Re: [External] Re: [PATCH v2 07/19] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page

[Muchun Song]

On Fri, Oct 30, 2020 at 6:00 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 10/28/20 11:13 PM, Muchun Song wrote:
> > On Thu, Oct 29, 2020 at 7:42 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> >>
> >> On 10/26/20 7:51 AM, Muchun Song wrote:
> >>> +
> >>> +static inline spinlock_t *vmemmap_pmd_lockptr(pmd_t *pmd)
> >>> +{
> >>> +     static DEFINE_SPINLOCK(pgtable_lock);
> >>> +
> >>> +     return &pgtable_lock;
> >>> +}
> >>
> >> This is just a global lock.  Correct?  And hugetlb specific?
> >
> > Yes, it is a global lock. Originally, I wanted to use the pmd lock(e.g.
> > pmd_lockptr()). But we need to allocate memory for the spinlock and
> > initialize it when ALLOC_SPLIT_PTLOCKS. It may increase the
> > complexity.
> >
> > And I think that here alloc/free hugetlb pages is not a frequent operation.
> > So I finally use a global lock. Maybe it is enough.
> >
> >>
> >> It should be OK as the page table entries for huegtlb pages will not
> >> overlap with other entries.
> >
> > Does "hugetlb specific" mean the pmd lock? or per hugetlb lock?
> > If it is pmd lock, this is fine to me. If not, it may not be enough.
> > Because the lock also guards the splitting of pmd pgtable.
>
> By "hugetlb specific", I was trying to say that only hugetlb code would
> use this lock.  It is not a concern now.  However, there has been talk
> about other code doing something similar to remove struct pages.  If that
> ever happens then we will need a different locking scheme.

Agreed, It is not a concern now :)

>
> Disregard my statement about there being no overlap.  I was confusing
> page tables for huge pages with page tables for mappings mmap entries
> of huge pages.
> --
> Mike Kravetz



-- 
Yours,
Muchun