Re: [External] Re: [PATCH v4 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers

[Muchun Song]

On Tue, Nov 17, 2020 at 11:06 PM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Fri, Nov 13, 2020 at 06:59:36PM +0800, Muchun Song wrote:
> > +#define page_huge_pte(page)          ((page)->pmd_huge_pte)
>
> Seems you do not need this one anymore.
>
> > +void vmemmap_pgtable_free(struct page *page)
> > +{
> > +     struct page *pte_page, *t_page;
> > +
> > +     list_for_each_entry_safe(pte_page, t_page, &page->lru, lru) {
> > +             list_del(&pte_page->lru);
> > +             pte_free_kernel(&init_mm, page_to_virt(pte_page));
> > +     }
> > +}
> > +
> > +int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> > +{
> > +     unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> > +
> > +     /* Store preallocated pages on huge page lru list */
> > +     INIT_LIST_HEAD(&page->lru);
> > +
> > +     while (nr--) {
> > +             pte_t *pte_p;
> > +
> > +             pte_p = pte_alloc_one_kernel(&init_mm);
> > +             if (!pte_p)
> > +                     goto out;
> > +             list_add(&virt_to_page(pte_p)->lru, &page->lru);
> > +     }
>
> Definetely this looks better and easier to handle.
> Btw, did you explore Matthew's hint about instead of allocating a new page,
> using one of the ones you are going to free to store the ptes?
> I am not sure whether it is feasible at all though.

Hi Oscar and Matthew,

I have started an investigation about this. Finally, I think that it
may not be feasible. If we use a vmemmap page frame as a
page table when we split the PMD table firstly, in this stage,
we need to set 512 pte entry to the vmemmap page frame. If
someone reads the tail struct page struct of the HugeTLB,
it can get the arbitrary value (I am not sure it actually exists,
maybe the memory compaction module can do this). So on
the safe side, I think that allocating a new page is a good
choice.

Thanks.

>
>
> > --- a/mm/hugetlb_vmemmap.h
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -9,12 +9,24 @@
> >  #ifndef _LINUX_HUGETLB_VMEMMAP_H
> >  #define _LINUX_HUGETLB_VMEMMAP_H
> >  #include <linux/hugetlb.h>
> > +#include <linux/mm.h>
>
> why do we need this here?
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun