Hugetlbfs Reservation Overview
------------------------------
Huge pages as described at 'Documentation/vm/hugetlbpage.txt' are typically
preallocated for application use.  These huge pages are instantiated in a
task's address space at page fault time if the VMA indicates huge pages are
to be used.  If no huge page exists at page fault time, the task is sent
a SIGBUS and often dies an unhappy death.  Shortly after huge page support
was added, it was determined that it would be better to detect a shortage
of huge pages at mmap() time.  The idea is that if there were not enough
huge pages to cover the mapping, the mmap() would fail.  This was first
done with a simple check in the code at mmap() time to determine if there
were enough free huge pages to cover the mapping.  Like most things in the
kernel, the code has evolved over time.  However, the basic idea was to
'reserve' huge pages at mmap() time to ensure that huge pages would be
available for page faults in that mapping.  The description below attempts to
describe how huge page reserve processing is done in the v4.10 kernel.


Audience
--------
This description is primarily targeted at kernel developers who are modifying
hugetlbfs code.


The Data Structures
-------------------
resv_huge_pages
	This is a global (per-hstate) count of reserved huge pages.  Reserved
	huge pages are only available to the task which reserved them.
	Therefore, the number of huge pages generally available is computed
	as (free_huge_pages - resv_huge_pages).
Reserve Map
	A reserve map is described by the structure:
	struct resv_map {
		struct kref refs;
		spinlock_t lock;
		struct list_head regions;
		long adds_in_progress;
		struct list_head region_cache;
		long region_cache_count;
	};
	There is one reserve map for each huge page mapping in the system.
	The regions list within the resv_map describes the regions within
	the mapping.  A region is described as:
	struct file_region {
		struct list_head link;
		long from;
		long to;
	};
	The 'from' and 'to' fields of the file region structure are huge page
	indices into the mapping.  Depending on the type of mapping, a
	region in the reserv_map may indicate reservations exist for the
	range, or reservations do not exist.
Flags for MAP_PRIVATE Reservations
	These are stored in the bottom bits of the reservation map pointer.
	#define HPAGE_RESV_OWNER    (1UL << 0) Indicates this task is the
		owner of the reservations associated with the mapping.
	#define HPAGE_RESV_UNMAPPED (1UL << 1) Indicates task originally
		mapping this range (and creating reserves) has unmapped a
		page from this task (the child) due to a failed COW.
Page Flags
	The PagePrivate page flag is used to indicate that a huge page
	reservation must be restored when the huge page is freed.  More
	details will be discussed in the "Freeing huge pages" section.


Reservation Map Location (Private or Shared)
--------------------------------------------
A huge page mapping or segment is either private or shared.  If private,
it is typically only available to a single address space (task).  If shared,
it can be mapped into multiple address spaces (tasks).  The location and
semantics of the reservation map is significantly different for two types
of mappings.  Location differences are:
- For private mappings, the reservation map hangs off the the VMA structure.
  Specifically, vma->vm_private_data.  This reserve map is created at the
  time the mapping (mmap(MAP_PRIVATE)) is created.
- For shared mappings, the reservation map hangs off the inode.  Specifically,
  inode->i_mapping->private_data.  Since shared mappings are always backed
  by files in the hugetlbfs filesystem, the hugetlbfs code ensures each inode
  contains a reservation map.  As a result, the reservation map is allocated
  when the inode is created.


Creating Reservations
---------------------
Reservations are created when a huge page backed shared memory segment is
created (shmget(SHM_HUGETLB)) or a mapping is created via mmap(MAP_HUGETLB).
These operations result in a call to the routine hugetlb_reserve_pages()

int hugetlb_reserve_pages(struct inode *inode,
					long from, long to,
					struct vm_area_struct *vma,
					vm_flags_t vm_flags)

The first thing hugetlb_reserve_pages() does is check for the NORESERVE
flag was specified in either the shmget() or mmap() call.  If NORESERVE
was specified, then this routine returns immediately as no reservation
are desired.

