+cc Suren, linux-doc sorry, forgetting cc's all over this evening... (Friday etc. :) Suren - could you take a look at the VMA lock stuff + check it's sane/correct any mistakes? I generated output from this change and uploaded to my website for review convenience [0]. Thanks! [0] https://ljs.io/output/mm/vma_locks On Fri, Nov 01, 2024 at 06:50:33PM +0000, Lorenzo Stoakes wrote: > Locking around VMAs is complicated and confusing. While we have a number of > disparate comments scattered around the place, we seem to be reaching a > level of complexity that justifies a serious effort at clearly documenting > how locks are expected to be interacted with when it comes to interacting > with mm_struct and vm_area_struct objects. > > This is especially pertinent as regards efforts to find sensible > abstractions for these fundamental objects within the kernel rust > abstraction whose compiler strictly requires some means of expressing these > rules (and through this expression can help self-document these > requirements as well as enforce them which is an exciting concept). > > The document limits scope to mmap and VMA locks and those that are > immediately adjacent and relevant to them - so additionally covers page > table locking as this is so very closely tied to VMA operations (and relies > upon us handling these correctly). > > The document tries to cover some of the nastier and more confusing edge > cases and concerns especially around lock ordering and page table teardown. > > The document also provides some VMA lock internals, which are up to date > and inclusive of recent changes to recent sequence number changes. > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > --- > > REVIEWERS NOTES: > You can speed up doc builds by running `make SPHINXDIRS=mm htmldocs`. I > also uploaded a copy of this to my website at > https://ljs.io/output/mm/vma_locks to make it easier to have a quick > read through. Thanks! > > > Documentation/mm/index.rst | 1 + > Documentation/mm/vma_locks.rst | 527 +++++++++++++++++++++++++++++++++ > 2 files changed, 528 insertions(+) > create mode 100644 Documentation/mm/vma_locks.rst > > diff --git a/Documentation/mm/index.rst b/Documentation/mm/index.rst > index 0be1c7503a01..da5f30acaca5 100644 > --- a/Documentation/mm/index.rst > +++ b/Documentation/mm/index.rst > @@ -64,3 +64,4 @@ documentation, or deleted if it has served its purpose. > vmemmap_dedup > z3fold > zsmalloc > + vma_locks > diff --git a/Documentation/mm/vma_locks.rst b/Documentation/mm/vma_locks.rst > new file mode 100644 > index 000000000000..52b9d484376a > --- /dev/null > +++ b/Documentation/mm/vma_locks.rst > @@ -0,0 +1,527 @@ > +VMA Locking > +=========== > + > +Overview > +-------- > + > +Userland memory ranges are tracked by the kernel via Virtual Memory Areas or > +'VMA's of type `struct vm_area_struct`. > + > +Each VMA describes a virtually contiguous memory range with identical > +attributes, each of which described by a `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 `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 `mm`. > + > +Each mm object contains a maple tree data structure which describes all VMAs > +within the virtual address space. > + > +The kernel is designed to be highly scalable against concurrent access to > +userland memory, so a complicated set of locks are required to ensure no data > +races or memory corruption occurs. > + > +This document explores this locking in detail. > + > +.. note:: > + > + There are three different things that a user might want to achieve via > + locks - the first of which is **stability**. That is - ensuring that the VMA > + won't be freed or modified in any way from underneath us. > + > + All MM and VMA locks ensure stability. > + > + Secondly we have locks which allow **reads** but not writes (and which might > + be held concurrent with other CPUs who also hold the read lock). > + > + Finally, we have locks which permit exclusive access to the VMA to allow for > + **writes** to the VMA. > + > +MM and VMA locks > +---------------- > + > +There are two key classes of lock utilised when reading and manipulating VMAs - > +the `mmap_lock` which is a read/write semaphore maintained at the `mm_struct` > +level of granularity and, if CONFIG_PER_VMA_LOCK is set, a per-VMA lock at the > +VMA level of granularity. > + > +.. 