Linux 6.14-rc1

[linux.git] / Documentation / mm / process_addrs.rst

.. SPDX-License-Identifier: GPL-2.0

=================
Process Addresses
=================

.. toctree::
   :maxdepth: 3


Userland memory ranges are tracked by the kernel via Virtual Memory Areas or
'VMA's of type :c:struct:`!struct vm_area_struct`.

Each VMA describes a virtually contiguous memory range with identical
attributes, each described by a :c:struct:`!struct vm_area_struct`
object. Userland access outside of VMAs is invalid except in the case where an
adjacent stack VMA could be extended to contain the accessed address.

All VMAs are contained within one and only one virtual address space, described
by a :c:struct:`!struct mm_struct` object which is referenced by all tasks (that is,
threads) which share the virtual address space. We refer to this as the
:c:struct:`!mm`.

Each mm object contains a maple tree data structure which describes all VMAs
within the virtual address space.

.. note:: An exception to this is the 'gate' VMA which is provided by
          architectures which use :c:struct:`!vsyscall` and is a global static
          object which does not belong to any specific mm.

-------
Locking
-------

The kernel is designed to be highly scalable against concurrent read operations
on VMA **metadata** so a complicated set of locks are required to ensure memory
corruption does not occur.

.. note:: Locking VMAs for their metadata does not have any impact on the memory
          they describe nor the page tables that map them.

Terminology
-----------

* **mmap locks** - Each MM has a read/write semaphore :c:member:`!mmap_lock`
  which locks at a process address space granularity which can be acquired via
  :c:func:`!mmap_read_lock`, :c:func:`!mmap_write_lock` and variants.
* **VMA locks** - The VMA lock is at VMA granularity (of course) which behaves
  as a read/write semaphore in practice. A VMA read lock is obtained via
  :c:func:`!lock_vma_under_rcu` (and unlocked via :c:func:`!vma_end_read`) and a
  write lock via :c:func:`!vma_start_write` (all VMA write locks are unlocked
  automatically when the mmap write lock is released). To take a VMA write lock
  you **must** have already acquired an :c:func:`!mmap_write_lock`.
* **rmap locks** - When trying to access VMAs through the reverse mapping via a
  :c:struct:`!struct address_space` or :c:struct:`!struct anon_vma` object
  (reachable from a folio via :c:member:`!folio->mapping`). VMAs must be stabilised via
  :c:func:`!anon_vma_[try]lock_read` or :c:func:`!anon_vma_[try]lock_write` for
  anonymous memory and :c:func:`!i_mmap_[try]lock_read` or
  :c:func:`!i_mmap_[try]lock_write` for file-backed memory. We refer to these
  locks as the reverse mapping locks, or 'rmap locks' for brevity.

We discuss page table locks separately in the dedicated section below.

The first thing **any** of these locks achieve is to **stabilise** the VMA
within the MM tree. That is, guaranteeing that the VMA object will not be
deleted from under you nor modified (except for some specific fields
described below).

Stabilising a VMA also keeps the address space described by it around.

Lock usage
----------

If you want to **read** VMA metadata fields or just keep the VMA stable, you
must do one of the following:

* Obtain an mmap read lock at the MM granularity via :c:func:`!mmap_read_lock` (or a
  suitable variant), unlocking it with a matching :c:func:`!mmap_read_unlock` when
  you're done with the VMA, *or*
* Try to obtain a VMA read lock via :c:func:`!lock_vma_under_rcu`. This tries to
  acquire the lock atomically so might fail, in which case fall-back logic is
  required to instead obtain an mmap read lock if this returns :c:macro:`!NULL`,
  *or*
* Acquire an rmap lock before traversing the locked interval tree (whether
  anonymous or file-backed) to obtain the required VMA.

If you want to **write** VMA metadata fields, then things vary depending on the
field (we explore each VMA field in detail below). For the majority you must:

* Obtain an mmap write lock at the MM granularity via :c:func:`!mmap_write_lock` (or a
  suitable variant), unlocking it with a matching :c:func:`!mmap_write_unlock` when
  you're done with the VMA, *and*
* Obtain a VMA write lock via :c:func:`!vma_start_write` for each VMA you wish to
  modify, which will be released automatically when :c:func:`!mmap_write_unlock` is
  called.
* If you want to be able to write to **any** field, you must also hide the VMA
  from the reverse mapping by obtaining an **rmap write lock**.

VMA locks are special in that you must obtain an mmap **write** lock **first**
in order to obtain a VMA **write** lock. A VMA **read** lock however can be
obtained without any other lock (:c:func:`!lock_vma_under_rcu` will acquire then
release an RCU lock to lookup the VMA for you).

This constrains the impact of writers on readers, as a writer can interact with
one VMA while a reader interacts with another simultaneously.