The arguments 'from' and 'to' are huge page indices into the mapping or
underlying file.  For shmget(), 'from' is always 0 and 'to' corresponds to
the length of the segment/mapping.  For mmap(), the offset argument could
be used to specify the offset into the underlying file.  In such a case
the 'from' and 'to' arguments have been adjusted by this offset.

One of the big differences between PRIVATE and SHARED mappings is the way
in which reservations are represented in the reservation map.
- For shared mappings, an entry in the reservation map indicates a reservation
  exists or did exist for the corresponding page.  As reservations are
  consumed, the reservation map is not modified.
- For private mappings, the lack of an entry in the reservation map indicates
  a reservation exists for the corresponding page.  As reservations are
  consumed, entries are added to the reservation map.  Therefore, the
  reservation map can also be used to determine which reservations have
  been consumed.

For private mappings, hugetlb_reserve_pages() creates the reservation map and
hangs it off the VMA structure.  In addition, the HPAGE_RESV_OWNER flag is set
to indicate this VMA owns the reservations.

The reservation map is consulted to determine how many huge page reservations
are needed for the current mapping/segment.  For private mappings, this is
always the value (to - from).  However, for shared mappings it is possible that some reservations may already exist within the range (to - from).  See the
section "Reservation Map Modifications" for details on how this is accomplished.

The mapping may be associated with a subpool.  If so, the subpool is consulted
to ensure there is sufficient space for the mapping.  It is possible that the
subpool has set aside reservations that can be used for the mapping.  See the
section "Subpool Reservations" for more details.

After consulting the reservation map and subpool, the number of needed new
reservations is known.  The routine hugetlb_acct_memory() is called to check
for and take the requested number of reservations.  hugetlb_acct_memory()
calls into routines that potentially allocate and adjust surplus page counts.
However, within those routines the code is simply checking to ensure there
are enough free huge pages to accommodate the reservation.  If there are,
the global reservation count resv_huge_pages is adjusted something like the
following.
	if (resv_needed <= (resv_huge_pages - free_huge_pages))
		resv_huge_pages += resv_needed;
Note that the global lock hugetlb_lock is held when checking and adjusting
these counters.

If there were enough free huge pages and the global count resv_huge_pages
was adjusted, then the reservation map associated with the mapping is
modified to reflect the reservations.  In the case of a shared mapping, a
file_region will exist that includes the range 'from' 'to'.  For private
mappings, no modifications are made to the reservation map as lack of an
entry indicates a reservation exists.

If hugetlb_reserve_pages() was successful, the global reservation count and
reservation map associated with the mapping will be modified as required to
ensure reservations exist for the range 'from' - 'to'.


Consuming Reservations/Allocating a Huge Page
---------------------------------------------
Reservations are consumed when huge pages associated with the reservations
are allocated and instantiated in the corresponding mapping.  The allocation
is performed within the routine alloc_huge_page().
struct page *alloc_huge_page(struct vm_area_struct *vma,
                                    unsigned long addr, int avoid_reserve)
alloc_huge_page is passed a VMA pointer and a virtual address, so it can
consult the reservation map to determine if a reservation exists.  In addition,
alloc_huge_page takes the argument avoid_reserve which indicates reserves
should not be used even if it appears they have been set aside for the
specified address.  The avoid_reserve argument is most often used in the case
of Copy on Write and Page Migration where additional copies of an existing
page are being allocated.

