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Diffstat (limited to 'Documentation/vm/page_frags')
| -rw-r--r-- | Documentation/vm/page_frags | 42 |
1 files changed, 0 insertions, 42 deletions
diff --git a/Documentation/vm/page_frags b/Documentation/vm/page_frags deleted file mode 100644 index a6714565dbf9..000000000000 --- a/Documentation/vm/page_frags +++ /dev/null @@ -1,42 +0,0 @@ -Page fragments --------------- - -A page fragment is an arbitrary-length arbitrary-offset area of memory -which resides within a 0 or higher order compound page. Multiple -fragments within that page are individually refcounted, in the page's -reference counter. - -The page_frag functions, page_frag_alloc and page_frag_free, provide a -simple allocation framework for page fragments. This is used by the -network stack and network device drivers to provide a backing region of -memory for use as either an sk_buff->head, or to be used in the "frags" -portion of skb_shared_info. - -In order to make use of the page fragment APIs a backing page fragment -cache is needed. This provides a central point for the fragment allocation -and tracks allows multiple calls to make use of a cached page. The -advantage to doing this is that multiple calls to get_page can be avoided -which can be expensive at allocation time. However due to the nature of -this caching it is required that any calls to the cache be protected by -either a per-cpu limitation, or a per-cpu limitation and forcing interrupts -to be disabled when executing the fragment allocation. - -The network stack uses two separate caches per CPU to handle fragment -allocation. The netdev_alloc_cache is used by callers making use of the -__netdev_alloc_frag and __netdev_alloc_skb calls. The napi_alloc_cache is -used by callers of the __napi_alloc_frag and __napi_alloc_skb calls. The -main difference between these two calls is the context in which they may be -called. The "netdev" prefixed functions are usable in any context as these -functions will disable interrupts, while the "napi" prefixed functions are -only usable within the softirq context. - -Many network device drivers use a similar methodology for allocating page -fragments, but the page fragments are cached at the ring or descriptor -level. In order to enable these cases it is necessary to provide a generic -way of tearing down a page cache. For this reason __page_frag_cache_drain -was implemented. It allows for freeing multiple references from a single -page via a single call. The advantage to doing this is that it allows for -cleaning up the multiple references that were added to a page in order to -avoid calling get_page per allocation. - -Alexander Duyck, Nov 29, 2016. |