On Tue, Mar 23, 2021 at 7:34 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 17-03-21 18:12:55, Johannes Weiner wrote:
> [...]
> > Here is an idea of how it could work:
> >
> > struct page already has
> >
> > struct { /* page_pool used by netstack */
> > /**
> > * @dma_addr: might require a 64-bit value even on
> > * 32-bit architectures.
> > */
> > dma_addr_t dma_addr;
> > };
> >
> > and as you can see from its union neighbors, there is quite a bit more
> > room to store private data necessary for the page pool.
> >
> > When a page's refcount hits zero and it's a networking page, we can
> > feed it back to the page pool instead of the page allocator.
> >
> > From a first look, we should be able to use the PG_owner_priv_1 page
> > flag for network pages (see how this flag is overloaded, we can add a
> > PG_network alias). With this, we can identify the page in __put_page()
> > and __release_page(). These functions are already aware of different
> > types of pages and do their respective cleanup handling. We can
> > similarly make network a first-class citizen and hand pages back to
> > the network allocator from in there.
>
> For compound pages we have a concept of destructors. Maybe we can extend
> that for order-0 pages as well. The struct page is heavily packed and
> compound_dtor shares the storage without other metadata
> int pages; /* 16 4 */
> unsigned char compound_dtor; /* 16 1 */
> atomic_t hpage_pinned_refcount; /* 16 4 */
> pgtable_t pmd_huge_pte; /* 16 8 */
> void * zone_device_data; /* 16 8 */
>
> But none of those should really require to be valid when a page is freed
> unless I am missing something. It would really require to check their
> users whether they can leave the state behind. But if we can establish a
> contract that compound_dtor can be always valid when a page is freed
> this would be really a nice and useful abstraction because you wouldn't
> have to care about the specific type of page.
>
> But maybe I am just overlooking the real complexity there.
> --
For now probably the easiest way is to have network pages be first
class with a specific flag as previously discussed and have concrete
handling for it, rather than trying to establish the contract across
page types.
Thanks,
-Arjun
> Michal Hocko
> SUSE Labs