The helper routine vma_needs_reservation() is called to determine if a
reservation exists for the address within the mapping(vma).  See the section
"Reservation Map Helper Routines" for detailed information on what this
routine does.  The value returned from vma_needs_reservation() is generally
0 or 1.  0 if a reservation exists for the address, 1 if no reservation exists.
If a reservation does not exist, and there is a subpool associated with the
mapping the subpool is consulted to determine if it contains reservations.
If the subpool contains reservations, one can be used for this allocation.
However, in every case the avoid_reserve argument overrides the use of
a reservation for the allocation.  After determining whether a reservation
exists and can be used for the allocation, the routine dequeue_huge_page_vma()
is called.  This routine takes two arguments related to reservations:
- avoid_reserve, this is the same value/argument passed to alloc_huge_page()
- chg, even though this argument is of type long only the values 0 or 1 are
  passed to dequeue_huge_page_vma.  If the value is 0, it indicates a
  reservation exists (see the section "Memory Policy and Reservations" for
  possible issues).  If the value is 1, it indicates a reservation does not
  exist and the page must be taken from the global free pool if possible.
The free lists associated with the memory policy of the VMA are searched for
a free page.  If a page is found, the value free_huge_pages is decremented
when the page is removed from the free list.  If there was a reservation
associated with the page, the following adjustments are made:
	SetPagePrivate(page);	/* Indicates allocating this page consumed
				 * a reservation, and if an error is
				 * encountered such that the page must be
				 * freed, the reservation will be restored. */
	resv_huge_pages--;	/* Decrement the global reservation count */
Note, if no huge page can be found that satisfies the VMA's memory policy
an attempt will be made to allocate one using the buddy allocator.  This
brings up the issue of surplus huge pages and overcommit which is beyond
the scope reservations.  Even if a surplus page is allocated, the same
reservation based adjustments as above will be made: SetPagePrivate(page) and
resv_huge_pages--.

After obtaining a new huge page, (page)->private is set to the value of
the subpool associated with the page if it exists.  This will be used for
subpool accounting when the page is freed.

The routine vma_commit_reservation() is then called to adjust the reserve
map based on the consumption of the reservation.  In general, this involves
ensuring the page is represented within a file_region structure of the region
map.  For shared mappings where the the reservation was present, an entry
in the reserve map already existed so no change is made.  However, if there
was no reservation in a shared mapping or this was a private mapping a new
entry must be created.

It is possible that the reserve map could have been changed between the call
to vma_needs_reservation() at the beginning of alloc_huge_page() and the
call to vma_commit_reservation() after the page was allocated.  This would
be possible if hugetlb_reserve_pages was called for the same page in a shared
mapping.  In such cases, the reservation count and subpool free page count
will be off by one.  This rare condition can be identified by comparing the
return value from vma_needs_reservation and vma_commit_reservation.  If such
a race is detected, the subpool and global reserve counts are adjusted to
compensate.  See the section "Reservation Map Helper Routines" for more
information on these routines.


Instantiate Huge Pages
----------------------
After huge page allocation, the page is typically added to the page tables
of the allocating task.  Before this, pages in a shared mapping are added
to the page cache and pages in private mappings are added to an anonymous
reverse mapping.  In both cases, the PagePrivate flag is cleared.  Therefore,
when a huge page that has been instantiated is freed no adjustment is made
to the global reservation count (resv_huge_pages).


Freeing Huge Pages
------------------
Huge page freeing is performed by the routine free_huge_page().  This routine
is the destructor for hugetlbfs compound pages.  As a result, it is only
passed a pointer to the page struct.  When a huge page is freed, reservation
accounting may need to be performed.  This would be the case if the page was
associated with a subpool that contained reserves, or the page is being freed
on an error path where a global reserve count must be restored.

The page->private field points to any subpool associated with the page.
If the PagePrivate flag is set, it indicates the global reserve count should
be adjusted (see the section "Consuming Reservations/Allocating a Huge Page"
for information on how these are set).

The routine first calls hugepage_subpool_put_pages() for the page.  If this
routine returns a value of 0 (which does not equal the value passed 1) it
indicates reserves are associated with the subpool, and this newly free page
must be used to keep the number of subpool reserves above the minimum size.
Therefore, the global resv_huge_pages counter is incremented in this case.

If the PagePrivate flag was set in the page, the global resv_huge_pages counter
will always be incremented.


Subpool Reservations
--------------------
There is a struct hstate associated with each huge page size.  The hstate
tracks all huge pages of the specified size.  A subpool represents a subset
of pages within a hstate that is associated with a mounted hugetlbfs
filesystem.

