|RFC-0204: VMO Reference Child|
VMO child type for reference tracking.
|Date submitted (year-month-day)||2023-01-04|
|Date reviewed (year-month-day)||2023-01-03|
The objective of this document is to introduce a new VMO child type meant for explicitly tracking references to a VMO.
Filesystems that represent files in memory using VMOs need to track when it is safe to garbage collect these VMOs. This might include activities like flushing out modified VMO contents to disk, detaching the VMO from a userpager service, tearing down the VMO itself, thereby freeing its pages, and also releasing any internal metadata the filesystem might be using to track the open file.
There are several ways that filesystems can share file VMOs with clients. They
can vend out duplicate handles to the VMO, which clients can use to read from
and write to pages in the VMO directly. Clients can also create a VM mapping
using the VMO handle with
zx_vmar_map(), drop the VMO handle, and then
continue accessing the VMO's pages through addresses in the VM mapping.
Filesystems can also create streams using the VMO with
hand out stream handles to clients. A unified reference counting mechanism is
required to track all these various kinds of references.
Blobfs, which is the immutable filesystem that serves executables on Fuchsia,
attempts to solve this reference counting problem by creating Copy-on-Write
ZX_VMO_CHILD_SNAPSHOT_AT_LEAST_ON_WRITE) clones of the file VMO. The only
handle to the file VMO is held by
blobfs, which is used to create CoW clones
each time a file is requested by a client. It is a handle to the clone that
blobfs hands out, not the original VMO. Reference counting then maps to
tracking the number of outstanding clones, and
blobfs can garbage collect when
the clone count drops to zero, by waiting on the
to become active on the original VMO. This works with VM mappings as well
because a VMO will internally be held alive by the mapping even if the VMO
handle is dropped.
Blobfs does not hand out streams today.
This strategy of reference counting using CoW clones works fine for an immutable
filesystem, but breaks when applied to a mutable one like
a mutable filesystem, any changes that are made to a client's reference to a
file should be reflected back in the original file. In other words, any writes
performed in the CoW clone should make it back to the parent VMO. This violates
CoW semantics because the moment a page is written in the CoW clone the kernel
creates a copy of the parent's page in the clone; the clone's changes are never
visible in the parent. There is a similar problem with snapshot clones
ZX_VMO_CHILD_SNAPSHOT) as well. Updates in the child are never visible in the
parent and vice versa.
This leaves us with the
ZX_VMO_CHILD_SLICE VMO child type. On the surface it
might appear that slices are what we need here, since they provide a direct
window to the parent's pages. However, slices have the limitation that they
cannot be created against VMOs that are resizable, and cannot be resized
themselves. A mutable filesystem would want to allow clients to resize its file
VMOs. Therefore all three VMO child types available today fall short in a
reference counting scheme using VMO children.
- firstname.lastname@example.org, email@example.com
- firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
The design was socialized with the Zircon team and discussed in the Kernel Evolution Working Group.
ZX_VMO_CHILD_REFERENCE is a new type of VMO child intended to track
outstanding references to a VMO. The intent is that each time the filesystem
needs a new reference to a file VMO for a client, it will create a reference
child and treat it as the original VMO. Streams would be created against a
reference child instead of the original VMO. Similarly VM mappings would be
created for reference children. This gives the kernel a way to track the number
of references to a VMO by using the existing VMO child tracking mechanism.
Filesystems can use the
ZX_VMO_ZERO_CHILDREN signal to determine when no
references remain and it is safe to destroy the VMO.
Note that reusing the
ZX_VMO_ZERO_CHILDREN signal means that filesystems would
not be able to tear down the original VMO as long as any type of child exists,
not just a reference child. This, however, is fine and allows filesystems to mix
different child types per their requirements. For example,
blobfs can continue
to use CoW clones for data segments where writes need to be restricted to the
client, and use reference children (created read-only with
ZX_VMO_CHILD_NO_WRITE) for text segments. It would still be able to use the
ZX_VMO_ZERO_CHILDREN signal for a unified reference counting scheme.
