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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	= Transparent Hugepage Support =
2	
3	== Objective ==
4	
5	Performance critical computing applications dealing with large memory
6	working sets are already running on top of libhugetlbfs and in turn
7	hugetlbfs. Transparent Hugepage Support is an alternative means of
8	using huge pages for the backing of virtual memory with huge pages
9	that supports the automatic promotion and demotion of page sizes and
10	without the shortcomings of hugetlbfs.
11	
12	Currently it only works for anonymous memory mappings and tmpfs/shmem.
13	But in the future it can expand to other filesystems.
14	
15	The reason applications are running faster is because of two
16	factors. The first factor is almost completely irrelevant and it's not
17	of significant interest because it'll also have the downside of
18	requiring larger clear-page copy-page in page faults which is a
19	potentially negative effect. The first factor consists in taking a
20	single page fault for each 2M virtual region touched by userland (so
21	reducing the enter/exit kernel frequency by a 512 times factor). This
22	only matters the first time the memory is accessed for the lifetime of
23	a memory mapping. The second long lasting and much more important
24	factor will affect all subsequent accesses to the memory for the whole
25	runtime of the application. The second factor consist of two
26	components: 1) the TLB miss will run faster (especially with
27	virtualization using nested pagetables but almost always also on bare
28	metal without virtualization) and 2) a single TLB entry will be
29	mapping a much larger amount of virtual memory in turn reducing the
30	number of TLB misses. With virtualization and nested pagetables the
31	TLB can be mapped of larger size only if both KVM and the Linux guest
32	are using hugepages but a significant speedup already happens if only
33	one of the two is using hugepages just because of the fact the TLB
34	miss is going to run faster.
35	
36	== Design ==
37	
38	- "graceful fallback": mm components which don't have transparent hugepage
39	  knowledge fall back to breaking huge pmd mapping into table of ptes and,
40	  if necessary, split a transparent hugepage. Therefore these components
41	  can continue working on the regular pages or regular pte mappings.
42	
43	- if a hugepage allocation fails because of memory fragmentation,
44	  regular pages should be gracefully allocated instead and mixed in
45	  the same vma without any failure or significant delay and without
46	  userland noticing
47	
48	- if some task quits and more hugepages become available (either
49	  immediately in the buddy or through the VM), guest physical memory
50	  backed by regular pages should be relocated on hugepages
51	  automatically (with khugepaged)
52	
53	- it doesn't require memory reservation and in turn it uses hugepages
54	  whenever possible (the only possible reservation here is kernelcore=
55	  to avoid unmovable pages to fragment all the memory but such a tweak
56	  is not specific to transparent hugepage support and it's a generic
57	  feature that applies to all dynamic high order allocations in the
58	  kernel)
59	
60	Transparent Hugepage Support maximizes the usefulness of free memory
61	if compared to the reservation approach of hugetlbfs by allowing all
62	unused memory to be used as cache or other movable (or even unmovable
63	entities). It doesn't require reservation to prevent hugepage
64	allocation failures to be noticeable from userland. It allows paging
65	and all other advanced VM features to be available on the
66	hugepages. It requires no modifications for applications to take
67	advantage of it.
68	
69	Applications however can be further optimized to take advantage of
70	this feature, like for example they've been optimized before to avoid
71	a flood of mmap system calls for every malloc(4k). Optimizing userland
72	is by far not mandatory and khugepaged already can take care of long
73	lived page allocations even for hugepage unaware applications that
74	deals with large amounts of memory.
75	
76	In certain cases when hugepages are enabled system wide, application
77	may end up allocating more memory resources. An application may mmap a
78	large region but only touch 1 byte of it, in that case a 2M page might
79	be allocated instead of a 4k page for no good. This is why it's
80	possible to disable hugepages system-wide and to only have them inside
81	MADV_HUGEPAGE madvise regions.
82	
83	Embedded systems should enable hugepages only inside madvise regions
84	to eliminate any risk of wasting any precious byte of memory and to
85	only run faster.
86	
87	Applications that gets a lot of benefit from hugepages and that don't
88	risk to lose memory by using hugepages, should use
89	madvise(MADV_HUGEPAGE) on their critical mmapped regions.