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. > + > +If CONFIG_PER_VMA_LOCK is not set, then things are relatively simple - a write > +mmap lock gives you exclusive write access to a VMA, and a read lock gives you > +concurrent read-only access. > + > +In the presence of CONFIG_PER_VMA_LOCK, i.e. VMA locks, things are more > +complicated. In this instance, a write semaphore is no longer enough to gain > +exclusive access to a VMA, a VMA write lock is also required. > + > +The VMA lock is implemented via the use of both a read/write semaphore and > +per-VMA and per-mm sequence numbers. We go into detail on this in the VMA lock > +internals section below, so for the time being it is important only to note that > +we can obtain either a VMA read or write lock. > + > +.. note:: > + > + VMAs under VMA **read** lock are obtained by the `lock_vma_under_rcu()` > + function, and **no** existing mmap or VMA lock must be held, This function > + either returns a read-locked VMA, or NULL if the lock could not be > + acquired. As the name suggests, the VMA will be acquired under RCU, though > + once obtained, remains stable. > + > + This kind of locking is entirely optimistic - if the lock is contended or a > + competing write has started, then we do not obtain a read lock. > + > + The `lock_vma_under_rcu()` function first calls `rcu_read_lock()` to ensure > + that the VMA is acquired in an RCU critical section, then attempts to VMA > + lock it via `vma_start_read()`, before releasing the RCU lock via > + `rcu_read_unlock()`. > + > + VMA read locks hold the a read lock on the `vma->vm_lock` semaphore for their > + duration and the caller of `lock_vma_under_rcu()` must release it via > + `vma_end_read()`. > + > + VMA **write** locks are acquired via `vma_start_write()` in instances where a > + VMA is about to be modified, unlike `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 `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 (we explore this in detail > + in the VMA lock internals section below). > + > + This ensures the semantics we require - VMA write locks provide exclusive > + write access to the VMA. > + > +Examining all valid lock state and what each implies: > + > +.. list-table:: > + :header-rows: 1 > + > + * - mmap lock > + - VMA lock > + - Stable? > + - Can read safely? > + - Can write safely? > + * - \- > + - \- > + - N > + - N > + - N > + * - R > + - \- > + - Y > + - Y > + - N > + * - \- > + - R > + - Y > + - Y > + - N > + * - W > + - \- > + - Y > + - Y > + - N > + * - W > + - W > + - Y > + - Y > + - Y > + > +Note that there are some exceptions to this - the `anon_vma` field is permitted > +to be written to under mmap read lock and is instead serialised by the `struct > +mm_struct` field `page_table_lock`. In addition the `vm_mm` and all > +lock-specific fields are permitted to be read under RCU alone (though stability cannot > +be expected in this instance). > + > +.. note:: > + The most notable place to use the VMA read lock is on page table faults on > + the x86-64 architecture, which importantly means that without a VMA write > + lock, page faults can race against you even if you hold an mmap write lock. > + > +VMA Fields > +---------- > + > +We examine each field of the `struct vm_area_struct` type in detail in the table > +below. > + > +Reading of each field requires either an mmap read lock or a VMA read lock to be > +held, except where 'unstable RCU read' is specified, in which case unstable > +access to the field is permitted under RCU alone. > + > +The table specifies which write locks must be held to write to the field. > + > +.. list-table:: > + :widths: 20 10 22 5 20 > + :header-rows: 1 > + > + * - Field > + - Config > + - Description > + - Unstable RCU read? > + - Write Lock > + * - vm_start > + - > + - Inclusive start virtual address of range VMA describes. > + - > + - mmap write, VMA write > + * - vm_end > + - > + - Exclusive end virtual address of range VMA describes. > + - > + - mmap write, VMA write > + * - vm_rcu > + - vma lock > + - RCU list head, in union with vma_start, vma_end. RCU implementation detail. > + - N/A > + - N/A > + * - vm_mm > + - > + - Containing mm_struct. > + - Y > + - (Static) > + * - vm_page_prot > + - > + - Architecture-specific page table protection bits determined from VMA > + flags > + - > + - mmap write, VMA write > + * - vm_flags > + - > + - Read-only access to VMA flags describing attributes of VMA, in union with > + private writable `__vm_flags`. > + - > + - N/A > + * - __vm_flags > + - > + - Private, writable access to VMA flags field, updated by vm_flags_*() > + functions. > + - > + - mmap write, VMA write > + * - detached > + - vma lock > + - VMA lock implementation detail - indicates whether the VMA has been > + detached from the tree. > + - Y > + - mmap write, VMA write > + * - vm_lock_seq > + - vma lock > + - VMA lock implementation detail - A sequence number used to serialise the > + VMA lock, see the VMA lock section below. > + - Y > + - mmap write, VMA write > + * - vm_lock > + - vma lock > + - VMA