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

[Muchun Song]

On Tue, Apr 5, 2022 at 11:34 AM Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:
>
>
>
> On 4/4/22 17:31, Muchun Song wrote:
> > On Mon, Apr 4, 2022 at 5:25 PM Anshuman Khandual
> > <anshuman.khandual@xxxxxxx> 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 ?

It's unfortunate, we're deadlocked and will have to try again later :-(

>
> > 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.

Yep.  I'm working on this [1] and will cc you in the next version.

[1] https://lore.kernel.org/all/20220330153745.20465-1-songmuchun@xxxxxxxxxxxxx/

Thanks.