Lines Matching +full:write +full:- +full:only
1 .. SPDX-License-Identifier: GPL-2.0
19 All VMAs are contained within one and only one virtual address space, described
31 -------
33 -------
43 -----------
45 * **mmap locks** - Each MM has a read/write semaphore :c:member:`!mmap_lock`
48 * **VMA locks** - The VMA lock is at VMA granularity (of course) which behaves
49 as a read/write semaphore in practice. A VMA read lock is obtained via
51 write lock via :c:func:`!vma_start_write` (all VMA write locks are unlocked
52 automatically when the mmap write lock is released). To take a VMA write lock
54 * **rmap locks** - When trying to access VMAs through the reverse mapping via a
56 (reachable from a folio via :c:member:`!folio->mapping`). VMAs must be stabilised via
59 :c:func:`!i_mmap_[try]lock_write` for file-backed memory. We refer to these
72 ----------
81 acquire the lock atomically so might fail, in which case fall-back logic is
85 anonymous or file-backed) to obtain the required VMA.
87 If you want to **write** VMA metadata fields, then things vary depending on the
90 * Obtain an mmap write lock at the MM granularity via :c:func:`!mmap_write_lock` (or a
93 * Obtain a VMA write lock via :c:func:`!vma_start_write` for each VMA you wish to
96 * If you want to be able to write to **any** field, you must also hide the VMA
97 from the reverse mapping by obtaining an **rmap write lock**.
99 VMA locks are special in that you must obtain an mmap **write** lock **first**
100 in order to obtain a VMA **write** lock. A VMA **read** lock however can be
108 means that without a VMA write lock, page faults will run concurrent with
116 mmap lock VMA lock rmap lock Stable? Read? Write most? Write all?
118 \- \- \- N N N N
119 \- R \- Y Y N N
120 \- \- R/W Y Y N N
121 R/W \-/R \-/R/W Y Y N N
122 W W \-/R Y Y Y N
127 attempting to do the reverse is invalid as it can result in deadlock - if
128 another task already holds an mmap write lock and attempts to acquire a VMA
129 write lock that will deadlock on the VMA read lock.
131 All of these locks behave as read/write semaphores in practice, so you can
132 obtain either a read or a write lock for each of these.
134 .. note:: Generally speaking, a read/write semaphore is a class of lock which
135 permits concurrent readers. However a write lock can only be obtained
139 This renders read locks on a read/write semaphore concurrent with other
140 readers and write locks exclusive against all others holding the semaphore.
148 .. note:: We exclude VMA lock-specific fields here to avoid confusion, as these
154 Field Description Write lock
156 :c:member:`!vm_start` Inclusive start virtual address of range mmap write,
157 VMA describes. VMA write,
158 rmap write.
159 :c:member:`!vm_end` Exclusive end virtual address of range mmap write,
160 VMA describes. VMA write,
161 rmap write.
162 :c:member:`!vm_pgoff` Describes the page offset into the file, mmap write,
163 the original page offset within the VMA write,
164 virtual address space (prior to any rmap write.
177 Field Description Write lock
179 :c:member:`!vm_mm` Containing mm_struct. None - written once on
181 :c:member:`!vm_page_prot` Architecture-specific page table mmap write, VMA write.
184 :c:member:`!vm_flags` Read-only access to VMA flags describing N/A
188 :c:member:`!__vm_flags` Private, writable access to VMA flags mmap write, VMA write.
191 :c:member:`!vm_file` If the VMA is file-backed, points to a None - written once on
195 :c:member:`!vm_ops` If the VMA is file-backed, then either None - Written once on
196 the driver or file-system provides a initial map by
197 :c:struct:`!struct vm_operations_struct` :c:func:`!f_ops->mmap()`.
201 driver-specific metadata.
