Re: [PATCH v4 2/2] arm64: mm: hugetlb: Enable HUGETLB_PAGE_FREE_VMEMMAP for arm64

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/4/22 17:31, Muchun Song wrote:
> On Mon, Apr 4, 2022 at 5:25 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>> Hello Muchun,
>>
>> On 3/31/22 12:26, Muchun Song wrote:
>>> The feature of minimizing overhead of struct page associated with each
>>> HugeTLB page aims to free its vmemmap pages (used as struct page) to
>>> save memory, where is ~14GB/16GB per 1TB HugeTLB pages (2MB/1GB type).
>>
>> Enabling this feature saves us around 1.4/1.6 % memory but looking from
>> other way around, unavailability of vmemmap backing pages (~1.4GB) when
>> freeing up a corresponding HugeTLB page, could prevent ~1TB memory from
>> being used as normal page form (requiring their own struct pages), thus
>> forcing the HugeTLB page to remain as such ? Is not this problematic ?
>>
>> These additional 1TB memory in normal pages, from a HugeTLB dissolution
>> could have eased the system's memory pressure without this feature being
>> enabled.
> 
> You are right. If the system is already under heavy memory pressure, it could
> prevent the user from freeing HugeTLB pages to the buddy allocator. If the
> HugeTLB page are allocated from non-movable zone, this scenario may be
> not problematic since once a HugeTLB page is freed, then the system will

But how can even the first HugeTLB page be freed without vmemmmap which is
throttled due to lack of sufficient memory ?

> have memory to be allocated to be used as vmemmap pages, subsequent
> freeing of HugeTLB pages may be getting easier.  However, if the HUgeTLB
> pages are allocated from the movable zone, then the thing becomes terrible,
> which is documented in Documentation/admin-guide/mm/memory-hotplug.rst.
> 
> So there is a cmdline "hugetlb_free_vmemmap" to control if enabling this
> feature.  The user should enable/disable this depending on their workload.

Should there also be a sysfs interface for this knob as well ? Perhaps the
system usage might change on the way, without requiring a reboot.