.. note:: The primary users of VMA read locks are page fault handlers, which
          means that without a VMA write lock, page faults will run concurrent with
          whatever you are doing.

Examining all valid lock states:

.. table::

   ========= ======== ========= ======= ===== =========== ==========
   mmap lock VMA lock rmap lock Stable? Read? Write most? Write all?
   ========= ======== ========= ======= ===== =========== ==========
   \-        \-       \-        N       N     N           N
   \-        R        \-        Y       Y     N           N
   \-        \-       R/W       Y       Y     N           N
   R/W       \-/R     \-/R/W    Y       Y     N           N
   W         W        \-/R      Y       Y     Y           N
   W         W        W         Y       Y     Y           Y
   ========= ======== ========= ======= ===== =========== ==========

.. warning:: While it's possible to obtain a VMA lock while holding an mmap read lock,
             attempting to do the reverse is invalid as it can result in deadlock - if
             another task already holds an mmap write lock and attempts to acquire a VMA
             write lock that will deadlock on the VMA read lock.

All of these locks behave as read/write semaphores in practice, so you can
obtain either a read or a write lock for each of these.

.. note:: Generally speaking, a read/write semaphore is a class of lock which
          permits concurrent readers. However a write lock can only be obtained
          once all readers have left the critical region (and pending readers
          made to wait).

          This renders read locks on a read/write semaphore concurrent with other
          readers and write locks exclusive against all others holding the semaphore.

VMA fields
^^^^^^^^^^

We can subdivide :c:struct:`!struct vm_area_struct` fields by their purpose, which makes it
easier to explore their locking characteristics:

.. note:: We exclude VMA lock-specific fields here to avoid confusion, as these
          are in effect an internal implementation detail.

.. table:: Virtual layout fields

   ===================== ======================================== ===========
   Field                 Description                              Write lock
   ===================== ======================================== ===========
   :c:member:`!vm_start` Inclusive start virtual address of range mmap write,
                         VMA describes.                           VMA write,
                                                                  rmap write.
   :c:member:`!vm_end`   Exclusive end virtual address of range   mmap write,
                         VMA describes.                           VMA write,
                                                                  rmap write.
   :c:member:`!vm_pgoff` Describes the page offset into the file, mmap write,
                         the original page offset within the      VMA write,
                         virtual address space (prior to any      rmap write.
                         :c:func:`!mremap`), or PFN if a PFN map
                         and the architecture does not support
                         :c:macro:`!CONFIG_ARCH_HAS_PTE_SPECIAL`.
   ===================== ======================================== ===========

These fields describes the size, start and end of the VMA, and as such cannot be
modified without first being hidden from the reverse mapping since these fields
are used to locate VMAs within the reverse mapping interval trees.

.. table:: Core fields

   ============================ ======================================== =========================
   Field                        Description                              Write lock
   ============================ ======================================== =========================
   :c:member:`!vm_mm`           Containing mm_struct.                    None - written once on
                                                                         initial map.
   :c:member:`!vm_page_prot`    Architecture-specific page table         mmap write, VMA write.
                                protection bits determined from VMA
                                flags.
   :c:member:`!vm_flags`        Read-only access to VMA flags describing N/A
                                attributes of the VMA, in union with
                                private writable
                                :c:member:`!__vm_flags`.
   :c:member:`!__vm_flags`      Private, writable access to VMA flags    mmap write, VMA write.
                                field, updated by
                                :c:func:`!vm_flags_*` functions.
   :c:member:`!vm_file`         If the VMA is file-backed, points to a   None - written once on
                                struct file object describing the        initial map.
                                underlying file, if anonymous then
                                :c:macro:`!NULL`.
   :c:member:`!vm_ops`          If the VMA is file-backed, then either   None - Written once on
                                the driver or file-system provides a     initial map by
                                :c:struct:`!struct vm_operations_struct` :c:func:`!f_ops->mmap()`.
                                object describing callbacks to be
                                invoked on VMA lifetime events.
   :c:member:`!vm_private_data` A :c:member:`!void *` field for          Handled by driver.
                                driver-specific metadata.
   ============================ ======================================== =========================

These are the core fields which describe the MM the VMA belongs to and its attributes.