206 .. table:: Config-specific fields
209 … Configuration option Description Write lock
211 …:`!anon_name` CONFIG_ANON_VMA_NAME A field for storing a mmap write,
212 … :c:struct:`!struct anon_vma_name` VMA write.
215 is set or the VMA is file-backed. The
220 … to perform readahead. This field is swap-specific
222 …:`!vm_policy` CONFIG_NUMA :c:type:`!mempolicy` object which mmap write,
223 … describes the NUMA behaviour of the VMA write.
227 … describes the current state of numab-specific
231 …:`!vm_userfaultfd_ctx` CONFIG_USERFAULTFD Userfaultfd context wrapper object of mmap write,
232 … type :c:type:`!vm_userfaultfd_ctx`, VMA write.
246 Field Description Write lock
248 …c:member:`!shared.rb` A red/black tree node used, if the mmap write, VMA write,
249 mapping is file-backed, to place the VMA i_mmap write.
251 :c:member:`!struct address_space->i_mmap`
253 …c:member:`!shared.rb_subtree_last` Metadata used for management of the mmap write, VMA write,
254 interval tree if the VMA is file-backed. i_mmap write.
255 …mber:`!anon_vma_chain` List of pointers to both forked/CoW’d mmap read, anon_vma write.
257 :c:member:`!vma->anon_vma` if it is
258 non-:c:macro:`!NULL`.
260 … anonymous folios mapped exclusively to setting non-:c:macro:`NULL`:
263 … by the :c:macro:`!page_table_lock`. This When non-:c:macro:`NULL` and
265 … mmap write, VMA write,
266 anon_vma write.
274 .. note:: If a file-backed mapping is mapped with :c:macro:`!MAP_PRIVATE` set
280 -----------
290 In Linux these are divided into five levels - PGD, P4D, PUD, PMD and PTE. Huge
303 1. **Traversing** page tables - Simply reading page tables in order to traverse
304 them. This only requires that the VMA is kept stable, so a lock which
307 2. **Installing** page table mappings - Whether creating a new mapping or
311 3. **Zapping/unmapping** page table entries - This is what the kernel calls
312 clearing page table mappings at the leaf level only, whilst leaving all page
317 The VMA need only be kept stable for this operation.
318 4. **Freeing** page tables - When finally the kernel removes page tables from a
330 locks described in the terminology section above - that is the mmap lock, the
333 That is - as long as you keep the relevant VMA **stable** - you are good to go
336 serialise - see the page table implementation detail section for more details).
342 .. warning:: Page tables are normally only traversed in regions covered by VMAs.
359 -------------
378 .. code-block::
380 inode->i_rwsem (while writing or truncating, not reading or faulting)
381 mm->mmap_lock
382 mapping->invalidate_lock (in filemap_fault)
386 mapping->i_mmap_rwsem
387 anon_vma->rwsem
388 mm->page_table_lock or pte_lock
391 mapping->private_lock (in block_dirty_folio)
393 lruvec->lru_lock (in folio_lruvec_lock_irq)
394 inode->i_lock (in set_page_dirty's __mark_inode_dirty)
395 bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
396 sb_lock (within inode_lock in fs/fs-writeback.c)
398 in arch-dependent flush_dcache_mmap_lock,
399 within bdi.wb->list_lock in __sync_single_inode)
401 There is also a file-system specific lock ordering comment located at the top of
404 .. code-block::
406 ->i_mmap_rwsem (truncate_pagecache)
407 ->private_lock (__free_pte->block_dirty_folio)
408 ->swap_lock (exclusive_swap_page, others)
409 ->i_pages lock
411 ->i_rwsem
412 ->invalidate_lock (acquired by fs in truncate path)
413 ->i_mmap_rwsem (truncate->unmap_mapping_range)
415 ->mmap_lock
