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Date:   Tue, 23 Mar 2021 15:34:20 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Arjun Roy <arjunroy@google.com>,
        Arjun Roy <arjunroy.kdev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Miller <davem@davemloft.net>,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Shakeel Butt <shakeelb@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Soheil Hassas Yeganeh <soheil@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Yang Shi <shy828301@gmail.com>, Roman Gushchin <guro@fb.com>
Subject: Re: [mm, net-next v2] mm: net: memcg accounting for TCP rx zerocopy
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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