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Date:   Thu,  7 Dec 2023 16:52:41 -0800
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Subject: [net-next v1 10/16] page_pool: don't release iov on elevanted refcount
From:   Mina Almasry <almasrymina@google.com>
To:     Shailend Chand <shailend@google.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org,
        bpf@vger.kernel.org, linux-media@vger.kernel.org,
        dri-devel@lists.freedesktop.org
Cc:     Mina Almasry <almasrymina@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Jeroen de Borst <jeroendb@google.com>,
        Praveen Kaligineedi <pkaligineedi@google.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Ahern <dsahern@kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Shuah Khan <shuah@kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        "=?UTF-8?q?Christian=20K=C3=B6nig?=" <christian.koenig@amd.com>,
        Yunsheng Lin <linyunsheng@huawei.com>,
        Harshitha Ramamurthy <hramamurthy@google.com>,
        Shakeel Butt <shakeelb@google.com>
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Precedence: bulk

Currently the page_pool behavior is that a page is considered for
recycling only once, the first time __page_pool_put_page() is called on
it.

This works because in practice the net stack only holds 1 reference to
the skb frags. In that case, the page_pool recycling works as expected,
as the skb frags will have 1 reference on the pages from the net stack
when __page_pool_put_page() is called (if the driver is not holding
extra references for recycling), and so the page will be recycled.

However, this is not compatible with devmem TCP. For devmem TCP, the net
stack holds 2 references for each frag, 1 reference is part of the SKB,
and the second reference is for the user holding the frag until they
call SO_DEVMEM_DONTNEED. This causes a bug in the page_pool recycling
where, when the skb is freed, the reference count goes from 2->1, the
page_pool sees a pending reference, releases the page, and so no devmem
iovs get recycled.

To fix this, don't release iovs on elevated refcount.

Signed-off-by: Mina Almasry <almasrymina@google.com>
---
 net/core/page_pool.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index f0148d66371b..dc2a148f5b06 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -731,6 +731,29 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
 		/* Page found as candidate for recycling */
 		return page;
 	}
+
+	if (page_is_page_pool_iov(page)) {
+		/* With devmem TCP and ppiovs, we can't release pages if the
+		 * refcount is > 1. This is because the net stack holds
+		 * 2 references:
+		 *	- 1 for the skb, and
+		 *	- 1 for the user until they call SO_DEVMEM_DONTNEED.
+		 * Releasing pages for elevated refcounts completely disables
+		 * page_pool recycling. Instead, simply don't release pages and
+		 * the next call to napi_pp_put_page() via SO_DEVMEM_DONTNEED
+		 * will consider the page again for recycling. As a result,
+		 * devmem TCP incompatible with drivers doing refcnt based
+		 * recycling unless those drivers:
+		 *
+		 * - don't mark skb_mark_for_recycle()
+		 * - are sure to release the last reference with
+		 *   page_pool_put_full_page() to consider the page for
+		 *   page_pool recycling.
+		 */
+		page_pool_page_put_many(page, 1);
+		return NULL;
+	}
+
 	/* Fallback/non-XDP mode: API user have elevated refcnt.
 	 *
 	 * Many drivers split up the page into fragments, and some
-- 
2.43.0.472.g3155946c3a-goog