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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , linux-kernel@vger.kernel.org Subject: [PATCH v2] page_pool: add a comment explaining the fragment counter usage Date: Tue, 14 Feb 2023 12:43:16 +0200 Message-Id: <20230214104316.2254814-1-ilias.apalodimas@linaro.org> X-Mailer: git-send-email 2.38.1 MIME-Version: 1.0 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1757803087002247335?= X-GMAIL-MSGID: =?utf-8?q?1757803087002247335?= When reading the page_pool code the first impression is that keeping two separate counters, one being the page refcnt and the other being fragment pp_frag_count, is counter-intuitive. However without that fragment counter we don't know when to reliably destroy or sync the outstanding DMA mappings. So let's add a comment explaining this part. Signed-off-by: Ilias Apalodimas Reviewed-by: Alexander Duyck --- Changes since v1: - Update the comment withe the correct description for pp_frag_count include/net/page_pool.h | 10 ++++++++++ 1 file changed, 10 insertions(+) -- 2.38.1 diff --git a/include/net/page_pool.h b/include/net/page_pool.h index 34bf531ffc8d..277e215cfb58 100644 --- a/include/net/page_pool.h +++ b/include/net/page_pool.h @@ -277,6 +277,16 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page, unsigned int dma_sync_size, bool allow_direct); +/* pp_frag_count represents the number of writers who can update the page + * either by updating skb->data or via DMA mappings for the device. + * We can't rely on the page refcnt, as we don't know who might be + * holding page references and we can't reliably destroy or sync DMA mappings + * of the fragments. + * + * When pp_frag_count reaches 0 we can either recycle the page, if the page + * refcnt is 1, or return it back to the memory allocator and destroy any + * mappings we have. + */ static inline void page_pool_fragment_page(struct page *page, long nr) { atomic_long_set(&page->pp_frag_count, nr);