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