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 11:48 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 09.12.20 16:13, Muchun Song wrote:
> > 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:
> >
>
> Hm, from what I understand you cannot use sizeof() in #if etc. So it
> might not be possible after all. At least the compiler should optimize
> code like
>
> if (!is_power_of_2(sizeof(struct page))) {
>         // either this
> } else {
>         // or that
> }
>
> that can never be reached

Got it. Thanks so much.

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


-- 
Yours,
Muchun