linux-kernel - Re: [PATCH v3 3/4] mm: shmem: override mTHP shmem default with a kernel parameter

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Message-ID: <95c48a30-0696-4110-950e-e81afb4ffc2a@igalia.com>
Date: Thu, 31 Oct 2024 10:24:01 -0300
From: Maíra Canal <mcanal@...lia.com>
To: David Hildenbrand <david@...hat.com>, Jonathan Corbet <corbet@....net>,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
 Barry Song <baohua@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Lance Yang <ioworker0@...il.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-dev@...lia.com
Subject: Re: [PATCH v3 3/4] mm: shmem: override mTHP shmem default with a
 kernel parameter

Hi David,

On 31/10/24 09:57, David Hildenbrand wrote:
> On 31.10.24 13:51, Maíra Canal wrote:
>> Hi David,
>>
>> On 31/10/24 09:37, David Hildenbrand wrote:
>>> On 30.10.24 13:58, Maíra Canal wrote:
>>>> Add the ``thp_shmem=`` kernel command line to allow specifying the
>>>> default policy of each supported shmem hugepage size. The kernel
>>>> parameter
>>>> accepts the following format:
>>>>
>>>> thp_shmem=<size>[KMG],<size>[KMG]:<policy>;<size>[KMG]-
>>>> <size>[KMG]:<policy>
>>>>
>>>> For example,
>>>>
>>>> thp_shmem=16K-64K:always;128K,512K:inherit;256K:advise;1M-2M:never;4M-8M:within_size
>>>>
>>>> By configuring the default policy of several shmem hugepages, the user
>>>> can take advantage of mTHP before it's been configured through sysfs.
>>>>
>>>> Signed-off-by: Maíra Canal <mcanal@...lia.com>
>>>> ---
>>>>    .../admin-guide/kernel-parameters.txt         |  10 ++
>>>>    Documentation/admin-guide/mm/transhuge.rst    |  17 +++
>>>>    mm/shmem.c                                    | 109 +++++++++++++ 
>>>> ++++-
>>>>    3 files changed, 135 insertions(+), 1 deletion(-)
>>>>
>>
>> [...]
>>
>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>> index dfcc88ec6e34..c2299fa0b345 100644
>>>> --- a/mm/shmem.c
>>>> +++ b/mm/shmem.c
>>>> @@ -136,6 +136,7 @@ static unsigned long huge_shmem_orders_always
>>>> __read_mostly;
>>>>    static unsigned long huge_shmem_orders_madvise __read_mostly;
>>>>    static unsigned long huge_shmem_orders_inherit __read_mostly;
>>>>    static unsigned long huge_shmem_orders_within_size __read_mostly;
>>>> +static bool shmem_orders_configured __initdata;
>>>>    #endif
>>>>    #ifdef CONFIG_TMPFS
>>>> @@ -5027,7 +5028,8 @@ void __init shmem_init(void)
>>>>         * Default to setting PMD-sized THP to inherit the global
>>>> setting and
>>>>         * disable all other multi-size THPs.
>>>>         */
>>>> -    huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
>>>> +    if (!shmem_orders_configured)
>>>> +        huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
>>>>    #endif
>>>>        return;
>>>> @@ -5180,6 +5182,26 @@ struct kobj_attribute 
>>>> thpsize_shmem_enabled_attr =
>>>>    #if defined(CONFIG_TRANSPARENT_HUGEPAGE)
>>>> +static inline int get_order_from_str(const char *size_str)
>>>> +{
>>>> +    unsigned long size;
>>>> +    char *endptr;
>>>> +    int order;
>>>> +
>>>> +    size = memparse(size_str, &endptr);
>>>> +
>>>> +    if (!is_power_of_2(size))
>>>> +        goto err;
>>>> +    order = get_order(size);
>>>> +    if (BIT(order) & ~THP_ORDERS_ALL_FILE_DEFAULT)
>>>> +        goto err;
>>>> +
>>>> +    return order;
>>>> +err:
>>>> +    pr_err("invalid size %s in thp_shmem boot parameter\n", size_str);
>>>> +    return -EINVAL;
>>>> +}
>>>
>>> Hm, mostly copy and paste. You could reuse existing get_order_from_str()
>>> simply by passing in the supported orders and moving error reporting to
>>> the caller.
>>>
>>
>> Can I use functions from mm/huge_memory.c here?
> 
> Yes, that's the idea.
> 

Unfortunately, it isn't possible without adding the function to a
header.

I deleted `get_order_from_str()` from mm/shmem.c just to test it:

mm/shmem.c:5230:13: error: call to undeclared function 
'get_order_from_str'; ISO C99 and later do not support implicit function 
declarations [-Wimplicit-function-declaration]
  5230 |                                 start = 
get_order_from_str(start_size);
       |                                         ^
mm/shmem.c:5233:19: error: call to undeclared function 
'get_order_from_str'; ISO C99 and later do not support implicit function 
declarations [-Wimplicit-function-declaration]
  5233 |                                 start = end = 
get_order_from_str(subtoken);
       |                                               ^
2 errors generated.
make[3]: *** [scripts/Makefile.build:229: mm/shmem.o] Error 1
make[2]: *** [scripts/Makefile.build:478: mm] Error 2
make[2]: *** Waiting for unfinished jobs....

Please, check the discussion I had with Barry about the pros and cons of
copy and paste and code refactor [1][2]. We ended up deciding to
duplicate the code.

[1] 
https://lore.kernel.org/linux-mm/CAGsJ_4wJ2xoNRLMNkmLL3x1t2YiqQJ1=saXa+HNxRUbSNivRCw@mail.gmail.com/
[2] 
https://lore.kernel.org/linux-mm/CAGsJ_4yp89one_hoB_87poU2wrpJh_0NVicRKW538eE6yo1kZw@mail.gmail.com/

Best Regards,
- Maíra