Re: [External] Re: [PATCH v7 05/15] mm/bootmem_info: Introduce {free,prepare}_vmemmap_page()

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Dec 7, 2020 at 8:39 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 30.11.20 16:18, Muchun Song wrote:
> > In the later patch, we can use the free_vmemmap_page() to free the
> > unused vmemmap pages and initialize a page for vmemmap page using
> > via prepare_vmemmap_page().
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/linux/bootmem_info.h | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
> > index 4ed6dee1adc9..239e3cc8f86c 100644
> > --- a/include/linux/bootmem_info.h
> > +++ b/include/linux/bootmem_info.h
> > @@ -3,6 +3,7 @@
> >  #define __LINUX_BOOTMEM_INFO_H
> >
> >  #include <linux/mmzone.h>
> > +#include <linux/mm.h>
> >
> >  /*
> >   * Types for free bootmem stored in page->lru.next. These have to be in
> > @@ -22,6 +23,29 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
> >  void get_page_bootmem(unsigned long info, struct page *page,
> >                     unsigned long type);
> >  void put_page_bootmem(struct page *page);
> > +
> > +static inline void free_vmemmap_page(struct page *page)
> > +{
> > +     VM_WARN_ON(!PageReserved(page) || page_ref_count(page) != 2);
> > +
> > +     /* bootmem page has reserved flag in the reserve_bootmem_region */
> > +     if (PageReserved(page)) {
> > +             unsigned long magic = (unsigned long)page->freelist;
> > +
> > +             if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> > +                     put_page_bootmem(page);
> > +             else
> > +                     WARN_ON(1);
> > +     }
> > +}
> > +
> > +static inline void prepare_vmemmap_page(struct page *page)
> > +{
> > +     unsigned long section_nr = pfn_to_section_nr(page_to_pfn(page));
> > +
> > +     get_page_bootmem(section_nr, page, SECTION_INFO);
> > +     mark_page_reserved(page);
> > +}
>
> Can you clarify in the description when exactly these functions are
> called and on which type of pages?
>
> Would indicating "bootmem" in the function names make it clearer what we
> are dealing with?
>
> E.g., any memory allocated via the memblock allocator and not via the
> buddy will be makred reserved already in the memmap. It's unclear to me
> why we need the mark_page_reserved() here - can you enlighten me? :)

Sorry for ignoring this question. Because the vmemmap pages are allocated
from the bootmem allocator which is marked as PG_reserved. For those bootmem
pages, we should call put_page_bootmem for free. You can see that we
clear the PG_reserved in the put_page_bootmem. In order to be consistent,
the prepare_vmemmap_page also marks the page as PG_reserved.

Thanks.

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


-- 
Yours,
Muchun