page_frags.rst (2317B)
1.. _page_frags: 2 3============== 4Page fragments 5============== 6 7A page fragment is an arbitrary-length arbitrary-offset area of memory 8which resides within a 0 or higher order compound page. Multiple 9fragments within that page are individually refcounted, in the page's 10reference counter. 11 12The page_frag functions, page_frag_alloc and page_frag_free, provide a 13simple allocation framework for page fragments. This is used by the 14network stack and network device drivers to provide a backing region of 15memory for use as either an sk_buff->head, or to be used in the "frags" 16portion of skb_shared_info. 17 18In order to make use of the page fragment APIs a backing page fragment 19cache is needed. This provides a central point for the fragment allocation 20and tracks allows multiple calls to make use of a cached page. The 21advantage to doing this is that multiple calls to get_page can be avoided 22which can be expensive at allocation time. However due to the nature of 23this caching it is required that any calls to the cache be protected by 24either a per-cpu limitation, or a per-cpu limitation and forcing interrupts 25to be disabled when executing the fragment allocation. 26 27The network stack uses two separate caches per CPU to handle fragment 28allocation. The netdev_alloc_cache is used by callers making use of the 29netdev_alloc_frag and __netdev_alloc_skb calls. The napi_alloc_cache is 30used by callers of the __napi_alloc_frag and __napi_alloc_skb calls. The 31main difference between these two calls is the context in which they may be 32called. The "netdev" prefixed functions are usable in any context as these 33functions will disable interrupts, while the "napi" prefixed functions are 34only usable within the softirq context. 35 36Many network device drivers use a similar methodology for allocating page 37fragments, but the page fragments are cached at the ring or descriptor 38level. In order to enable these cases it is necessary to provide a generic 39way of tearing down a page cache. For this reason __page_frag_cache_drain 40was implemented. It allows for freeing multiple references from a single 41page via a single call. The advantage to doing this is that it allows for 42cleaning up the multiple references that were added to a page in order to 43avoid calling get_page per allocation. 44 45Alexander Duyck, Nov 29, 2016.