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

[David Hildenbrand]

On 09.12.20 08:36, Muchun Song wrote:
> On Mon, Dec 7, 2020 at 8:39 PM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>> ---
>>>  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.

I don't think that really makes sense.

After put_page_bootmem() put the last reference, it clears PG_reserved
and hands the page over to the buddy via free_reserved_page(). From that
point on, further get_page_bootmem() would be completely wrong and
dangerous.

Both, put_page_bootmem() and get_page_bootmem() rely on the fact that
they are dealing with memblock allcoations - marked via PG_reserved. If
prepare_vmemmap_page() would be called on something that's *not* coming
from the memblock allocator, it would be completely broken - or am I
missing something?

AFAIKT, there should rather be a BUG_ON(!PageReserved(page)) in
prepare_vmemmap_page() - or proper handling to deal with !memblock
allocations.


And as I said, indicating "bootmem" as part of the function names might
make it clearer that this is not for getting any vmemmap pages (esp.
allocated when hotplugging memory).

-- 
Thanks,

David / dhildenb