When a hugetlbfs filesystem is mounted a min_size option can be specified
which indicates the minimum number of huge pages required by the filesystem.
If this option is specified, the number of huge pages corresponding to
min_size are reserved for use by the filesystem.  This number is tracked in
the min_hpages field of a struct hugepage_subpool.  At mount time,
hugetlb_acct_memory(min_hpages) is called to reserve the specified number of
huge pages.  If they can not be reserved, the mount fails.

The routines hugepage_subpool_get/put_pages() are called when pages are
obtained from or released back to a subpool.  They perform all subpool
accounting, and track any reservations associated with the subpool.
hugepage_subpool_get/put_pages are passed the number of huge pages by which
to adjust the subpool 'used page' count (down for get, up for put).  Normally,
they return the same value that was passed or an error if not enough pages
exist in the subpool.

However, if reserves are associated with the subpool a return value less
than the passed value may be returned.  This return value indicates the
number of additional global pool adjustments which must be made.  For example,
suppose a subpool contains 3 reserved huge pages and someone asks for 5.
The 3 reserved pages associated with the subpool can be used to satisfy part
of the request.  But, 2 pages must be obtained from the global pools.  To
relay this information to the caller, the value 2 is returned.  The caller
is then responsible for attempting to obtain the additional two pages from
the global pools.


COW and Reservations
--------------------
Since shared mappings all point to and use the same underlying pages, the
biggest reservation concern for COW is private mappings.  In this case,
two tasks can be pointing at the same previously allocated page.  One task
attempts to write to the page, so a new page must be allocated so that each
task points to its own page.

When the page was originally allocated, the reservation for that page was
consumed.  When an attempt to allocate a new page is made as a result of
COW, it is possible that no free huge pages are free and the allocation
will fail.

When the private mapping was originally created, the owner of the mapping
was noted by setting the HPAGE_RESV_OWNER bit in the pointer to the reservation
map of the owner.  Since the owner created the mapping, the owner owns all
the reservations associated with the mapping.  Therefore, when a write fault
occurs and there is no page available, different action is taken for the owner
and non-owner of the reservation.

In the case where the faulting task is not the owner, the fault will fail and
the task will typically receive a SIGBUS.

If the owner is the faulting task, we want it to succeed since it owned the
original reservation.  To accomplish this, the page is unmapped from the
non-owning task.  In this way, the only reference is from the owning task.
In addition, the HPAGE_RESV_UNMAPPED bit is set in the reservation map pointer
of the non-owning task.  The non-owning task may receive a SIGBUS if it later
faults on a non-present page.  But, the original owner of the
mapping/reservation will behave as expected.


Reservation Map Modifications
-----------------------------
The following low level routines are used to make modifications to a
reservation map.  Typically, these routines are not called directly.  Rather,
a reservation map helper routine is called which calls one of these low level
routines.  These low level routines are fairly well documented in the source
code (mm/hugetlb.c).  These routines are:
long region_chg(struct resv_map *resv, long f, long t);
long region_add(struct resv_map *resv, long f, long t);
void region_abort(struct resv_map *resv, long f, long t);
long region_count(struct resv_map *resv, long f, long t);

Operations on the reservation map typically involve two operations:
1) region_chg() is called to examine the reserve map and determine how
   many pages in the specified range [f, t) are NOT currently represented.

   The calling code performs global checks and allocations to determine if
   there are enough huge pages for the operation to succeed.

2a) If the operation can succeed, region_add() is called to actually modify
    the reservation map for the same range [f, t) previously passed to
    region_chg().
2b) If the operation can not succeed, region_abort is called for the same range
    [f, t) to abort the operation.

Note that this is a two step process where region_add() and region_abort()
are guaranteed to succeed after a prior call to region_chg() for the same
range.  region_chg() is responsible for pre-allocating any data structures
necessary to ensure the subsequent operations (specifically region_add()))
will succeed.