.. table:: Config-specific fields

   ================================= ===================== ======================================== ===============
   Field                             Configuration option  Description                              Write lock
   ================================= ===================== ======================================== ===============
   :c:member:`!anon_name`            CONFIG_ANON_VMA_NAME  A field for storing a                    mmap write,
                                                           :c:struct:`!struct anon_vma_name`        VMA write.
                                                           object providing a name for anonymous
                                                           mappings, or :c:macro:`!NULL` if none
                                                           is set or the VMA is file-backed. The
                                                           underlying object is reference counted
                                                           and can be shared across multiple VMAs
                                                           for scalability.
   :c:member:`!swap_readahead_info`  CONFIG_SWAP           Metadata used by the swap mechanism      mmap read,
                                                           to perform readahead. This field is      swap-specific
                                                           accessed atomically.                     lock.
   :c:member:`!vm_policy`            CONFIG_NUMA           :c:type:`!mempolicy` object which        mmap write,
                                                           describes the NUMA behaviour of the      VMA write.
                                                           VMA. The underlying object is reference
                                                           counted.
   :c:member:`!numab_state`          CONFIG_NUMA_BALANCING :c:type:`!vma_numab_state` object which  mmap read,
                                                           describes the current state of           numab-specific
                                                           NUMA balancing in relation to this VMA.  lock.
                                                           Updated under mmap read lock by
                                                           :c:func:`!task_numa_work`.
   :c:member:`!vm_userfaultfd_ctx`   CONFIG_USERFAULTFD    Userfaultfd context wrapper object of    mmap write,
                                                           type :c:type:`!vm_userfaultfd_ctx`,      VMA write.
                                                           either of zero size if userfaultfd is
                                                           disabled, or containing a pointer
                                                           to an underlying
                                                           :c:type:`!userfaultfd_ctx` object which
                                                           describes userfaultfd metadata.
   ================================= ===================== ======================================== ===============

These fields are present or not depending on whether the relevant kernel
configuration option is set.

.. table:: Reverse mapping fields

   =================================== ========================================= ============================
   Field                               Description                               Write lock
   =================================== ========================================= ============================
   :c:member:`!shared.rb`              A red/black tree node used, if the        mmap write, VMA write,
                                       mapping is file-backed, to place the VMA  i_mmap write.
                                       in the
                                       :c:member:`!struct address_space->i_mmap`
                                       red/black interval tree.
   :c:member:`!shared.rb_subtree_last` Metadata used for management of the       mmap write, VMA write,
                                       interval tree if the VMA is file-backed.  i_mmap write.
   :c:member:`!anon_vma_chain`         List of pointers to both forked/CoW’d     mmap read, anon_vma write.
                                       :c:type:`!anon_vma` objects and
                                       :c:member:`!vma->anon_vma` if it is
                                       non-:c:macro:`!NULL`.
   :c:member:`!anon_vma`               :c:type:`!anon_vma` object used by        When :c:macro:`NULL` and
                                       anonymous folios mapped exclusively to    setting non-:c:macro:`NULL`:
                                       this VMA. Initially set by                mmap read, page_table_lock.
                                       :c:func:`!anon_vma_prepare` serialised
                                       by the :c:macro:`!page_table_lock`. This  When non-:c:macro:`NULL` and
                                       is set as soon as any page is faulted in. setting :c:macro:`NULL`:
                                                                                 mmap write, VMA write,
                                                                                 anon_vma write.
   =================================== ========================================= ============================

These fields are used to both place the VMA within the reverse mapping, and for
anonymous mappings, to be able to access both related :c:struct:`!struct anon_vma` objects
and the :c:struct:`!struct anon_vma` in which folios mapped exclusively to this VMA should
reside.

.. note:: If a file-backed mapping is mapped with :c:macro:`!MAP_PRIVATE` set
          then it can be in both the :c:type:`!anon_vma` and :c:type:`!i_mmap`
          trees at the same time, so all of these fields might be utilised at
          once.

Page tables
-----------

We won't speak exhaustively on the subject but broadly speaking, page tables map
virtual addresses to physical ones through a series of page tables, each of
which contain entries with physical addresses for the next page table level
(along with flags), and at the leaf level the physical addresses of the
underlying physical data pages or a special entry such as a swap entry,
migration entry or other special marker. Offsets into these pages are provided
by the virtual address itself.

In Linux these are divided into five levels - PGD, P4D, PUD, PMD and PTE. Huge
pages might eliminate one or two of these levels, but when this is the case we
typically refer to the leaf level as the PTE level regardless.

.. note:: In instances where the architecture supports fewer page tables than
          five the kernel cleverly 'folds' page table levels, that is stubbing
          out functions related to the skipped levels. This allows us to
          conceptually act as if there were always five levels, even if the
          compiler might, in practice, eliminate any code relating to missing
          ones.