Reference child creation
zx_vmo_create_child() syscall will support a new option flag,
ZX_VMO_CHILD_REFERENCE. Since this is conceptually a reference to the parent
length parameters are not meaningful, and both will be
required to be set to
0. A reference child will always refer to the entirety
of the parent VMO. An alternative would be to have the caller specify
length as the current size of the VMO, however this places an undue
burden on the caller of always being aware of the parent VMO's size, which might
zx_vmo_get_size() call. In scenarios where filesystem clients are
constantly changing the VMO size (for example, by appending to the file), the
filesystem might be required to make several
zx_vmo_get_size() calls until it
sees a stable size, which is cumbersome.
The same rules governing the
ZX_VMO_ZERO_CHILDREN signal will apply to
reference children as existing child types. The creation of a reference child
will deactivate the signal (if it was previously active), and the destruction of
the last child will activate the signal.
ZX_VMO_CHILD_RESIZABLE will only be permitted if the parent VMO was created
ZX_VMO_RESIZABLE. It is possible to create a non-resizable reference or a
resizable reference of a resizable VMO. However, it is not possible to create a
resizable reference of a non-resizable VMO; a reference child can simply be
thought of as a reference to the parent VMO, and if the parent VMO is not
resizable, there would exist no way for a reference child to be resized.
References will inherit the
ZX_VMO_RESIZABLE option from the parent.
Resizability through the reference handle itself will be determined by a new
ZX_RIGHT_RESIZE right on the handle, which will be added only if the reference
was created with
ZX_VMO_CHILD_RESIZABLE. A non-resizable reference can still
get resized by the parent; the non-resizability here is only restricting the
ability to directly resize through the reference. Separating the
ZX_RIGHT_RESIZE right from the
ZX_VMO_RESIZABLE option flag captures this
Introducing a new
ZX_RIGHT_RESIZE would require integrating it into all
existing VMO APIs where applicable. This can be done by adding it to the default
set of rights on a VMO handle if it is created with the
ZX_VMO_CHILD_RESIZABLE in the case of VMO children.
All VM mappings created for references of a resizable VMO will require the
ZX_VM_ALLOW_FAULTS flag, regardless of whether the reference handle itself
Difference from VMO slices
A lot of the functionality provided by reference children overlaps with
ZX_VMO_CHILD_SLICE. There are a couple of key differences:
- Slices can provide a window to a sub-range in the VMO. References always span the entire VMO.
- Slices cannot be created for resizable VMOs and cannot be resizable themselves. References can be created for resizable VMOs and can be resizable themselves (if the parent was resizable).
Supporting VMO types
Reference children can be created for all VMOs that were created with
zx_pager_create_vmo() and descendants of such VMOs. This
excludes VMOs created with
zx_vmo_create_physical(), however, it does not preclude those use cases
entirely. Both contiguous and physical VMOs are not resizable, and so the user
would be able to create slices over the entire VMO instead to obtain equivalent
Interaction with VMO operations
All VMO operations on references will simply be forwarded to the parent. For example:
- VMO reads and writes will function as if performed directly on the parent VMO.
- Commits and decommits will be forwarded to the parent as well.
- VM mappings when created against reference children will map corresponding pages in the parent.
Creating children of references
The ability to create children of references will be controlled by the same set
of rules as child creation on the parent VMO. For example, if the parent VMO is
a pager-backed VMO and hence does not support
reference child will not support
When a child is created against a reference, the child's
point to the reference, even though the child can conceptually be thought of as
a child of the original VMO. The intent is to accurately represent the creation
chain of a descendant VMO; this behavior is in line with existing behavior of
child creation in the case of nested slices.
Since all pages in the reference refer to pages in the parent VMO, the parent
will continue being attributed for all the pages, as reported by
zx_object_get_info()). A reference does not directly hold
any pages and will always have a page attribution count of zero. This is in line
with the existing model for VMO page attribution where each page is attributed
to exactly one VMO.
