMG]:<state>;<size>-<size>[KMG]:<state>``,
>> +passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``,
>>   where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and
>>   supported anonymous THP)  and ``<state>`` is one of ``always``, ``madvise``,
>>   ``never`` or ``inherit``.
>> --
>> 2.46.2
>>