416 ->i_mmap_rwsem
417 ->page_table_lock or pte_lock (various, mainly in memory.c)
418 ->i_pages lock (arch-dependent flush_dcache_mmap_lock)
420 ->mmap_lock
421 ->invalidate_lock (filemap_fault)
422 ->lock_page (filemap_fault, access_process_vm)
424 ->i_rwsem (generic_perform_write)
425 ->mmap_lock (fault_in_readable->do_page_fault)
427 bdi->wb.list_lock
428 sb_lock (fs/fs-writeback.c)
429 ->i_pages lock (__sync_single_inode)
431 ->i_mmap_rwsem
432 ->anon_vma.lock (vma_merge)
434 ->anon_vma.lock
435 ->page_table_lock or pte_lock (anon_vma_prepare and various)
437 ->page_table_lock or pte_lock
438 ->swap_lock (try_to_unmap_one)
439 ->private_lock (try_to_unmap_one)
440 ->i_pages lock (try_to_unmap_one)
441 ->lruvec->lru_lock (follow_page_mask->mark_page_accessed)
442 ->lruvec->lru_lock (check_pte_range->folio_isolate_lru)
443 ->private_lock (folio_remove_rmap_pte->set_page_dirty)
444 ->i_pages lock (folio_remove_rmap_pte->set_page_dirty)
445 bdi.wb->list_lock (folio_remove_rmap_pte->set_page_dirty)
446 ->inode->i_lock (folio_remove_rmap_pte->set_page_dirty)
447 bdi.wb->list_lock (zap_pte_range->set_page_dirty)
448 ->inode->i_lock (zap_pte_range->set_page_dirty)
449 ->private_lock (zap_pte_range->block_dirty_folio)
454 ------------------------------
456 ------------------------------
458 .. warning:: Locking rules for PTE-level page tables are very different from
462 --------------------------
467 * **Higher level page table locks** - Higher level page tables, that is PGD, P4D
469 :c:member:`!mm->page_table_lock` lock when modified.
471 * **Fine-grained page table locks** - PMDs and PTEs each have fine-grained locks
475 mapped into higher memory (if a 32-bit system) and carefully locked via
500 held (read or write), doing so with only rmap locks would be dangerous (see
518 PTE-level page tables are different from page tables at other levels, and there
521 * On 32-bit architectures, they may be in high memory (meaning they need to be
523 * When empty, they can be unlinked and RCU-freed while holding an mmap lock or
527 So accessing PTE-level page tables requires at least holding an RCU read lock;
528 but that only suffices for readers that can tolerate racing with concurrent
533 PMD entry still refers to the same PTE-level page table.
534 If the writer does not care whether it is the same PTE-level page table, it
539 To access PTE-level page tables, a helper like :c:func:`!pte_offset_map_lock` or
551 functionality like GUP-fast locklessly traverses (that is reads) page tables,
555 read must be performed once and only once or not depends on the architecture
556 (for instance x86-64 does not require any special precautions).
558 If a write is being performed, or if a read informs whether a write takes place
562 must retrieve page table entries once and only once.
564 If we are reading page table entries, then we need only ensure that the compiler
566 functions - :c:func:`!pgdp_get`, :c:func:`!p4dp_get`, :c:func:`!pudp_get`,
570 the page table entry only once.
577 GUP-fast (see :c:func:`!gup_fast` and its various page table level handlers like
583 by :c:func:`!set_pXX` functions - :c:func:`!set_pgd`, :c:func:`!set_p4d`,
587 as in :c:func:`!pXX_clear` functions - :c:func:`!pgd_clear`,
595 mmap or VMA lock in read or write mode (see the warning in the locking rules
599 PGD, P4D or PUD, the :c:member:`!mm->page_table_lock` must be held. This is
605 references the :c:member:`!mm->page_table_lock`.
