Re: [External] Re: [PATCH 3/5] mm: sparsemem: split the huge PMD mapping of vmemmap pages

[Muchun Song]

On Fri, Jun 11, 2021 at 6:35 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 6/9/21 5:13 AM, Muchun Song wrote:
> > If the vmemmap is huge PMD mapped, we should split the huge PMD firstly
> > and then we can change the PTE page table entry. In this patch, we add
> > the ability of splitting the huge PMD mapping of vmemmap pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  include/linux/mm.h   |  2 +-
> >  mm/hugetlb.c         | 42 ++++++++++++++++++++++++++++++++++--
> >  mm/hugetlb_vmemmap.c |  3 ++-
> >  mm/sparse-vmemmap.c  | 61 +++++++++++++++++++++++++++++++++++++++++++++-------
> >  4 files changed, 96 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index cadc8cc2c715..b97e1486c5c1 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3056,7 +3056,7 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> >  #endif
> >
> >  void vmemmap_remap_free(unsigned long start, unsigned long end,
> > -                     unsigned long reuse);
> > +                     unsigned long reuse, struct list_head *pgtables);
> >  int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> >                       unsigned long reuse, gfp_t gfp_mask);
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index c3b2a8a494d6..3137c72d9cc7 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1609,6 +1609,13 @@ static void __prep_account_new_huge_page(struct hstate *h, int nid)
> >  static void __prep_new_huge_page(struct hstate *h, struct page *page)
> >  {
> >       free_huge_page_vmemmap(h, page);
> > +     /*
> > +      * Because we store preallocated pages on @page->lru,
> > +      * vmemmap_pgtable_free() must be called before the
> > +      * initialization of @page->lru in INIT_LIST_HEAD().
> > +      */
> > +     vmemmap_pgtable_free(&page->lru);
> > +
> >       INIT_LIST_HEAD(&page->lru);
> >       set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> >       hugetlb_set_page_subpool(page, NULL);
> > @@ -1775,14 +1782,29 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
> >               nodemask_t *node_alloc_noretry)
> >  {
> >       struct page *page;
> > +     LIST_HEAD(pgtables);
> > +
> > +     if (vmemmap_pgtable_prealloc(h, &pgtables))
> > +             return NULL;
>
> In the previous two patches I asked:
> - Can we wait until later to prealloc vmemmap pages for gigantic pages
>   allocated from bootmem?
> - Should we fail to add a hugetlb page to the pool if we can not do
>   vmemmap optimization?
>
>
> Depending on the answers to those questions, we may be able to eliminate
> these vmemmap_pgtable_prealloc/vmemmap_pgtable_free calls in hugetlb.c.
> What about adding the calls to free_huge_page_vmemmap?
> At the beginning of free_huge_page_vmemmap, allocate any vmemmap pgtable
> pages.  If it fails, skip optimization.  We can free any pages before
> returning to the caller.

You are right because we've introduced HPageVmemmapOptimized flag.
It can be useful here. If failing to optimize vmemmap is allowed, we can
eliminate allocating/freeing page table helpers. Thanks for your reminder.

>
> Since we also know the page/address in the page table can we check to see
> if it is already PTE mapped.  If so, can we then skip allocation?

Good point. We need to allocate 512 page tables when splitting
1 GB huge page. If we fail to allocate page tables in the middle
of processing of remapping, we should restore the previous
mapping. I just want to clarify something for myself.

Thanks, Mike. I'll try in the next version.


> --
> Mike Kravetz