As mentioned above, region_chg() determines the number of pages in the range
which are NOT currently represented in the map.  This number is returned to
the caller.  region_add() returns the number of pages in the range added to
the map.  In most cases, the return value of region_add() is the same as the
return value of region_chg().  However, in the case of shared mappings it is
possible for changes to the reservation map to be made between the calls to
region_chg() and region_add().  In this case, the return value of region_add()
will not match the return value of region_chg().  It is likely that in such
cases global counts and subpool accounting will be incorrect and in need of
adjustment.  It is the responsibility of the caller to check for this condition
and make the appropriate adjustments.

The routine region_del() is called to remove regions from a reservation map.
It is typically called in the following situations:
- When a file in the hugetlbfs filesystem is being removed, the inode will
  be released and the reservation map freed.  Before freeing the reservation
  map, all the individual file_region structures must be freed.  In this case
  region_del is passed the range [0, LONG_MAX).
- When a hugetlbfs file is being truncated.  In this case, all allocated pages
  after the new file size must be freed.  In addition, any file_region entries
  in the reservation map past the new end of file must be deleted.  In this
  case, region_del is passed the range [new_end_of_file, LONG_MAX).
- When a hole is being punched in a hugetlbfs file.  In this case, huge pages
  are removed from the middle of the file one at a time.  As the pages are
  removed, region_del() is called to remove the corresponding entry from the
  reservation map.  In this case, region_del is passed the range
  [page_idx, page_idx + 1).
In every case, region_del() will return the number of pages removed from the
reservation map.  In VERY rare cases, region_del() can fail.  This can only
happen in the hole punch case where it has to split an existing file_region
entry and can not allocate a new structure.  In this error case, region_del()
will return -ENOMEM.  The problem here is that the reservation map will
indicate that there is a reservation for the page.  However, the subpool and
global reservation counts will not reflect the reservation.  To handle this
situation, the routine hugetlb_fix_reserve_counts() is called to adjust the
counters so that they correspond with the reservation map entry that could
not be deleted.

region_count() is called when unmapping a private huge page mapping.  In
private mappings, the lack of a entry in the reservation map indicates that
a reservation exists.  Therefore, by counting the number of entries in the
reservation map we know how many reservations were consumed and how many are
outstanding (outstanding = (end - start) - region_count(resv, start, end)).
Since the mapping is going away, the subpool and global reservation counts
are decremented by the number of outstanding reservations.


Reservation Map Helper Routines
-------------------------------
Several helper routines exist to query and modify the reservation maps.
These routines are only interested with reservations for a specific huge
page, so they just pass in an address instead of a range.  In addition,
they pass in the associated VMA.  From the VMA, the type of mapping (private
or shared) and the location of the reservation map (inode or VMA) can be
determined.  These routines simply call the underlying routines described
in the section "Reservation Map Modifications".  However, they do take into
account the 'opposite' meaning of reservation map entries for private and
shared mappings and hide this detail from the caller.

long vma_needs_reservation(struct hstate *h,
				struct vm_area_struct *vma, unsigned long addr)
This routine calls region_chg() for the specified page.  If no reservation
exists, 1 is returned.  If a reservation exists, 0 is returned.

long vma_commit_reservation(struct hstate *h,
				struct vm_area_struct *vma, unsigned long addr)
This calls region_add() for the specified page.  As in the case of region_chg
and region_add, this routine is to be called after a previous call to
vma_needs_reservation.  It will add a reservation entry for the page.  It
returns 1 if the reservation was added and 0 if not.  The return value should
be compared with the return value of the previous call to
vma_needs_reservation.  An unexpected difference indicates the reservation
map was modified between calls.

void vma_end_reservation(struct hstate *h,
				struct vm_area_struct *vma, unsigned long addr)
This calls region_abort() for the specified page.  As in the case of region_chg
and region_abort, this routine is to be called after a previous call to
vma_needs_reservation.  It will abort/end the in progress reservation add
operation.