There are four key operations typically performed on page tables:

1. **Traversing** page tables - Simply reading page tables in order to traverse
   them. This only requires that the VMA is kept stable, so a lock which
   establishes this suffices for traversal (there are also lockless variants
   which eliminate even this requirement, such as :c:func:`!gup_fast`).
2. **Installing** page table mappings - Whether creating a new mapping or
   modifying an existing one in such a way as to change its identity. This
   requires that the VMA is kept stable via an mmap or VMA lock (explicitly not
   rmap locks).
3. **Zapping/unmapping** page table entries - This is what the kernel calls
   clearing page table mappings at the leaf level only, whilst leaving all page
   tables in place. This is a very common operation in the kernel performed on
   file truncation, the :c:macro:`!MADV_DONTNEED` operation via
   :c:func:`!madvise`, and others. This is performed by a number of functions
   including :c:func:`!unmap_mapping_range` and :c:func:`!unmap_mapping_pages`.
   The VMA need only be kept stable for this operation.
4. **Freeing** page tables - When finally the kernel removes page tables from a
   userland process (typically via :c:func:`!free_pgtables`) extreme care must
   be taken to ensure this is done safely, as this logic finally frees all page
   tables in the specified range, ignoring existing leaf entries (it assumes the
   caller has both zapped the range and prevented any further faults or
   modifications within it).

.. note:: Modifying mappings for reclaim or migration is performed under rmap
          lock as it, like zapping, does not fundamentally modify the identity
          of what is being mapped.

**Traversing** and **zapping** ranges can be performed holding any one of the
locks described in the terminology section above - that is the mmap lock, the
VMA lock or either of the reverse mapping locks.

That is - as long as you keep the relevant VMA **stable** - you are good to go
ahead and perform these operations on page tables (though internally, kernel
operations that perform writes also acquire internal page table locks to
serialise - see the page table implementation detail section for more details).

When **installing** page table entries, the mmap or VMA lock must be held to
keep the VMA stable. We explore why this is in the page table locking details
section below.

.. warning:: Page tables are normally only traversed in regions covered by VMAs.
             If you want to traverse page tables in areas that might not be
             covered by VMAs, heavier locking is required.
             See :c:func:`!walk_page_range_novma` for details.

**Freeing** page tables is an entirely internal memory management operation and
has special requirements (see the page freeing section below for more details).

.. warning:: When **freeing** page tables, it must not be possible for VMAs
             containing the ranges those page tables map to be accessible via
             the reverse mapping.

             The :c:func:`!free_pgtables` function removes the relevant VMAs
             from the reverse mappings, but no other VMAs can be permitted to be
             accessible and span the specified range.

Lock ordering
-------------

As we have multiple locks across the kernel which may or may not be taken at the
same time as explicit mm or VMA locks, we have to be wary of lock inversion, and
the **order** in which locks are acquired and released becomes very important.

.. note:: Lock inversion occurs when two threads need to acquire multiple locks,
   but in doing so inadvertently cause a mutual deadlock.

   For example, consider thread 1 which holds lock A and tries to acquire lock B,
   while thread 2 holds lock B and tries to acquire lock A.

   Both threads are now deadlocked on each other. However, had they attempted to
   acquire locks in the same order, one would have waited for the other to
   complete its work and no deadlock would have occurred.

The opening comment in :c:macro:`!mm/rmap.c` describes in detail the required
ordering of locks within memory management code:

.. code-block::

  inode->i_rwsem        (while writing or truncating, not reading or faulting)
    mm->mmap_lock
      mapping->invalidate_lock (in filemap_fault)
        folio_lock
          hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share, see hugetlbfs below)
            vma_start_write
              mapping->i_mmap_rwsem
                anon_vma->rwsem
                  mm->page_table_lock or pte_lock
                    swap_lock (in swap_duplicate, swap_info_get)
                      mmlist_lock (in mmput, drain_mmlist and others)
                      mapping->private_lock (in block_dirty_folio)
                          i_pages lock (widely used)
                            lruvec->lru_lock (in folio_lruvec_lock_irq)
                      inode->i_lock (in set_page_dirty's __mark_inode_dirty)
                      bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
                        sb_lock (within inode_lock in fs/fs-writeback.c)
                        i_pages lock (widely used, in set_page_dirty,
                                  in arch-dependent flush_dcache_mmap_lock,
                                  within bdi.wb->list_lock in __sync_single_inode)

There is also a file-system specific lock ordering comment located at the top of
:c:macro:`!mm/filemap.c`:

.. code-block::

  ->i_mmap_rwsem                        (truncate_pagecache)
    ->private_lock                      (__free_pte->block_dirty_folio)
      ->swap_lock                       (exclusive_swap_page, others)
        ->i_pages lock

  ->i_rwsem
    ->invalidate_lock                   (acquired by fs in truncate path)
      ->i_mmap_rwsem                    (truncate->unmap_mapping_range)

  ->mmap_lock
    ->i_mmap_rwsem
      ->page_table_lock or pte_lock     (various, mainly in memory.c)
        ->i_pages lock                  (arch-dependent flush_dcache_mmap_lock)

  ->mmap_lock
    ->invalidate_lock                   (filemap_fault)
      ->lock_page                       (filemap_fault, access_process_vm)

  ->i_rwsem                             (generic_perform_write)
    ->mmap_lock                         (fault_in_readable->do_page_fault)

  bdi->wb.list_lock
    sb_lock                             (fs/fs-writeback.c)
    ->i_pages lock                      (__sync_single_inode)