607 Allocating a PTE will either use the :c:member:`!mm->page_table_lock` or, if
629 When modifying data in ranges we typically only wish to allocate higher page
636 above is empty, if so, only then acquiring the page table lock and checking
639 This allows for a traversal with page table locks only being taken when
648 PTE-specific lock, and then *again* checking that the PMD entry is as expected.
653 Installing entries this way ensures mutual exclusion on write.
662 It is insufficient to simply hold an mmap write lock and VMA lock (which will
663 prevent racing faults, and rmap operations), as a file-backed mapping can be
664 truncated under the :c:struct:`!struct address_space->i_mmap_rwsem` alone.
667 through the :c:struct:`!struct anon_vma->rb_root` or the :c:member:`!struct
668 address_space->i_mmap` interval trees) can have its page tables torn down.
671 either the mmap write lock has been taken (as specified by its
706 ------------------
711 VMA read locking is entirely optimistic - if the lock is contended or a competing
712 write has started, then we do not obtain a read lock.
719 VMA read locks hold the read lock on the :c:member:`!vma->vm_lock` semaphore for
723 VMA **write** locks are acquired via :c:func:`!vma_start_write` in instances where a
725 acquired. An mmap write lock **must** be held for the duration of the VMA write
726 lock, releasing or downgrading the mmap write lock also releases the VMA write
729 Note that a semaphore write lock is not held across a VMA lock. Rather, a
730 sequence number is used for serialisation, and the write semaphore is only
731 acquired at the point of write lock to update this.
733 This ensures the semantics we require - VMA write locks provide exclusive write
741 read/write semaphore and sequence numbers belonging to the containing
751 read lock acquisition. Once acquired, the RCU lock can be released as it is only
755 Writing requires the mmap to be write-locked and the VMA lock to be acquired via
756 :c:func:`!vma_start_write`, however the write lock is released by the termination or
757 downgrade of the mmap write lock so no :c:func:`!vma_end_write` is required.
759 All this is achieved by the use of per-mm and per-VMA sequence counts, which are
760 used in order to reduce complexity, especially for operations which write-lock
763 If the mm sequence count, :c:member:`!mm->mm_lock_seq` is equal to the VMA
764 sequence count :c:member:`!vma->vm_lock_seq` then the VMA is write-locked. If
767 Each time the mmap write lock is released in :c:func:`!mmap_write_unlock` or
769 also increments :c:member:`!mm->mm_lock_seq` via
772 This way, we ensure that, regardless of the VMA's sequence number, a write lock
773 is never incorrectly indicated and that when we release an mmap write lock we
774 efficiently release **all** VMA write locks contained within the mmap at the
777 Since the mmap write lock is exclusive against others who hold it, the automatic
779 keep VMAs locked across entirely separate write operations. It also maintains
783 :c:member:`!vma->vm_lock` read/write semaphore and hold it, while checking that
793 On the write side, we acquire a write lock on the :c:member:`!vma->vm_lock`
794 read/write semaphore, before setting the VMA's sequence number under this lock,
795 also simultaneously holding the mmap write lock.
801 complexity with a long-term held write lock.
803 This clever combination of a read/write semaphore and sequence count allows for
804 fast RCU-based per-VMA lock acquisition (especially on page fault, though
807 mmap write lock downgrading
808 ---------------------------
810 When an mmap write lock is held one has exclusive access to resources within the
811 mmap (with the usual caveats about requiring VMA write locks to avoid races with
814 It is then possible to **downgrade** from a write lock to a read lock via
816 implicitly terminates all VMA write locks via :c:func:`!vma_end_write_all`, but
822 have to acquire a write lock first to downgrade it, and the downgraded lock
823 prevents a new write lock from being obtained until the original lock is
826 For clarity, we map read (R)/downgraded write (D)/write (W) locks against one
829 .. list-table:: Lock exclusivity
831 :header-rows: 1
832 :stub-columns: 1
834 * -
835 - R
836 - D
837 - W
838 * - R
839 - N
840 - N
841 - Y
842 * - D
843 - N
844 - Y
845 - Y
846 * - W
847 - Y
848 - Y
849 - Y
855 ---------------