long vma_add_reservation(struct hstate *h,
				struct vm_area_struct *vma, unsigned long addr)
This is a special wrapper routine to help facilitate reservation cleanup
on error paths.  It is only called from the routine restore_reserve_on_error().
This routine is used in conjunction with vma_needs_reservation in an attempt
to add a reservation to the reservation map.  It takes into account the
different reservation map semantics for private and shared mappings.  Hence,
region_add is called for shared mappings (as an entry present in the map
indicates a reservation), and region_del is called for private mappings (as
the absence of an entry in the map indicates a reservation).  See the section
"Reservation cleanup in error paths" for more information on what needs to
be done on error paths.


Reservation Cleanup in Error Paths
----------------------------------
As mentioned in the section "Reservation Map Helper Routines", reservation
map modifications are performed in two steps.  First vma_needs_reservation
is called before a page is allocated.  If the allocation is successful,
then vma_commit_reservation is called.  If not, vma_end_reservation is called.
Global and subpool reservation counts are adjusted based on success or failure
of the operation and all is well.

Additionally, after a huge page is instantiated the PagePrivate flag is
cleared so that accounting when the page is ultimately freed is correct.

However, there are several instances where errors are encountered after a huge
page is allocated but before it is instantiated.  In this case, the page
allocation has consumed the reservation and made the appropriate subpool,
reservation map and global count adjustments.  If the page is freed at this
time (before instantiation and clearing of PagePrivate), then free_huge_page
will increment the global reservation count.  However, the reservation map
indicates the reservation was consumed.  This resulting inconsistent state
will cause the 'leak' of a reserved huge page.  The global reserve count will
be  higher than it should and prevent allocation of a pre-allocated page.

The routine restore_reserve_on_error() attempts to handle this situation.  It
is fairly well documented.  The intention of this routine is to restore
the reservation map to the way it was before the page allocation.   In this
way, the state of the reservation map will correspond to the global reservation
count after the page is freed.

The routine restore_reserve_on_error itself may encounter errors while
attempting to restore the reservation map entry.  In this case, it will
simply clear the PagePrivate flag of the page.  In this way, the global
reserve count will not be incremented when the page is freed.  However, the
reservation map will continue to look as though the reservation was consumed.
A page can still be allocated for the address, but it will not use a reserved
page as originally intended.

There is some code (most notably userfaultfd) which can not call
restore_reserve_on_error.  In this case, it simply modifies the PagePrivate
so that a reservation will not be leaked when the huge page is freed.


Reservations and Memory Policy
------------------------------
Per-node huge page lists existed in struct hstate when git was first used
to manage Linux code.  The concept of reservations was added some time later.
When reservations were added, no attempt was made to take memory policy
into account.  While cpusets are not exactly the same as memory policy, this
comment in hugetlb_acct_memory sums up the interaction between reservations
and cpusets/memory policy.
	/*
	 * When cpuset is configured, it breaks the strict hugetlb page
	 * reservation as the accounting is done on a global variable. Such
	 * reservation is completely rubbish in the presence of cpuset because
	 * the reservation is not checked against page availability for the
	 * current cpuset. Application can still potentially OOM'ed by kernel
	 * with lack of free htlb page in cpuset that the task is in.
	 * Attempt to enforce strict accounting with cpuset is almost
	 * impossible (or too ugly) because cpuset is too fluid that
	 * task or memory node can be dynamically moved between cpusets.
	 *
	 * The change of semantics for shared hugetlb mapping with cpuset is
	 * undesirable. However, in order to preserve some of the semantics,
	 * we fall back to check against current free page availability as
	 * a best attempt and hopefully to minimize the impact of changing
	 * semantics that cpuset has.
	 */

Huge page reservations were added to prevent unexpected page allocation
failures (OOM) at page fault time.  However, if an application makes use
of cpusets or memory policy there is no guarantee that huge pages will be
available on the required nodes.  This is true even if there are a sufficient
number of global reservations.


Mike Kravetz, 7 April 2017