Re: [External] Re: [PATCH v7 06/15] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two

[Muchun Song]

On Wed, Dec 9, 2020 at 6:10 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 09.12.20 11:06, David Hildenbrand wrote:
> > On 09.12.20 11:03, Muchun Song wrote:
> >> On Wed, Dec 9, 2020 at 5:57 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>>
> >>> On 30.11.20 16:18, Muchun Song wrote:
> >>>> We only can free the tail vmemmap pages of HugeTLB to the buddy allocator
> >>>> when the size of struct page is a power of two.
> >>>>
> >>>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >>>> ---
> >>>>  mm/hugetlb_vmemmap.c | 5 +++++
> >>>>  1 file changed, 5 insertions(+)
> >>>>
> >>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> >>>> index 51152e258f39..ad8fc61ea273 100644
> >>>> --- a/mm/hugetlb_vmemmap.c
> >>>> +++ b/mm/hugetlb_vmemmap.c
> >>>> @@ -111,6 +111,11 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> >>>>       unsigned int nr_pages = pages_per_huge_page(h);
> >>>>       unsigned int vmemmap_pages;
> >>>>
> >>>> +     if (!is_power_of_2(sizeof(struct page))) {
> >>>> +             pr_info("disable freeing vmemmap pages for %s\n", h->name);
> >>>
> >>> I'd just drop that pr_info(). Users are able to observe that it's
> >>> working (below), so they are able to identify that it's not working as well.
> >>
> >> The below is just a pr_debug. Do you suggest converting it to pr_info?
> >
> > Good question. I wonder if users really have to know in most cases.
> > Maybe pr_debug() is good enough in environments where we want to debug
> > why stuff is not working as expected.
> >
>
> Oh, another thought, can we glue availability of
> HUGETLB_PAGE_FREE_VMEMMAP (or a new define based on the config and the
> size of a stuct page) to the size of struct page somehow?
>
> I mean, it's known at compile time that this will never work.

I want to define a macro which indicates the size of the
struct page. There is place (kernel/bounds.c) where can
do similar things. When I added the following code in
that file.

        DEFINE(STRUCT_PAGE_SIZE, sizeof(struct page));

Then the compiler will output a message like:

       make[2]: Circular kernel/bounds.s <- include/generated/bounds.h
dependency dropped.

Then I realise that the size of the struct page also depends
on include/generated/bounds.h. But this file is not generated.

Hi David,

Do you have some idea about this?

>
> --
> Thanks,
>
> David / dhildenb
>


-- 
Yours,
Muchun