Re: [PATCH 1/3] mm: vmalloc: Let user to control huge vmalloc default behavior

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2021/12/27 11:19, Matthew Wilcox wrote:
> On Mon, Dec 27, 2021 at 09:44:24AM +0800, Kefeng Wang wrote:
>> On 2021/12/27 1:36, Christophe Leroy wrote:
>>> Le 26/12/2021 à 09:39, Kefeng Wang a écrit :
>>>> Add HUGE_VMALLOC_DEFAULT_ENABLED to let user to choose whether or
>>>> not enable huge vmalloc mappings by default, and this could make
>>>> more architectures to enable huge vmalloc mappings feature but
>>>> don't want to enable it by default.
>>>>
>>>> Add hugevmalloc=on/off parameter to enable or disable this feature
>>>> at boot time, nohugevmalloc is still supported and equivalent to
>>>> hugevmalloc=off.
>>> Is there a real added value to have the user be able to select that ?
>>>
>>> If the architecture supports it, is there any good reason to not use it ?
>> There are some disadvantages[1],  one of the main concerns is the possible
>>
>> memory waste, we have backported this feature to our kernel 5.10, but our
>>
>> downstream in our some scenario(especially in embedded), they don't want
>>
>> it enabled by default, and others want it, this is why patch1 comes.
>>
>>> Why not just do like PPC and enable it by default ? Why should it be
>>> enabled by default on PPC but disabled by default on ARM64 and X86 ?
>> The PPC is default enabled, we don't changes this behavior.
>>
>> Maybe upstream is not care about this, as I said in cover-letter, if
>> arm64/x86
>>
>> don't want patch1, we could only just select config to enable it.
>>
>> Let's wait for more feedback.
> We should not have different defaults by architecture.  Either we change
> the default for PPC, or x86 & arm should have the same default as PPC.

Ok, since HUGE_VMALLOC_DEFAULT_ENABLED is introduced, we could make it

default y, not only select it on PPC, then the ppc/arm64/x86 have same 
default value.

And if someone don't want it, they could not enable this config.

Meanwhile hugevmalloc=on/off to make this feature to enable/disable at 
boot time.

I will add some explanation and resend it, thanks.

> .