  ->i_mmap_rwsem
    ->anon_vma.lock                     (vma_merge)

  ->anon_vma.lock
    ->page_table_lock or pte_lock       (anon_vma_prepare and various)

  ->page_table_lock or pte_lock
    ->swap_lock                         (try_to_unmap_one)
    ->private_lock                      (try_to_unmap_one)
    ->i_pages lock                      (try_to_unmap_one)
    ->lruvec->lru_lock                  (follow_page_mask->mark_page_accessed)
    ->lruvec->lru_lock                  (check_pte_range->folio_isolate_lru)
    ->private_lock                      (folio_remove_rmap_pte->set_page_dirty)
    ->i_pages lock                      (folio_remove_rmap_pte->set_page_dirty)
    bdi.wb->list_lock                   (folio_remove_rmap_pte->set_page_dirty)
    ->inode->i_lock                     (folio_remove_rmap_pte->set_page_dirty)
    bdi.wb->list_lock                   (zap_pte_range->set_page_dirty)
    ->inode->i_lock                     (zap_pte_range->set_page_dirty)
    ->private_lock                      (zap_pte_range->block_dirty_folio)

Please check the current state of these comments which may have changed since
the time of writing of this document.

------------------------------
Locking Implementation Details
------------------------------

.. warning:: Locking rules for PTE-level page tables are very different from
             locking rules for page tables at other levels.

Page table locking details
--------------------------

In addition to the locks described in the terminology section above, we have
additional locks dedicated to page tables:

* **Higher level page table locks** - Higher level page tables, that is PGD, P4D
  and PUD each make use of the process address space granularity
  :c:member:`!mm->page_table_lock` lock when modified.

* **Fine-grained page table locks** - PMDs and PTEs each have fine-grained locks
  either kept within the folios describing the page tables or allocated
  separated and pointed at by the folios if :c:macro:`!ALLOC_SPLIT_PTLOCKS` is
  set. The PMD spin lock is obtained via :c:func:`!pmd_lock`, however PTEs are
  mapped into higher memory (if a 32-bit system) and carefully locked via
  :c:func:`!pte_offset_map_lock`.

These locks represent the minimum required to interact with each page table
level, but there are further requirements.

Importantly, note that on a **traversal** of page tables, sometimes no such
locks are taken. However, at the PTE level, at least concurrent page table
deletion must be prevented (using RCU) and the page table must be mapped into
high memory, see below.

Whether care is taken on reading the page table entries depends on the
architecture, see the section on atomicity below.

Locking rules
^^^^^^^^^^^^^

We establish basic locking rules when interacting with page tables:

* When changing a page table entry the page table lock for that page table
  **must** be held, except if you can safely assume nobody can access the page
  tables concurrently (such as on invocation of :c:func:`!free_pgtables`).
* Reads from and writes to page table entries must be *appropriately*
  atomic. See the section on atomicity below for details.
* Populating previously empty entries requires that the mmap or VMA locks are
  held (read or write), doing so with only rmap locks would be dangerous (see
  the warning below).
* As mentioned previously, zapping can be performed while simply keeping the VMA
  stable, that is holding any one of the mmap, VMA or rmap locks.

.. warning:: Populating previously empty entries is dangerous as, when unmapping
             VMAs, :c:func:`!vms_clear_ptes` has a window of time between
             zapping (via :c:func:`!unmap_vmas`) and freeing page tables (via
             :c:func:`!free_pgtables`), where the VMA is still visible in the
             rmap tree. :c:func:`!free_pgtables` assumes that the zap has
             already been performed and removes PTEs unconditionally (along with
             all other page tables in the freed range), so installing new PTE
             entries could leak memory and also cause other unexpected and
             dangerous behaviour.

There are additional rules applicable when moving page tables, which we discuss
in the section on this topic below.

PTE-level page tables are different from page tables at other levels, and there
are extra requirements for accessing them:

* On 32-bit architectures, they may be in high memory (meaning they need to be
  mapped into kernel memory to be accessible).
* When empty, they can be unlinked and RCU-freed while holding an mmap lock or
  rmap lock for reading in combination with the PTE and PMD page table locks.
  In particular, this happens in :c:func:`!retract_page_tables` when handling
  :c:macro:`!MADV_COLLAPSE`.
  So accessing PTE-level page tables requires at least holding an RCU read lock;
  but that only suffices for readers that can tolerate racing with concurrent
  page table updates such that an empty PTE is observed (in a page table that
  has actually already been detached and marked for RCU freeing) while another
  new page table has been installed in the same location and filled with
  entries. Writers normally need to take the PTE lock and revalidate that the
  PMD entry still refers to the same PTE-level page table.
  If the writer does not care whether it is the same PTE-level page table, it
  can take the PMD lock and revalidate that the contents of pmd entry still meet
  the requirements. In particular, this also happens in :c:func:`!retract_page_tables`
  when handling :c:macro:`!MADV_COLLAPSE`.