90	
91	== sysfs ==
92	
93	Transparent Hugepage Support for anonymous memory can be entirely disabled
94	(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
95	regions (to avoid the risk of consuming more memory resources) or enabled
96	system wide. This can be achieved with one of:
97	
98	echo always >/sys/kernel/mm/transparent_hugepage/enabled
99	echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
100	echo never >/sys/kernel/mm/transparent_hugepage/enabled
101	
102	It's also possible to limit defrag efforts in the VM to generate
103	anonymous hugepages in case they're not immediately free to madvise
104	regions or to never try to defrag memory and simply fallback to regular
105	pages unless hugepages are immediately available. Clearly if we spend CPU
106	time to defrag memory, we would expect to gain even more by the fact we
107	use hugepages later instead of regular pages. This isn't always
108	guaranteed, but it may be more likely in case the allocation is for a
109	MADV_HUGEPAGE region.
110	
111	echo always >/sys/kernel/mm/transparent_hugepage/defrag
112	echo defer >/sys/kernel/mm/transparent_hugepage/defrag
113	echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag
114	echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
115	echo never >/sys/kernel/mm/transparent_hugepage/defrag
116	
117	"always" means that an application requesting THP will stall on allocation
118	failure and directly reclaim pages and compact memory in an effort to
119	allocate a THP immediately. This may be desirable for virtual machines
120	that benefit heavily from THP use and are willing to delay the VM start
121	to utilise them.
122	
123	"defer" means that an application will wake kswapd in the background
124	to reclaim pages and wake kcompactd to compact memory so that THP is
125	available in the near future. It's the responsibility of khugepaged
126	to then install the THP pages later.
127	
128	"defer+madvise" will enter direct reclaim and compaction like "always", but
129	only for regions that have used madvise(MADV_HUGEPAGE); all other regions
130	will wake kswapd in the background to reclaim pages and wake kcompactd to
131	compact memory so that THP is available in the near future.
132	
133	"madvise" will enter direct reclaim like "always" but only for regions
134	that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.
135	
136	"never" should be self-explanatory.
137	
138	By default kernel tries to use huge zero page on read page fault to
139	anonymous mapping. It's possible to disable huge zero page by writing 0
140	or enable it back by writing 1:
141	
142	echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
143	echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
144	
145	Some userspace (such as a test program, or an optimized memory allocation
146	library) may want to know the size (in bytes) of a transparent hugepage:
147	
148	cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size
149	
150	khugepaged will be automatically started when
151	transparent_hugepage/enabled is set to "always" or "madvise, and it'll
152	be automatically shutdown if it's set to "never".
153	
154	khugepaged runs usually at low frequency so while one may not want to
155	invoke defrag algorithms synchronously during the page faults, it
156	should be worth invoking defrag at least in khugepaged. However it's
157	also possible to disable defrag in khugepaged by writing 0 or enable
158	defrag in khugepaged by writing 1:
159	
160	echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
161	echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
162	
163	You can also control how many pages khugepaged should scan at each
164	pass:
165	
166	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
167	
168	and how many milliseconds to wait in khugepaged between each pass (you
169	can set this to 0 to run khugepaged at 100% utilization of one core):
170	
171	/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
172	
173	and how many milliseconds to wait in khugepaged if there's an hugepage
174	allocation failure to throttle the next allocation attempt.
175	
176	/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
177	
178	The khugepaged progress can be seen in the number of pages collapsed:
179	
180	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
181	
182	for each pass:
183	
184	/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
185	
186	max_ptes_none specifies how many extra small pages (that are
187	not already mapped) can be allocated when collapsing a group
188	of small pages into one large page.
189	
190	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
191	
192	A higher value leads to use additional memory for programs.
193	A lower value leads to gain less thp performance. Value of
194	max_ptes_none can waste cpu time very little, you can
195	ignore it.
196	
197	max_ptes_swap specifies how many pages can be brought in from
198	swap when collapsing a group of pages into a transparent huge page.
199	
200	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
201	
202	A higher value can cause excessive swap IO and waste
203	memory. A lower value can prevent THPs from being
204	collapsed, resulting fewer pages being collapsed into
205	THPs, and lower memory access performance.
206	
207	== Boot parameter ==
208	
209	You can change the sysfs boot time defaults of Transparent Hugepage
210	Support by passing the parameter "transparent_hugepage=always" or
211	"transparent_hugepage=madvise" or "transparent_hugepage=never"
212	(without "") to the kernel command line.