lock implementation detail - A pointer to the VMA lock read/write > + semaphore. > + - Y > + - None required > + * - shared.rb > + - > + - A red/black tree node used, if the mapping is file-backed, to place the > + VMA in the `struct address_space->i_mmap` red/black interval tree. > + - > + - mmap write, VMA write, i_mmap write > + * - shared.rb_subtree_last > + - > + - Metadata used for management of the interval tree if the VMA is > + file-backed. > + - > + - mmap write, VMA write, i_mmap write > + * - anon_vma_chain > + - > + - List of links to forked/CoW'd `anon_vma` objects. > + - > + - mmap read or above, anon_vma write lock > + * - anon_vma > + - > + - `anon_vma` object used by anonymous folios mapped exclusively to this VMA. > + - > + - mmap read or above, page_table_lock > + * - vm_ops > + - > + - If the VMA is file-backed, then either the driver or file-system provides > + a `struct vm_operations_struct` object describing callbacks to be invoked > + on specific VMA lifetime events. > + - > + - (Static) > + * - vm_pgoff > + - > + - Describes the page offset into the file, the original page offset within > + the virtual address space (prior to any `mremap()`), or PFN if a PFN map. > + - > + - mmap write, VMA write > + * - vm_file > + - > + - If the VMA is file-backed, points to a `struct file` object describing > + the underlying file, if anonymous then `NULL`. > + - > + - (Static) > + * - vm_private_data > + - > + - A `void *` field for driver-specific metadata. > + - > + - Driver-mandated. > + * - anon_name > + - anon name > + - A field for storing a `struct anon_vma_name` object providing a name for > + anonymous mappings, or `NULL` if none is set or the VMA is file-backed. > + - > + - mmap write, VMA write > + * - swap_readahead_info > + - swap > + - Metadata used by the swap mechanism to perform readahead. > + - > + - mmap read > + * - vm_region > + - nommu > + - The containing region for the VMA for architectures which do not > + possess an MMU. > + - N/A > + - N/A > + * - vm_policy > + - numa > + - `mempolicy` object which describes NUMA behaviour of the VMA. > + - > + - mmap write, VMA write > + * - numab_state > + - numab > + - `vma_numab_state` object which describes the current state of NUMA > + balancing in relation to this VMA. > + - > + - mmap write, VMA write > + * - vm_userfaultfd_ctx > + - > + - Userfaultfd context wrapper object of type `vm_userfaultfd_ctx`, either > + of zero size if userfaultfd is disabled, or containing a pointer to an > + underlying `userfaultfd_ctx` object which describes userfaultfd metadata. > + - > + - mmap write, VMA write > + > +.. note:: > + > + In the config column 'vma lock' configuration means CONFIG_PER_VMA_LOCK, > + 'anon name' means CONFIG_ANON_VMA_NAME, 'swap' means CONFIG_SWAP, 'nommu' > + means that CONFIG_MMU is not set, 'numa' means CONFIG_NUMA and 'numab' means > + CONFIG_NUMA_BALANCING'. > + > + In the write lock column '(Static)' means that the field is set only once > + upon initialisation of the VMA and not changed after this, the VMA would > + either have been under an mmap write and VMA write lock at the time or not > + yet inserted into any tree. > + > +Page table locks > +---------------- > + > +When allocating a P4D, PUD or PMD and setting the relevant entry in the above > +PGD, P4D or PUD, the `mm->page_table_lock` is acquired to do so. This is > +acquired in `__p4d_alloc()`, `__pud_alloc()` and `__pmd_alloc()` respectively. > + > +.. note:: > + `__pmd_alloc()` actually invokes `pud_lock()` and `pud_lockptr()` in turn, > + however at the time of writing it ultimately references the > + `mm->page_table_lock`. > + > +Allocating a PTE will either use the `mm->page_table_lock` or, if > +`USE_SPLIT_PMD_PTLOCKS` is defined, used a lock embedded in the PMD physical > +page metadata in the form of a `struct ptdesc`, acquired by `pmd_ptdesc()` > +called from `pmd_lock()` and ultimately `__pte_alloc()`. > + > +Finally, modifying the contents of the PTE has special treatment, as this is a > +lock that we must acquire whenever we want stable and exclusive access to > +entries pointing to data pages within a PTE, especially when we wish to modify > +them. > + > +This is performed via `pte_offset_map_lock()` which carefully checks to ensure > +that the PTE hasn't changed from under us, ultimately invoking `pte_lockptr()` > +to obtain a spin lock at PTE granularity contained within the `struct ptdesc` > +associated with the physical PTE page. The lock must be released via > +`pte_unmap_unlock()`. > + > +.. note:: > + There are some variants on this, such as `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 `__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). > + > +Page