46 строки
2.3 KiB
ReStructuredText
46 строки
2.3 KiB
ReStructuredText
.. _page_frags:
|
|
|
|
==============
|
|
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.
|