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4d09d0f4 AD |
1 | Page fragments |
2 | -------------- | |
3 | ||
4 | A page fragment is an arbitrary-length arbitrary-offset area of memory | |
5 | which resides within a 0 or higher order compound page. Multiple | |
6 | fragments within that page are individually refcounted, in the page's | |
7 | reference counter. | |
8 | ||
9 | The page_frag functions, page_frag_alloc and page_frag_free, provide a | |
10 | simple allocation framework for page fragments. This is used by the | |
11 | network stack and network device drivers to provide a backing region of | |
12 | memory for use as either an sk_buff->head, or to be used in the "frags" | |
13 | portion of skb_shared_info. | |
14 | ||
15 | In order to make use of the page fragment APIs a backing page fragment | |
16 | cache is needed. This provides a central point for the fragment allocation | |
17 | and tracks allows multiple calls to make use of a cached page. The | |
18 | advantage to doing this is that multiple calls to get_page can be avoided | |
19 | which can be expensive at allocation time. However due to the nature of | |
20 | this caching it is required that any calls to the cache be protected by | |
21 | either a per-cpu limitation, or a per-cpu limitation and forcing interrupts | |
22 | to be disabled when executing the fragment allocation. | |
23 | ||
24 | The network stack uses two separate caches per CPU to handle fragment | |
25 | allocation. The netdev_alloc_cache is used by callers making use of the | |
26 | __netdev_alloc_frag and __netdev_alloc_skb calls. The napi_alloc_cache is | |
27 | used by callers of the __napi_alloc_frag and __napi_alloc_skb calls. The | |
28 | main difference between these two calls is the context in which they may be | |
29 | called. The "netdev" prefixed functions are usable in any context as these | |
30 | functions will disable interrupts, while the "napi" prefixed functions are | |
31 | only usable within the softirq context. | |
32 | ||
33 | Many network device drivers use a similar methodology for allocating page | |
34 | fragments, but the page fragments are cached at the ring or descriptor | |
35 | level. In order to enable these cases it is necessary to provide a generic | |
36 | way of tearing down a page cache. For this reason __page_frag_cache_drain | |
37 | was implemented. It allows for freeing multiple references from a single | |
38 | page via a single call. The advantage to doing this is that it allows for | |
39 | cleaning up the multiple references that were added to a page in order to | |
40 | avoid calling get_page per allocation. | |
41 | ||
42 | Alexander Duyck, Nov 29, 2016. |