Re: [External] Re: [PATCH v8 03/12] mm/bootmem_info: Introduce free_bootmem_page helper

[Muchun Song]

On Thu, Dec 10, 2020 at 10:16 PM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Dec 10, 2020 at 11:55:17AM +0800, Muchun Song wrote:
> > Any memory allocated via the memblock allocator and not via the buddy
> > will be makred reserved already in the memmap. For those pages, we can
>          marked

Thanks.

> > call free_bootmem_page() to free it to buddy allocator.
> >
> > Becasue we wan to free some vmemmap pages of the HugeTLB to the buddy
> Because     want
> > allocator, we can use this helper to do that in the later patchs.
>                                                            patches
>

Thanks.

> To be honest, I think if would be best to introduce this along with
> patch#4, so we get to see where it gets used.
>
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  include/linux/bootmem_info.h | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
> > index 4ed6dee1adc9..20a8b0df0c39 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>
>
> <linux/mm.h> already includes <linux/mmzone.h>

Yeah. Can remove this.

>
> > +static inline void free_bootmem_page(struct page *page)
> > +{
> > +     unsigned long magic = (unsigned long)page->freelist;
> > +
> > +     /* bootmem page has reserved flag in the reserve_bootmem_region */
> reserve_bootmem_region sets the reserved flag on bootmem pages?

Right.

>
> > +     VM_WARN_ON(!PageReserved(page) || page_ref_count(page) != 2);
>
> We do check for PageReserved in patch#4 before calling in here.
> Do we need yet another check here? IOW, do we need to be this paranoid?

Yeah, do not need to check again. We can remove it.

>
> > +     if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> > +             put_page_bootmem(page);
> > +     else
> > +             WARN_ON(1);
>
> Lately, some people have been complaining about using WARN_ON as some
> systems come with panic_on_warn set.
>
> I would say that in this case it does not matter much as if the vmemmap
> pages are not either SECTION_INFO or MIX_SECTION_INFO it means that a
> larger corruption happened elsewhere.
>
> But I think I would align the checks here.
> It does not make sense to me to only scream under DEBUG_VM if page's
> refcount differs from 2, and have a WARN_ON if the page we are trying
> to free was not used for the memmap array.
> Both things imply a corruption, so I would set the checks under the same
> configurations.

Do you suggest changing them all to VM_DEBUG_ON?

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun