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

[Barry Song <21cnbao@gmail.com>]

On Fri, Nov 1, 2024 at 3:20 AM Maíra Canal <mcanal@igalia.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@igalia.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.

>
> Best Regards,
> - Maíra
>

Thanks
Barry