diff options
Diffstat (limited to 'Documentation/vm')
-rw-r--r-- | Documentation/vm/page_frags | 42 | ||||
-rw-r--r-- | Documentation/vm/transhuge.txt | 10 |
2 files changed, 50 insertions, 2 deletions
diff --git a/Documentation/vm/page_frags b/Documentation/vm/page_frags new file mode 100644 index 000000000000..a6714565dbf9 --- /dev/null +++ b/Documentation/vm/page_frags @@ -0,0 +1,42 @@ +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. diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index c4171e4519c2..cd28d5ee5273 100644 --- a/Documentation/vm/transhuge.txt +++ b/Documentation/vm/transhuge.txt @@ -110,6 +110,7 @@ MADV_HUGEPAGE region. echo always >/sys/kernel/mm/transparent_hugepage/defrag echo defer >/sys/kernel/mm/transparent_hugepage/defrag +echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag echo madvise >/sys/kernel/mm/transparent_hugepage/defrag echo never >/sys/kernel/mm/transparent_hugepage/defrag @@ -120,10 +121,15 @@ that benefit heavily from THP use and are willing to delay the VM start to utilise them. "defer" means that an application will wake kswapd in the background -to reclaim pages and wake kcompact to compact memory so that THP is +to reclaim pages and wake kcompactd to compact memory so that THP is available in the near future. It's the responsibility of khugepaged to then install the THP pages later. +"defer+madvise" will enter direct reclaim and compaction like "always", but +only for regions that have used madvise(MADV_HUGEPAGE); all other regions +will wake kswapd in the background to reclaim pages and wake kcompactd to +compact memory so that THP is available in the near future. + "madvise" will enter direct reclaim like "always" but only for regions that are have used madvise(MADV_HUGEPAGE). This is the default behaviour. @@ -296,7 +302,7 @@ thp_split_page is incremented every time a huge page is split into base reason is that a huge page is old and is being reclaimed. This action implies splitting all PMD the page mapped with. -thp_split_page_failed is is incremented if kernel fails to split huge +thp_split_page_failed is incremented if kernel fails to split huge page. This can happen if the page was pinned by somebody. thp_deferred_split_page is incremented when a huge page is put onto split |