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Tue, 23 Mar 2021 11:48:06 -0700 (PDT) MIME-Version: 1.0 References: <20210316041645.144249-1-arjunroy.kdev@gmail.com> In-Reply-To: From: Arjun Roy Date: Tue, 23 Mar 2021 11:47:54 -0700 Message-ID: Subject: Re: [mm, net-next v2] mm: net: memcg accounting for TCP rx zerocopy To: Michal Hocko Cc: Johannes Weiner , Arjun Roy , Andrew Morton , David Miller , netdev , Linux Kernel Mailing List , Cgroups , Linux MM , Shakeel Butt , Eric Dumazet , Soheil Hassas Yeganeh , Jakub Kicinski , Yang Shi , Roman Gushchin Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: F36F080192EC X-Stat-Signature: 6jibu6z1ozqcuxbdtdm64a133pbdrond Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf16; identity=mailfrom; envelope-from=""; helo=mail-pj1-f47.google.com; client-ip=209.85.216.47 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616525285-599933 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Mar 23, 2021 at 7:34 AM Michal Hocko 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