213	
214	== Hugepages in tmpfs/shmem ==
215	
216	You can control hugepage allocation policy in tmpfs with mount option
217	"huge=". It can have following values:
218	
219	  - "always":
220	    Attempt to allocate huge pages every time we need a new page;
221	
222	  - "never":
223	    Do not allocate huge pages;
224	
225	  - "within_size":
226	    Only allocate huge page if it will be fully within i_size.
227	    Also respect fadvise()/madvise() hints;
228	
229	  - "advise:
230	    Only allocate huge pages if requested with fadvise()/madvise();
231	
232	The default policy is "never".
233	
234	"mount -o remount,huge= /mountpoint" works fine after mount: remounting
235	huge=never will not attempt to break up huge pages at all, just stop more
236	from being allocated.
237	
238	There's also sysfs knob to control hugepage allocation policy for internal
239	shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
240	is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
241	MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
242	
243	In addition to policies listed above, shmem_enabled allows two further
244	values:
245	
246	  - "deny":
247	    For use in emergencies, to force the huge option off from
248	    all mounts;
249	  - "force":
250	    Force the huge option on for all - very useful for testing;
251	
252	== Need of application restart ==
253	
254	The transparent_hugepage/enabled values and tmpfs mount option only affect
255	future behavior. So to make them effective you need to restart any
256	application that could have been using hugepages. This also applies to the
257	regions registered in khugepaged.
258	
259	== Monitoring usage ==
260	
261	The number of anonymous transparent huge pages currently used by the
262	system is available by reading the AnonHugePages field in /proc/meminfo.
263	To identify what applications are using anonymous transparent huge pages,
264	it is necessary to read /proc/PID/smaps and count the AnonHugePages fields
265	for each mapping.
266	
267	The number of file transparent huge pages mapped to userspace is available
268	by reading ShmemPmdMapped and ShmemHugePages fields in /proc/meminfo.
269	To identify what applications are mapping file transparent huge pages, it
270	is necessary to read /proc/PID/smaps and count the FileHugeMapped fields
271	for each mapping.
272	
273	Note that reading the smaps file is expensive and reading it
274	frequently will incur overhead.
275	
276	There are a number of counters in /proc/vmstat that may be used to
277	monitor how successfully the system is providing huge pages for use.
278	
279	thp_fault_alloc is incremented every time a huge page is successfully
280		allocated to handle a page fault. This applies to both the
281		first time a page is faulted and for COW faults.
282	
283	thp_collapse_alloc is incremented by khugepaged when it has found
284		a range of pages to collapse into one huge page and has
285		successfully allocated a new huge page to store the data.
286	
287	thp_fault_fallback is incremented if a page fault fails to allocate
288		a huge page and instead falls back to using small pages.
289	
290	thp_collapse_alloc_failed is incremented if khugepaged found a range
291		of pages that should be collapsed into one huge page but failed
292		the allocation.
293	
294	thp_file_alloc is incremented every time a file huge page is successfully
295		allocated.
296	
297	thp_file_mapped is incremented every time a file huge page is mapped into
298		user address space.
299	
300	thp_split_page is incremented every time a huge page is split into base
301		pages. This can happen for a variety of reasons but a common
302		reason is that a huge page is old and is being reclaimed.
303		This action implies splitting all PMD the page mapped with.
304	
305	thp_split_page_failed is incremented if kernel fails to split huge
306		page. This can happen if the page was pinned by somebody.
307	
308	thp_deferred_split_page is incremented when a huge page is put onto split
309		queue. This happens when a huge page is partially unmapped and
310		splitting it would free up some memory. Pages on split queue are
311		going to be split under memory pressure.
312	
313	thp_split_pmd is incremented every time a PMD split into table of PTEs.
314		This can happen, for instance, when application calls mprotect() or
315		munmap() on part of huge page. It doesn't split huge page, only
316		page table entry.
317	
318	thp_zero_page_alloc is incremented every time a huge zero page is
319		successfully allocated. It includes allocations which where
320		dropped due race with other allocation. Note, it doesn't count
321		every map of the huge zero page, only its allocation.
322	
323	thp_zero_page_alloc_failed is incremented if kernel fails to allocate
324		huge zero page and falls back to using small pages.
325	
326	As the system ages, allocating huge pages may be expensive as the
327	system uses memory compaction to copy data around memory to free a
328	huge page for use. There are some counters in /proc/vmstat to help
329	monitor this overhead.
