On Tue 23-03-21 11:47:54, Arjun Roy wrote:

On Tue, Mar 23, 2021 at 7:34 AM Michal Hocko [off-list ref] wrote:

quoted

On Wed 17-03-21 18:12:55, Johannes Weiner wrote:
[...]

quoted

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.

If you are going to claim a page flag then it would be much better to
have it more generic. Flags are really scarce and if all you care about
is PageHasDestructor() and provide one via page->dtor then the similar
mechanism can be reused by somebody else. Or does anything prevent that?
-- 
Michal Hocko
SUSE Labs