Re: [External] Re: [PATCH v5 21/21] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two

[Muchun Song]

On Mon, Nov 23, 2020 at 3:00 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Fri, Nov 20, 2020 at 09:25:52AM +0100, Michal Hocko wrote:
> > On Fri 20-11-20 14:43:25, Muchun Song wrote:
> > > We only can free the unused vmemmap to the buddy system when the
> > > size of struct page is a power of two.
> >
> > Can we actually have !power_of_2 struct pages?
>
> Yes.  On x86-64, if you don't enable MEMCG, it's 56 bytes.  On SPARC64,
> if you do enable MEMCG, it's 72 bytes.  On 32-bit systems, it's
> anything from 32-44 bytes, depending on MEMCG, WANT_PAGE_VIRTUAL and
> LAST_CPUPID_NOT_IN_PAGE_FLAGS.
>

On x86-64, even if you do not enable MEMCG, it's also 64 bytes. Because
CONFIG_HAVE_ALIGNED_STRUCT_PAGE is defined if we use SLUB.



-- 
Yours,
Muchun