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

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Message-ID: <ec69b1ee-89f4-47d2-a223-f277b7aad76b@redhat.com>
Date: Thu, 31 Oct 2024 22:39:37 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, Maíra Canal
 <mcanal@...lia.com>
Cc: Jonathan Corbet <corbet@....net>,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.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 v3 3/4] mm: shmem: override mTHP shmem default with a
 kernel parameter

On 31.10.24 22:12, Barry Song wrote:
> On Fri, Nov 1, 2024 at 3:20 AM Maíra Canal <mcanal@...lia.com> wrote:
>>
>> On 31/10/24 10:33, David Hildenbrand wrote:
>>> On 31.10.24 14:24, Maíra Canal wrote:
>>>> 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.
>>>
>>> Well ... sure, what's the problem with that?
>>
>> David & Barry, how do you feel about adding `get_order_from_str()` to
>> lib/cmdline.c?
> 
> I'd vote to leave it as is. If, at some point, the controls for shared memory
> and anonymous memory are moved to a file, that would be the right time
> to call the same get_order_from_str() for both.
 > > This is too trivial to warrant being an exposed API in huge_memory.h
> or cmdline.

I ... don't quite agree. cmdline.c is probably a bit excessive and I 
wouldn't suggest that at this point.

This seems like a reasonable helper function to have as inline in 
mm/internal.h.

... unless I am missing something important and the obvious code 
duplication is warranted.

-- 
Cheers,

David / dhildenb