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Date:   Wed, 24 Mar 2021 10:12:46 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Arjun Roy <arjunroy@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        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 Tue 23-03-21 11:47:54, Arjun Roy wrote:
> On Tue, Mar 23, 2021 at 7:34 AM Michal Hocko <mhocko@suse.com> 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.

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