table teardown > +------------------- > + > +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 `struct address_space` i_mmap_lock alone. > + > +As a result, no VMA which can be accessed via the reverse mapping (either > +anon_vma or the `struct address_space->i_mmap` interval tree) can have its page > +tables torn down. > + > +The operation is typically performed via `free_pgtables()`, which assumes either > +the mmap write lock has been taken (as specified by its `mm_wr_locked` > +parameter), or that it the VMA is fully detached. > + > +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. > + > +As a result of these careful conditions, note that page table entries are > +cleared without page table locks, as it is assumed that all of these precautions > +have already been taken. > + > +mmap write lock downgrading > +--------------------------- > + > +While it is possible to obtain an mmap write or read lock using the > +`mm->mmap_lock` read/write semaphore, it is also possible to **downgrade** from > +a write lock to a read lock via `mmap_write_downgrade()`. > + > +Similar to `mmap_write_unlock()`, this implicitly terminates all VMA write locks > +via `vma_end_write_all()` (more or this behaviour in the VMA lock internals > +section below), but importantly does not relinquish the mmap lock while > +downgrading, therefore keeping the locked virtual address space stable. > + > +A subtlety here is that callers can assume, if they invoke an > +mmap_write_downgrade() operation, that they still have exclusive access to the > +virtual address space (excluding VMA read lock holders), as for another task to > +have downgraded they would have had to have exclusive access to the semaphore > +which can't be the case until the current task completes what it is doing. > + > +Stack expansion > +--------------- > + > +Stack expansion throws up additional complexities in that we cannot permit there > +to be racing page faults, as a result we invoke `vma_start_write()` to prevent > +this in `expand_downwards()` or `expand_upwards()`. > + > +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 `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) > + > +Please check the current state of this comment which may have changed since the > +time of writing of this document. > + > +VMA lock internals > +------------------ > + > +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 `struct > +mm_struct` and the VMA. > + > +Read locks are acquired via `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, `vma_end_read()` is called to > +release the VMA read lock. This can be done under RCU alone. > + > +Writing requires the mmap to be write-locked and the VMA lock to be acquired via > +`vma_start_write()`, however the write lock is released by the termination or > +downgrade of the mmap write lock so no `vma_end_write()` is required. > + > +All this is achieved by the use of per-mm and per-VMA sequence counts. This is > +used to reduce complexity, and potential especially around operations which > +write-lock multiple VMAs at once. > + > +If the mm sequence count, `mm->mm_lock_seq` is equal to the VMA sequence count > +`vma->vm_lock_seq` then the VMA is write-locked. If they differ, then they are > +not. > + > +Each time an mmap write lock is acquired in `mmap_write_lock()`, > +`mmap_write_lock_nested()`, `mmap_write_lock_killable()`, the `mm->mm_lock_seq` > +sequence number is incremented via `mm_lock_seqcount_begin()`. > + > +Each time the mmap write lock is released in `mmap_write_unlock()` or > +`mmap_write_downgrade()`, `vma_end_write_all()` is invoked which also increments > +`mm->mm_lock_seq` via `mm_lock_seqcount_end()`. > + > +This way, we ensure regardless of the VMA's sequence number count, that a write > +lock is not incorrectly indicated (since we increment the sequence counter on > +acquiring the mmap write lock, which is required in order to obtain a VMA write > +lock), and that when we release an mmap write lock, we efficiently release > +**all** VMA write locks contained within the mmap at the same time. > + > +The exclusivity of the mmap write lock ensures this is what we want, as there > +would never be a reason to persist per-VMA write locks across multiple mmap > +write lock acquisitions. > + > +Each time a VMA read lock is acquired, we acquire a read lock on the > +`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 `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 `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, `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 x86-64 page fault, though > +utilised elsewhere) with minimal complexity around lock ordering. > -- > 2.47.0
