Re: [External] Re: [PATCH v9 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song]

On Wed, Dec 16, 2020 at 11:45 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 12/15/20 5:03 PM, Mike Kravetz wrote:
> > On 12/13/20 7:45 AM, Muchun Song wrote:
> >> diff --git a/fs/Kconfig b/fs/Kconfig
> >> index 976e8b9033c4..4c3a9c614983 100644
> >> --- a/fs/Kconfig
> >> +++ b/fs/Kconfig
> >> @@ -245,6 +245,21 @@ config HUGETLBFS
> >>  config HUGETLB_PAGE
> >>      def_bool HUGETLBFS
> >>
> >> +config HUGETLB_PAGE_FREE_VMEMMAP
> >> +    def_bool HUGETLB_PAGE
> >> +    depends on X86_64
> >> +    depends on SPARSEMEM_VMEMMAP
> >> +    depends on HAVE_BOOTMEM_INFO_NODE
> >> +    help
> >> +      When using HUGETLB_PAGE_FREE_VMEMMAP, the system can save up some
> >> +      memory from pre-allocated HugeTLB pages when they are not used.
> >> +      6 pages per HugeTLB page of the pmd level mapping and (PAGE_SIZE - 2)
> >> +      pages per HugeTLB page of the pud level mapping.
> >> +
> >> +      When the pages are going to be used or freed up, the vmemmap array
> >> +      representing that range needs to be remapped again and the pages
> >> +      we discarded earlier need to be rellocated again.
> >
> > I see the previous discussion with David about wording here.  How about
> > leaving the functionality description general, and provide a specific
> > example for x86_64?  As mentioned we can always update when new arch support
> > is added.  Suggested text?
> >
> >       The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
> >       some vmemmap pages associated with pre-allocated HugeTLB pages.
> >       For example, on X86_64 6 vmemmap pages of size 4KB each can be
> >       saved for each 2MB HugeTLB page.  4094 vmemmap pages of size 4KB
> >       each can be saved for each 1GB HugeTLB page.
> >
> >       When a HugeTLB page is allocated or freed, the vmemmap array
> >       representing the range associated with the page will need to be
> >       remapped.  When a page is allocated, vmemmap pages are freed
> >       after remapping.  When a page is freed, previously discarded
> >       vmemmap pages must be allocated before before remapping.
>
> Sorry, I am slowly coming up to speed with discussions when I was away.
>
> It appears vmemmap is not being mapped with huge pages if the boot option
> hugetlb_free_vmemmap is on.   Is that correct?

Right.

>
> If that is correct, we should document the trade off of increased page
> table pages needed to map vmemmap vs the savings from freeing struct page
> pages.  If a user/sysadmin only uses a small number of hugetlb pages (as
> a percentage of system memory) they could end up using more memory with
> hugetlb_free_vmemmap on as opposed to off.  Perhaps, it should be part of
> the documentation for hugetlb_free_vmemmap?  If this is true, and people

Right, it is better to document it around hugetlb_free_vmemmap.
This should be a part of pathe #8. Thanks.


> think this should be documented, I can try to come up with something.
>
> --
> Mike Kravetz



-- 
Yours,
Muchun