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.
--
Michal Hocko
SUSE Labs
