On Fri, Nov 1, 2024 at 1:58 PM Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote: > > +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! I'll take a look over the weekend. Quite ironically, I'm currently working on some changes to vm_lock (moving it into vm_area_struct, making vm_area_struct SLAB_TYPESAFE_BY_RCU, etc). So... yeah, your timing is impeccable as usual! > > 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