To access PTE-level page tables, a helper like :c:func:`!pte_offset_map_lock` or
:c:func:`!pte_offset_map` can be used depending on stability requirements.
These map the page table into kernel memory if required, take the RCU lock, and
depending on variant, may also look up or acquire the PTE lock.
See the comment on :c:func:`!__pte_offset_map_lock`.

Atomicity
^^^^^^^^^

Regardless of page table locks, the MMU hardware concurrently updates accessed
and dirty bits (perhaps more, depending on architecture). Additionally, page
table traversal operations in parallel (though holding the VMA stable) and
functionality like GUP-fast locklessly traverses (that is reads) page tables,
without even keeping the VMA stable at all.

When performing a page table traversal and keeping the VMA stable, whether a
read must be performed once and only once or not depends on the architecture
(for instance x86-64 does not require any special precautions).

If a write is being performed, or if a read informs whether a write takes place
(on an installation of a page table entry say, for instance in
:c:func:`!__pud_install`), special care must always be taken. In these cases we
can never assume that page table locks give us entirely exclusive access, and
must retrieve page table entries once and only once.

If we are reading page table entries, then we need only ensure that the compiler
does not rearrange our loads. This is achieved via :c:func:`!pXXp_get`
functions - :c:func:`!pgdp_get`, :c:func:`!p4dp_get`, :c:func:`!pudp_get`,
:c:func:`!pmdp_get`, and :c:func:`!ptep_get`.

Each of these uses :c:func:`!READ_ONCE` to guarantee that the compiler reads
the page table entry only once.

However, if we wish to manipulate an existing page table entry and care about
the previously stored data, we must go further and use an hardware atomic
operation as, for example, in :c:func:`!ptep_get_and_clear`.

Equally, operations that do not rely on the VMA being held stable, such as
GUP-fast (see :c:func:`!gup_fast` and its various page table level handlers like
:c:func:`!gup_fast_pte_range`), must very carefully interact with page table
entries, using functions such as :c:func:`!ptep_get_lockless` and equivalent for
higher level page table levels.

Writes to page table entries must also be appropriately atomic, as established
by :c:func:`!set_pXX` functions - :c:func:`!set_pgd`, :c:func:`!set_p4d`,
:c:func:`!set_pud`, :c:func:`!set_pmd`, and :c:func:`!set_pte`.

Equally functions which clear page table entries must be appropriately atomic,
as in :c:func:`!pXX_clear` functions - :c:func:`!pgd_clear`,
:c:func:`!p4d_clear`, :c:func:`!pud_clear`, :c:func:`!pmd_clear`, and
:c:func:`!pte_clear`.

Page table installation
^^^^^^^^^^^^^^^^^^^^^^^

Page table installation is performed with the VMA held stable explicitly by an
mmap or VMA lock in read or write mode (see the warning in the locking rules
section for details as to why).

When allocating a P4D, PUD or PMD and setting the relevant entry in the above
PGD, P4D or PUD, the :c:member:`!mm->page_table_lock` must be held. This is
acquired in :c:func:`!__p4d_alloc`, :c:func:`!__pud_alloc` and
:c:func:`!__pmd_alloc` respectively.

.. note:: :c:func:`!__pmd_alloc` actually invokes :c:func:`!pud_lock` and
   :c:func:`!pud_lockptr` in turn, however at the time of writing it ultimately
   references the :c:member:`!mm->page_table_lock`.

Allocating a PTE will either use the :c:member:`!mm->page_table_lock` or, if
:c:macro:`!USE_SPLIT_PMD_PTLOCKS` is defined, a lock embedded in the PMD
physical page metadata in the form of a :c:struct:`!struct ptdesc`, acquired by
:c:func:`!pmd_ptdesc` called from :c:func:`!pmd_lock` and ultimately
:c:func:`!__pte_alloc`.

Finally, modifying the contents of the PTE requires special treatment, as the
PTE page table lock must be acquired whenever we want stable and exclusive
access to entries contained within a PTE, especially when we wish to modify
them.

This is performed via :c:func:`!pte_offset_map_lock` which carefully checks to
ensure that the PTE hasn't changed from under us, ultimately invoking
:c:func:`!pte_lockptr` to obtain a spin lock at PTE granularity contained within
the :c:struct:`!struct ptdesc` associated with the physical PTE page. The lock
must be released via :c:func:`!pte_unmap_unlock`.

.. note:: There are some variants on this, such as
   :c:func:`!pte_offset_map_rw_nolock` when we know we hold the PTE stable but
   for brevity we do not explore this.  See the comment for
   :c:func:`!__pte_offset_map_lock` for more details.