330	
331	compact_stall is incremented every time a process stalls to run
332		memory compaction so that a huge page is free for use.
333	
334	compact_success is incremented if the system compacted memory and
335		freed a huge page for use.
336	
337	compact_fail is incremented if the system tries to compact memory
338		but failed.
339	
340	compact_pages_moved is incremented each time a page is moved. If
341		this value is increasing rapidly, it implies that the system
342		is copying a lot of data to satisfy the huge page allocation.
343		It is possible that the cost of copying exceeds any savings
344		from reduced TLB misses.
345	
346	compact_pagemigrate_failed is incremented when the underlying mechanism
347		for moving a page failed.
348	
349	compact_blocks_moved is incremented each time memory compaction examines
350		a huge page aligned range of pages.
351	
352	It is possible to establish how long the stalls were using the function
353	tracer to record how long was spent in __alloc_pages_nodemask and
354	using the mm_page_alloc tracepoint to identify which allocations were
355	for huge pages.
356	
357	== get_user_pages and follow_page ==
358	
359	get_user_pages and follow_page if run on a hugepage, will return the
360	head or tail pages as usual (exactly as they would do on
361	hugetlbfs). Most gup users will only care about the actual physical
362	address of the page and its temporary pinning to release after the I/O
363	is complete, so they won't ever notice the fact the page is huge. But
364	if any driver is going to mangle over the page structure of the tail
365	page (like for checking page->mapping or other bits that are relevant
366	for the head page and not the tail page), it should be updated to jump
367	to check head page instead. Taking reference on any head/tail page would
368	prevent page from being split by anyone.
369	
370	NOTE: these aren't new constraints to the GUP API, and they match the
371	same constrains that applies to hugetlbfs too, so any driver capable
372	of handling GUP on hugetlbfs will also work fine on transparent
373	hugepage backed mappings.
374	
375	In case you can't handle compound pages if they're returned by
376	follow_page, the FOLL_SPLIT bit can be specified as parameter to
377	follow_page, so that it will split the hugepages before returning
378	them. Migration for example passes FOLL_SPLIT as parameter to
379	follow_page because it's not hugepage aware and in fact it can't work
380	at all on hugetlbfs (but it instead works fine on transparent
381	hugepages thanks to FOLL_SPLIT). migration simply can't deal with
382	hugepages being returned (as it's not only checking the pfn of the
383	page and pinning it during the copy but it pretends to migrate the
384	memory in regular page sizes and with regular pte/pmd mappings).
385	
386	== Optimizing the applications ==
387	
388	To be guaranteed that the kernel will map a 2M page immediately in any
389	memory region, the mmap region has to be hugepage naturally
390	aligned. posix_memalign() can provide that guarantee.
391	
392	== Hugetlbfs ==
393	
394	You can use hugetlbfs on a kernel that has transparent hugepage
395	support enabled just fine as always. No difference can be noted in
396	hugetlbfs other than there will be less overall fragmentation. All
397	usual features belonging to hugetlbfs are preserved and
398	unaffected. libhugetlbfs will also work fine as usual.
399	
400	== Graceful fallback ==
401	
402	Code walking pagetables but unaware about huge pmds can simply call
403	split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
404	pmd_offset. It's trivial to make the code transparent hugepage aware
405	by just grepping for "pmd_offset" and adding split_huge_pmd where
406	missing after pmd_offset returns the pmd. Thanks to the graceful
407	fallback design, with a one liner change, you can avoid to write
408	hundred if not thousand of lines of complex code to make your code
409	hugepage aware.
410	
411	If you're not walking pagetables but you run into a physical hugepage
412	but you can't handle it natively in your code, you can split it by
413	calling split_huge_page(page). This is what the Linux VM does before
414	it tries to swapout the hugepage for example. split_huge_page() can fail
415	if the page is pinned and you must handle this correctly.
416	
417	Example to make mremap.c transparent hugepage aware with a one liner
418	change:
419	
420	diff --git a/mm/mremap.c b/mm/mremap.c
421	--- a/mm/mremap.c
422	+++ b/mm/mremap.c
423	@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
424			return NULL;
425	
426		pmd = pmd_offset(pud, addr);
427	+	split_huge_pmd(vma, pmd, addr);
428		if (pmd_none_or_clear_bad(pmd))
429			return NULL;
430	
431	== Locking in hugepage aware code ==
432	
433	We want as much code as possible hugepage aware, as calling
434	split_huge_page() or split_huge_pmd() has a cost.
435	
436	To make pagetable walks huge pmd aware, all you need to do is to call
437	pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
438	mmap_sem in read (or write) mode to be sure an huge pmd cannot be
439	created from under you by khugepaged (khugepaged collapse_huge_page
440	takes the mmap_sem in write mode in addition to the anon_vma lock). If
441	pmd_trans_huge returns false, you just fallback in the old code
442	paths. If instead pmd_trans_huge returns true, you have to take the
443	page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
444	page table lock will prevent the huge pmd to be converted into a
445	regular pmd from under you (split_huge_pmd can run in parallel to the
446	pagetable walk). If the second pmd_trans_huge returns false, you
447	should just drop the page table lock and fallback to the old code as
448	before. Otherwise you can proceed to process the huge pmd and the
449	hugepage natively. Once finished you can drop the page table lock.
450	
451	== Refcounts and transparent huge pages ==
452	
453	Refcounting on THP is mostly consistent with refcounting on other compound
454	pages:
455	
456	  - get_page()/put_page() and GUP operate in head page's ->_refcount.
457	
458	  - ->_refcount in tail pages is always zero: get_page_unless_zero() never
459	    succeed on tail pages.
460	
461	  - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
462	    on relevant sub-page of the compound page.
463	
464	  - map/unmap of the whole compound page accounted in compound_mapcount
465	    (stored in first tail page). For file huge pages, we also increment
466	    ->_mapcount of all sub-pages in order to have race-free detection of
467	    last unmap of subpages.
468	
469	PageDoubleMap() indicates that the page is *possibly* mapped with PTEs.
470	
471	For anonymous pages PageDoubleMap() also indicates ->_mapcount in all
472	subpages is offset up by one. This additional reference is required to
473	get race-free detection of unmap of subpages when we have them mapped with
474	both PMDs and PTEs.
475	
476	This is optimization required to lower overhead of per-subpage mapcount
477	tracking. The alternative is alter ->_mapcount in all subpages on each
478	map/unmap of the whole compound page.
479	
480	For anonymous pages, we set PG_double_map when a PMD of the page got split
481	for the first time, but still have PMD mapping. The additional references
482	go away with last compound_mapcount.
483	
484	File pages get PG_double_map set on first map of the page with PTE and
485	goes away when the page gets evicted from page cache.
486	
487	split_huge_page internally has to distribute the refcounts in the head
488	page to the tail pages before clearing all PG_head/tail bits from the page
489	structures. It can be done easily for refcounts taken by page table
490	entries. But we don't have enough information on how to distribute any
491	additional pins (i.e. from get_user_pages). split_huge_page() fails any
492	requests to split pinned huge page: it expects page count to be equal to
493	sum of mapcount of all sub-pages plus one (split_huge_page caller must
494	have reference for head page).
495	
496	split_huge_page uses migration entries to stabilize page->_refcount and
497	page->_mapcount of anonymous pages. File pages just got unmapped.
498	
499	We safe against physical memory scanners too: the only legitimate way
500	scanner can get reference to a page is get_page_unless_zero().
501	
502	All tail pages have zero ->_refcount until atomic_add(). This prevents the
503	scanner from getting a reference to the tail page up to that point. After the
504	atomic_add() we don't care about the ->_refcount value. We already known how
505	many references should be uncharged from the head page.
506	
507	For head page get_page_unless_zero() will succeed and we don't mind. It's
508	clear where reference should go after split: it will stay on head page.
509	
510	Note that split_huge_pmd() doesn't have any limitation on refcounting:
511	pmd can be split at any point and never fails.
512	
513	== Partial unmap and deferred_split_huge_page() ==
514	
515	Unmapping part of THP (with munmap() or other way) is not going to free
516	memory immediately. Instead, we detect that a subpage of THP is not in use
517	in page_remove_rmap() and queue the THP for splitting if memory pressure
518	comes. Splitting will free up unused subpages.
519	
520	Splitting the page right away is not an option due to locking context in
521	the place where we can detect partial unmap. It's also might be
522	counterproductive since in many cases partial unmap happens during exit(2) if
523	a THP crosses a VMA boundary.
524	
525	Function deferred_split_huge_page() is used to queue page for splitting.
526	The splitting itself will happen when we get memory pressure via shrinker
527	interface.
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