application that could have been using hugepages. This also applies to
the regions registered in khugepaged.
+== Monitoring usage ==
+
+The number of transparent huge pages currently used by the system is
+available by reading the AnonHugePages field in /proc/meminfo. To
+identify what applications are using transparent huge pages, it is
+necessary to read /proc/PID/smaps and count the AnonHugePages fields
+for each mapping. Note that reading the smaps file is expensive and
+reading it frequently will incur overhead.
+
+There are a number of counters in /proc/vmstat that may be used to
+monitor how successfully the system is providing huge pages for use.
+
+thp_fault_alloc is incremented every time a huge page is successfully
+ allocated to handle a page fault. This applies to both the
+ first time a page is faulted and for COW faults.
+
+thp_collapse_alloc is incremented by khugepaged when it has found
+ a range of pages to collapse into one huge page and has
+ successfully allocated a new huge page to store the data.
+
+thp_fault_fallback is incremented if a page fault fails to allocate
+ a huge page and instead falls back to using small pages.
+
+thp_collapse_alloc_failed is incremented if khugepaged found a range
+ of pages that should be collapsed into one huge page but failed
+ the allocation.
+
+thp_split is incremented every time a huge page is split into base
+ pages. This can happen for a variety of reasons but a common
+ reason is that a huge page is old and is being reclaimed.
+
+As the system ages, allocating huge pages may be expensive as the
+system uses memory compaction to copy data around memory to free a
+huge page for use. There are some counters in /proc/vmstat to help
+monitor this overhead.
+
+compact_stall is incremented every time a process stalls to run
+ memory compaction so that a huge page is free for use.
+
+compact_success is incremented if the system compacted memory and
+ freed a huge page for use.
+
+compact_fail is incremented if the system tries to compact memory
+ but failed.
+
+compact_pages_moved is incremented each time a page is moved. If
+ this value is increasing rapidly, it implies that the system
+ is copying a lot of data to satisfy the huge page allocation.
+ It is possible that the cost of copying exceeds any savings
+ from reduced TLB misses.
+
+compact_pagemigrate_failed is incremented when the underlying mechanism
+ for moving a page failed.
+
+compact_blocks_moved is incremented each time memory compaction examines
+ a huge page aligned range of pages.
+
+It is possible to establish how long the stalls were using the function
+tracer to record how long was spent in __alloc_pages_nodemask and
+using the mm_page_alloc tracepoint to identify which allocations were
+for huge pages.
+
== get_user_pages and follow_page ==
get_user_pages and follow_page if run on a hugepage, will return the
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