When modifying data in ranges we typically only wish to allocate higher page
tables as necessary, using these locks to avoid races or overwriting anything,
and set/clear data at the PTE level as required (for instance when page faulting
or zapping).

A typical pattern taken when traversing page table entries to install a new
mapping is to optimistically determine whether the page table entry in the table
above is empty, if so, only then acquiring the page table lock and checking
again to see if it was allocated underneath us.

This allows for a traversal with page table locks only being taken when
required. An example of this is :c:func:`!__pud_alloc`.

At the leaf page table, that is the PTE, we can't entirely rely on this pattern
as we have separate PMD and PTE locks and a THP collapse for instance might have
eliminated the PMD entry as well as the PTE from under us.

This is why :c:func:`!__pte_offset_map_lock` locklessly retrieves the PMD entry
for the PTE, carefully checking it is as expected, before acquiring the
PTE-specific lock, and then *again* checking that the PMD entry is as expected.

If a THP collapse (or similar) were to occur then the lock on both pages would
be acquired, so we can ensure this is prevented while the PTE lock is held.

Installing entries this way ensures mutual exclusion on write.

Page table freeing
^^^^^^^^^^^^^^^^^^

Tearing down page tables themselves is something that requires significant
care. There must be no way that page tables designated for removal can be
traversed or referenced by concurrent tasks.

It is insufficient to simply hold an mmap write lock and VMA lock (which will
prevent racing faults, and rmap operations), as a file-backed mapping can be
truncated under the :c:struct:`!struct address_space->i_mmap_rwsem` alone.

As a result, no VMA which can be accessed via the reverse mapping (either
through the :c:struct:`!struct anon_vma->rb_root` or the :c:member:`!struct
address_space->i_mmap` interval trees) can have its page tables torn down.

The operation is typically performed via :c:func:`!free_pgtables`, which assumes
either the mmap write lock has been taken (as specified by its
:c:member:`!mm_wr_locked` parameter), or that the VMA is already unreachable.

It carefully removes the VMA from all reverse mappings, however it's important
that no new ones overlap these or any route remain to permit access to addresses
within the range whose page tables are being torn down.

Additionally, it assumes that a zap has already been performed and steps have
been taken to ensure that no further page table entries can be installed between
the zap and the invocation of :c:func:`!free_pgtables`.

Since it is assumed that all such steps have been taken, page table entries are
cleared without page table locks (in the :c:func:`!pgd_clear`, :c:func:`!p4d_clear`,
:c:func:`!pud_clear`, and :c:func:`!pmd_clear` functions.

.. note:: It is possible for leaf page tables to be torn down independent of
          the page tables above it as is done by
          :c:func:`!retract_page_tables`, which is performed under the i_mmap
          read lock, PMD, and PTE page table locks, without this level of care.

Page table moving
^^^^^^^^^^^^^^^^^

Some functions manipulate page table levels above PMD (that is PUD, P4D and PGD
page tables). Most notable of these is :c:func:`!mremap`, which is capable of
moving higher level page tables.

In these instances, it is required that **all** locks are taken, that is
the mmap lock, the VMA lock and the relevant rmap locks.

You can observe this in the :c:func:`!mremap` implementation in the functions
:c:func:`!take_rmap_locks` and :c:func:`!drop_rmap_locks` which perform the rmap
side of lock acquisition, invoked ultimately by :c:func:`!move_page_tables`.

VMA lock internals
------------------

Overview
^^^^^^^^

VMA read locking is entirely optimistic - if the lock is contended or a competing
write has started, then we do not obtain a read lock.

A VMA **read** lock is obtained by :c:func:`!lock_vma_under_rcu`, which first
calls :c:func:`!rcu_read_lock` to ensure that the VMA is looked up in an RCU
critical section, then attempts to VMA lock it via :c:func:`!vma_start_read`,
before releasing the RCU lock via :c:func:`!rcu_read_unlock`.

VMA read locks hold the read lock on the :c:member:`!vma->vm_lock` semaphore for
their duration and the caller of :c:func:`!lock_vma_under_rcu` must release it
via :c:func:`!vma_end_read`.

VMA **write** locks are acquired via :c:func:`!vma_start_write` in instances where a
VMA is about to be modified, unlike :c:func:`!vma_start_read` the lock is always
acquired. An mmap write lock **must** be held for the duration of the VMA write
lock, releasing or downgrading the mmap write lock also releases the VMA write
lock so there is no :c:func:`!vma_end_write` function.

Note that a semaphore write lock is not held across a VMA lock. Rather, a
sequence number is used for serialisation, and the write semaphore is only
acquired at the point of write lock to update this.

This ensures the semantics we require - VMA write locks provide exclusive write
access to the VMA.

Implementation details
^^^^^^^^^^^^^^^^^^^^^^

The VMA lock mechanism is designed to be a lightweight means of avoiding the use
of the heavily contended mmap lock. It is implemented using a combination of a
read/write semaphore and sequence numbers belonging to the containing
:c:struct:`!struct mm_struct` and the VMA.

Read locks are acquired via :c:func:`!vma_start_read`, which is an optimistic
operation, i.e. it tries to acquire a read lock but returns false if it is
unable to do so. At the end of the read operation, :c:func:`!vma_end_read` is
called to release the VMA read lock.

Invoking :c:func:`!vma_start_read` requires that :c:func:`!rcu_read_lock` has
been called first, establishing that we are in an RCU critical section upon VMA
read lock acquisition. Once acquired, the RCU lock can be released as it is only
required for lookup. This is abstracted by :c:func:`!lock_vma_under_rcu` which
is the interface a user should use.

Writing requires the mmap to be write-locked and the VMA lock to be acquired via
:c:func:`!vma_start_write`, however the write lock is released by the termination or
downgrade of the mmap write lock so no :c:func:`!vma_end_write` is required.

All this is achieved by the use of per-mm and per-VMA sequence counts, which are
used in order to reduce complexity, especially for operations which write-lock
multiple VMAs at once.

If the mm sequence count, :c:member:`!mm->mm_lock_seq` is equal to the VMA
sequence count :c:member:`!vma->vm_lock_seq` then the VMA is write-locked. If
they differ, then it is not.

Each time the mmap write lock is released in :c:func:`!mmap_write_unlock` or
:c:func:`!mmap_write_downgrade`, :c:func:`!vma_end_write_all` is invoked which
also increments :c:member:`!mm->mm_lock_seq` via
:c:func:`!mm_lock_seqcount_end`.

This way, we ensure that, regardless of the VMA's sequence number, a write lock
is never incorrectly indicated and that when we release an mmap write lock we
efficiently release **all** VMA write locks contained within the mmap at the
same time.

Since the mmap write lock is exclusive against others who hold it, the automatic
release of any VMA locks on its release makes sense, as you would never want to
keep VMAs locked across entirely separate write operations. It also maintains
correct lock ordering.

Each time a VMA read lock is acquired, we acquire a read lock on the
:c:member:`!vma->vm_lock` read/write semaphore and hold it, while checking that
the sequence count of the VMA does not match that of the mm.

If it does, the read lock fails. If it does not, we hold the lock, excluding
writers, but permitting other readers, who will also obtain this lock under RCU.

Importantly, maple tree operations performed in :c:func:`!lock_vma_under_rcu`
are also RCU safe, so the whole read lock operation is guaranteed to function
correctly.

On the write side, we acquire a write lock on the :c:member:`!vma->vm_lock`
read/write semaphore, before setting the VMA's sequence number under this lock,
also simultaneously holding the mmap write lock.

This way, if any read locks are in effect, :c:func:`!vma_start_write` will sleep
until these are finished and mutual exclusion is achieved.

After setting the VMA's sequence number, the lock is released, avoiding
complexity with a long-term held write lock.

This clever combination of a read/write semaphore and sequence count allows for
fast RCU-based per-VMA lock acquisition (especially on page fault, though
utilised elsewhere) with minimal complexity around lock ordering.

mmap write lock downgrading
---------------------------

When an mmap write lock is held one has exclusive access to resources within the
mmap (with the usual caveats about requiring VMA write locks to avoid races with
tasks holding VMA read locks).

It is then possible to **downgrade** from a write lock to a read lock via
:c:func:`!mmap_write_downgrade` which, similar to :c:func:`!mmap_write_unlock`,
implicitly terminates all VMA write locks via :c:func:`!vma_end_write_all`, but
importantly does not relinquish the mmap lock while downgrading, therefore
keeping the locked virtual address space stable.

An interesting consequence of this is that downgraded locks are exclusive
against any other task possessing a downgraded lock (since a racing task would
have to acquire a write lock first to downgrade it, and the downgraded lock
prevents a new write lock from being obtained until the original lock is
released).

For clarity, we map read (R)/downgraded write (D)/write (W) locks against one
another showing which locks exclude the others:

.. list-table:: Lock exclusivity
   :widths: 5 5 5 5
   :header-rows: 1
   :stub-columns: 1

   * -
     - R
     - D
     - W
   * - R
     - N
     - N
     - Y
   * - D
     - N
     - Y
     - Y
   * - W
     - Y
     - Y
     - Y

Here a Y indicates the locks in the matching row/column are mutually exclusive,
and N indicates that they are not.

Stack expansion
---------------

Stack expansion throws up additional complexities in that we cannot permit there
to be racing page faults, as a result we invoke :c:func:`!vma_start_write` to
prevent this in :c:func:`!expand_downwards` or :c:func:`!expand_upwards`.