= Transparent Hugepage Support =
  
  == Objective ==
  
  Performance critical computing applications dealing with large memory
  working sets are already running on top of libhugetlbfs and in turn
  hugetlbfs. Transparent Hugepage Support is an alternative means of
  using huge pages for the backing of virtual memory with huge pages
  that supports the automatic promotion and demotion of page sizes and
  without the shortcomings of hugetlbfs.

  Currently it only works for anonymous memory mappings and tmpfs/shmem.
  But in the future it can expand to other filesystems.

  
  The reason applications are running faster is because of two
  factors. The first factor is almost completely irrelevant and it's not
  of significant interest because it'll also have the downside of
  requiring larger clear-page copy-page in page faults which is a
  potentially negative effect. The first factor consists in taking a
  single page fault for each 2M virtual region touched by userland (so
  reducing the enter/exit kernel frequency by a 512 times factor). This
  only matters the first time the memory is accessed for the lifetime of
  a memory mapping. The second long lasting and much more important
  factor will affect all subsequent accesses to the memory for the whole
  runtime of the application. The second factor consist of two
  components: 1) the TLB miss will run faster (especially with
  virtualization using nested pagetables but almost always also on bare
  metal without virtualization) and 2) a single TLB entry will be
  mapping a much larger amount of virtual memory in turn reducing the
  number of TLB misses. With virtualization and nested pagetables the
  TLB can be mapped of larger size only if both KVM and the Linux guest
  are using hugepages but a significant speedup already happens if only
  one of the two is using hugepages just because of the fact the TLB
  miss is going to run faster.
  
  == Design ==

  - "graceful fallback": mm components which don't have transparent hugepage
    knowledge fall back to breaking huge pmd mapping into table of ptes and,
    if necessary, split a transparent hugepage. Therefore these components
    can continue working on the regular pages or regular pte mappings.

  
  - if a hugepage allocation fails because of memory fragmentation,
    regular pages should be gracefully allocated instead and mixed in
    the same vma without any failure or significant delay and without
    userland noticing
  
  - if some task quits and more hugepages become available (either
    immediately in the buddy or through the VM), guest physical memory
    backed by regular pages should be relocated on hugepages
    automatically (with khugepaged)
  
  - it doesn't require memory reservation and in turn it uses hugepages
    whenever possible (the only possible reservation here is kernelcore=
    to avoid unmovable pages to fragment all the memory but such a tweak
    is not specific to transparent hugepage support and it's a generic
    feature that applies to all dynamic high order allocations in the
    kernel)

  Transparent Hugepage Support maximizes the usefulness of free memory
  if compared to the reservation approach of hugetlbfs by allowing all
  unused memory to be used as cache or other movable (or even unmovable
  entities). It doesn't require reservation to prevent hugepage
  allocation failures to be noticeable from userland. It allows paging
  and all other advanced VM features to be available on the
  hugepages. It requires no modifications for applications to take
  advantage of it.
  
  Applications however can be further optimized to take advantage of
  this feature, like for example they've been optimized before to avoid
  a flood of mmap system calls for every malloc(4k). Optimizing userland
  is by far not mandatory and khugepaged already can take care of long
  lived page allocations even for hugepage unaware applications that
  deals with large amounts of memory.
  
  In certain cases when hugepages are enabled system wide, application
  may end up allocating more memory resources. An application may mmap a
  large region but only touch 1 byte of it, in that case a 2M page might
  be allocated instead of a 4k page for no good. This is why it's
  possible to disable hugepages system-wide and to only have them inside
  MADV_HUGEPAGE madvise regions.
  
  Embedded systems should enable hugepages only inside madvise regions
  to eliminate any risk of wasting any precious byte of memory and to
  only run faster.
  
  Applications that gets a lot of benefit from hugepages and that don't
  risk to lose memory by using hugepages, should use
  madvise(MADV_HUGEPAGE) on their critical mmapped regions.
  
  == sysfs ==

  Transparent Hugepage Support for anonymous memory can be entirely disabled
  (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
  regions (to avoid the risk of consuming more memory resources) or enabled
  system wide. This can be achieved with one of:

  
  echo always >/sys/kernel/mm/transparent_hugepage/enabled
  echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
  echo never >/sys/kernel/mm/transparent_hugepage/enabled
  
  It's also possible to limit defrag efforts in the VM to generate

  anonymous hugepages in case they're not immediately free to madvise
  regions or to never try to defrag memory and simply fallback to regular
  pages unless hugepages are immediately available. Clearly if we spend CPU
  time to defrag memory, we would expect to gain even more by the fact we
  use hugepages later instead of regular pages. This isn't always

  guaranteed, but it may be more likely in case the allocation is for a
  MADV_HUGEPAGE region.
  
  echo always >/sys/kernel/mm/transparent_hugepage/defrag

  echo defer >/sys/kernel/mm/transparent_hugepage/defrag

  echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
  echo never >/sys/kernel/mm/transparent_hugepage/defrag

  "always" means that an application requesting THP will stall on allocation
  failure and directly reclaim pages and compact memory in an effort to
  allocate a THP immediately. This may be desirable for virtual machines
  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
  available in the near future. It's the responsibility of khugepaged
  to then install the THP pages later.
  
  "madvise" will enter direct reclaim like "always" but only for regions
  that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.
  
  "never" should be self-explanatory.

  By default kernel tries to use huge zero page on read page fault to
  anonymous mapping. It's possible to disable huge zero page by writing 0
  or enable it back by writing 1:

  echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
  echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page

  khugepaged will be automatically started when
  transparent_hugepage/enabled is set to "always" or "madvise, and it'll
  be automatically shutdown if it's set to "never".
  
  khugepaged runs usually at low frequency so while one may not want to
  invoke defrag algorithms synchronously during the page faults, it
  should be worth invoking defrag at least in khugepaged. However it's

  also possible to disable defrag in khugepaged by writing 0 or enable
  defrag in khugepaged by writing 1:

  echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
  echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag

  
  You can also control how many pages khugepaged should scan at each
  pass:
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
  
  and how many milliseconds to wait in khugepaged between each pass (you
  can set this to 0 to run khugepaged at 100% utilization of one core):
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
  
  and how many milliseconds to wait in khugepaged if there's an hugepage
  allocation failure to throttle the next allocation attempt.
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
  
  The khugepaged progress can be seen in the number of pages collapsed:
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
  
  for each pass:
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans

  max_ptes_none specifies how many extra small pages (that are
  not already mapped) can be allocated when collapsing a group
  of small pages into one large page.
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
  
  A higher value leads to use additional memory for programs.
  A lower value leads to gain less thp performance. Value of
  max_ptes_none can waste cpu time very little, you can
  ignore it.

  max_ptes_swap specifies how many pages can be brought in from
  swap when collapsing a group of pages into a transparent huge page.
  
  /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
  
  A higher value can cause excessive swap IO and waste
  memory. A lower value can prevent THPs from being
  collapsed, resulting fewer pages being collapsed into
  THPs, and lower memory access performance.

  == Boot parameter ==
  
  You can change the sysfs boot time defaults of Transparent Hugepage
  Support by passing the parameter "transparent_hugepage=always" or
  "transparent_hugepage=madvise" or "transparent_hugepage=never"
  (without "") to the kernel command line.

  == Hugepages in tmpfs/shmem ==
  
  You can control hugepage allocation policy in tmpfs with mount option
  "huge=". It can have following values:
  
    - "always":
      Attempt to allocate huge pages every time we need a new page;
  
    - "never":
      Do not allocate huge pages;
  
    - "within_size":
      Only allocate huge page if it will be fully within i_size.
      Also respect fadvise()/madvise() hints;
  
    - "advise:
      Only allocate huge pages if requested with fadvise()/madvise();
  
  The default policy is "never".
  
  "mount -o remount,huge= /mountpoint" works fine after mount: remounting
  huge=never will not attempt to break up huge pages at all, just stop more
  from being allocated.
  
  There's also sysfs knob to control hugepage allocation policy for internal
  shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
  is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
  MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
  
  In addition to policies listed above, shmem_enabled allows two further
  values:
  
    - "deny":
      For use in emergencies, to force the huge option off from
      all mounts;
    - "force":
      Force the huge option on for all - very useful for testing;

  == Need of application restart ==

  The transparent_hugepage/enabled values and tmpfs mount option only affect
  future behavior. So to make them effective you need to restart any
  application that could have been using hugepages. This also applies to the
  regions registered in khugepaged.

  == Monitoring usage ==

  The number of anonymous transparent huge pages currently used by the
  system is available by reading the AnonHugePages field in /proc/meminfo.
  To identify what applications are using anonymous transparent huge pages,
  it is necessary to read /proc/PID/smaps and count the AnonHugePages fields
  for each mapping.
  
  The number of file transparent huge pages mapped to userspace is available
  by reading ShmemPmdMapped and ShmemHugePages fields in /proc/meminfo.
  To identify what applications are mapping file  transparent huge pages, it
  is necessary to read /proc/PID/smaps and count the FileHugeMapped 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_file_alloc is incremented every time a file huge page is successfully
  i	allocated.
  
  thp_file_mapped is incremented every time a file huge page is mapped into
  	user address space.

  thp_split_page 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.

  	This action implies splitting all PMD the page mapped with.
  
  thp_split_page_failed is 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
  	queue. This happens when a huge page is partially unmapped and
  	splitting it would free up some memory. Pages on split queue are
  	going to be split under memory pressure.

  thp_split_pmd is incremented every time a PMD split into table of PTEs.
  	This can happen, for instance, when application calls mprotect() or
  	munmap() on part of huge page. It doesn't split huge page, only
  	page table entry.

  thp_zero_page_alloc is incremented every time a huge zero page is
  	successfully allocated. It includes allocations which where
  	dropped due race with other allocation. Note, it doesn't count
  	every map of the huge zero page, only its allocation.
  
  thp_zero_page_alloc_failed is incremented if kernel fails to allocate
  	huge zero page and falls back to using small pages.

  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
  head or tail pages as usual (exactly as they would do on
  hugetlbfs). Most gup users will only care about the actual physical
  address of the page and its temporary pinning to release after the I/O
  is complete, so they won't ever notice the fact the page is huge. But
  if any driver is going to mangle over the page structure of the tail
  page (like for checking page->mapping or other bits that are relevant
  for the head page and not the tail page), it should be updated to jump

  to check head page instead. Taking reference on any head/tail page would
  prevent page from being split by anyone.

  
  NOTE: these aren't new constraints to the GUP API, and they match the
  same constrains that applies to hugetlbfs too, so any driver capable
  of handling GUP on hugetlbfs will also work fine on transparent
  hugepage backed mappings.
  
  In case you can't handle compound pages if they're returned by
  follow_page, the FOLL_SPLIT bit can be specified as parameter to
  follow_page, so that it will split the hugepages before returning
  them. Migration for example passes FOLL_SPLIT as parameter to
  follow_page because it's not hugepage aware and in fact it can't work
  at all on hugetlbfs (but it instead works fine on transparent
  hugepages thanks to FOLL_SPLIT). migration simply can't deal with
  hugepages being returned (as it's not only checking the pfn of the
  page and pinning it during the copy but it pretends to migrate the
  memory in regular page sizes and with regular pte/pmd mappings).
  
  == Optimizing the applications ==
  
  To be guaranteed that the kernel will map a 2M page immediately in any
  memory region, the mmap region has to be hugepage naturally
  aligned. posix_memalign() can provide that guarantee.
  
  == Hugetlbfs ==
  
  You can use hugetlbfs on a kernel that has transparent hugepage
  support enabled just fine as always. No difference can be noted in
  hugetlbfs other than there will be less overall fragmentation. All
  usual features belonging to hugetlbfs are preserved and
  unaffected. libhugetlbfs will also work fine as usual.
  
  == Graceful fallback ==

  Code walking pagetables but unaware about huge pmds can simply call

  split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by

  pmd_offset. It's trivial to make the code transparent hugepage aware

  by just grepping for "pmd_offset" and adding split_huge_pmd where

  missing after pmd_offset returns the pmd. Thanks to the graceful
  fallback design, with a one liner change, you can avoid to write
  hundred if not thousand of lines of complex code to make your code
  hugepage aware.
  
  If you're not walking pagetables but you run into a physical hugepage
  but you can't handle it natively in your code, you can split it by
  calling split_huge_page(page). This is what the Linux VM does before

  it tries to swapout the hugepage for example. split_huge_page() can fail
  if the page is pinned and you must handle this correctly.

