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