  
  Example to make mremap.c transparent hugepage aware with a one liner
  change:
  
  diff --git a/mm/mremap.c b/mm/mremap.c
  --- a/mm/mremap.c
  +++ b/mm/mremap.c
  @@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
  		return NULL;
  
  	pmd = pmd_offset(pud, addr);

  +	split_huge_pmd(vma, pmd, addr);

  	if (pmd_none_or_clear_bad(pmd))
  		return NULL;
  
  == Locking in hugepage aware code ==
  
  We want as much code as possible hugepage aware, as calling

  split_huge_page() or split_huge_pmd() has a cost.

  
  To make pagetable walks huge pmd aware, all you need to do is to call
  pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
  mmap_sem in read (or write) mode to be sure an huge pmd cannot be
  created from under you by khugepaged (khugepaged collapse_huge_page
  takes the mmap_sem in write mode in addition to the anon_vma lock). If
  pmd_trans_huge returns false, you just fallback in the old code
  paths. If instead pmd_trans_huge returns true, you have to take the

  page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
  page table lock will prevent the huge pmd to be converted into a
  regular pmd from under you (split_huge_pmd can run in parallel to the

  pagetable walk). If the second pmd_trans_huge returns false, you

  should just drop the page table lock and fallback to the old code as
  before. Otherwise you can proceed to process the huge pmd and the
  hugepage natively. Once finished you can drop the page table lock.
  
  == Refcounts and transparent huge pages ==
  
  Refcounting on THP is mostly consistent with refcounting on other compound
  pages:

    - get_page()/put_page() and GUP operate in head page's ->_refcount.

    - ->_refcount in tail pages is always zero: get_page_unless_zero() never

      succeed on tail pages.
  
    - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
      on relevant sub-page of the compound page.
  
    - map/unmap of the whole compound page accounted in compound_mapcount

      (stored in first tail page). For file huge pages, we also increment
      ->_mapcount of all sub-pages in order to have race-free detection of
      last unmap of subpages.

  PageDoubleMap() indicates that the page is *possibly* mapped with PTEs.
  
  For anonymous pages PageDoubleMap() also indicates ->_mapcount in all
  subpages is offset up by one. This additional reference is required to
  get race-free detection of unmap of subpages when we have them mapped with
  both PMDs and PTEs.

  
  This is optimization required to lower overhead of per-subpage mapcount
  tracking. The alternative is alter ->_mapcount in all subpages on each
  map/unmap of the whole compound page.

  For anonymous pages, we set PG_double_map when a PMD of the page got split
  for the first time, but still have PMD mapping. The additional references
  go away with last compound_mapcount.
  
  File pages get PG_double_map set on first map of the page with PTE and
  goes away when the page gets evicted from page cache.

  
  split_huge_page internally has to distribute the refcounts in the head

  page to the tail pages before clearing all PG_head/tail bits from the page
  structures. It can be done easily for refcounts taken by page table
  entries. But we don't have enough information on how to distribute any
  additional pins (i.e. from get_user_pages). split_huge_page() fails any
  requests to split pinned huge page: it expects page count to be equal to
  sum of mapcount of all sub-pages plus one (split_huge_page caller must
  have reference for head page).

  split_huge_page uses migration entries to stabilize page->_refcount and

  page->_mapcount of anonymous pages. File pages just got unmapped.

  
  We safe against physical memory scanners too: the only legitimate way
  scanner can get reference to a page is get_page_unless_zero().

  All tail pages have zero ->_refcount until atomic_add(). This prevents the
  scanner from getting a reference to the tail page up to that point. After the
  atomic_add() we don't care about the ->_refcount value.  We already known how
  many references should be uncharged from the head page.

  
  For head page get_page_unless_zero() will succeed and we don't mind. It's
  clear where reference should go after split: it will stay on head page.
  
  Note that split_huge_pmd() doesn't have any limitation on refcounting:
  pmd can be split at any point and never fails.
  
  == Partial unmap and deferred_split_huge_page() ==
  
  Unmapping part of THP (with munmap() or other way) is not going to free
  memory immediately. Instead, we detect that a subpage of THP is not in use
  in page_remove_rmap() and queue the THP for splitting if memory pressure
  comes. Splitting will free up unused subpages.
  
  Splitting the page right away is not an option due to locking context in
  the place where we can detect partial unmap. It's also might be
  counterproductive since in many cases partial unmap unmap happens during
  exit(2) if an THP crosses VMA boundary.
  
  Function deferred_split_huge_page() is used to queue page for splitting.
  The splitting itself will happen when we get memory pressure via shrinker
  interface.