Resizing a reference will resize the parent VMO. Similarly, a resize of the parent VMO will be reflected by the reference. References will also follow the content size of the parent VMO, and be able to access and manipulate it similar to the VMO size.
These are the broad changes that will be required in the kernel implementation of VMOs.
- A reference child will point to the same page container (
VmCowPages) as its parent. This has the desired effect of operations on the reference being forwarded to the parent, as they will both be performed against the same internal object.
- References will be tracked in the children list of the parent, allowing the
use of the
ZX_VMO_ZERO_CHILDRENsignal on the parent.
- The parent VMO will also maintain a new list of all reference children. This is required for certain operations like propagating updates to VM mappings, which are tracked outside of the page container.
- References will also share the same
ContentSizeManageras the parent in order to correctly update content sizes, which are used for reads and writes on streams created against the VMO.
- References will not hold the parent VMO alive. Once the parent goes away, references can continue to access the shared page container, which will have been kept alive since the references still point to it. The reference children will be rehomed in the grandparent (if any), as is the case for existing child types. The parent's reference list will be moved to one of the references so that VM mapping updates continue working correctly. All this ensures user observed behavior that is consistent with existing behavior in the case of slices today.
References can be implemented in a few small CLs that add support for the new
child type in VMO kernel internals, and then expose the new type flag through
ZX_RIGHT_RESIZE will be added to the default set of rights for
resizable VMOs at creation time. So enforcement of the right in
zx_vmo_set_size() should not break existing users of resizable VMOs. We can
start simple with requiring both
audit can then be performed to find existing cases where
being removed to prevent resizing, and have those cases remove
instead, or in addition to
ZX_RIGHT_WRITE. In the future,
can check only for
Operations on a reference occur as if performed directly on the parent VMO. This is facilitated by sharing the same internal page container in the kernel. So using references instead of the original VMO should not result in any observable performance impact for most operations. VM microbenchmarks will be written to validate this.
All operations that can be performed on the reference child can be performed on the parent VMO itself, a handle to which is required to create the reference. References are simply going to replace certain usages of the original VMO, so it would have been possible to perform these operations without references too.
Kernel unit tests and core tests will be added for reference children.
zx_vmo_create_child() syscall documentation will be updated with
ZX_VMO_CHILD_REFERENCE. Documentation for other VMO syscalls will need to be
updated to include the
ZX_RIGHT_RESIZE where applicable.
Drawbacks, alternatives, and unknowns
An alternative would be to introduce a generic handle counting scheme which activates a signal on the object when only one handle to the object remains. However, VMO handles are not an accurate representation of the number of outstanding references. It is possible to hold references to the VMO through VM mappings and streams without having to hold on to the VMO handle that was used to create them. We would also need to account for VM mapping, streams, and any new object in the future that might hold a reference to a VMO. This quickly becomes complicated and does not scale. Moreover, handle counts are inherently racy and it is not advisable to use them for purposes besides debugging.
The reference child approach captures the relationship between the filesystem and its clients better, which is in fact hierarchical. The filesystem's reference to the VMO can be viewed as the primary one, and all the other references it hands out to clients are secondary. The parent-child relationship represents this well. On the other hand, handles are symmetric; in this case not only do we care that only one reference remains, we also want that one remaining reference to be the one that the filesystem holds.
We could mint a new reference token object which the filesystem could hand out in addition to the VMO or stream. This could either be a new object that the kernel provides or something that the filesystem implements. However, this would require changing the filesystem API surface to support passing around a new reference token which might not be feasible. Reference children can be used interchangeably with VMOs, which makes supporting them a lot easier while retaining the existing filesystem API. Having an external notion of reference counting outside of the VMO is also bug-prone as it is possible to accidentally drop the token.
The only major way reference children differ from slices is the ability to resize. So an alternative could be to define the semantics of resize in the context of slices, insteading of creating a new VMO child type. Slices can span sub-ranges in the parent however, which makes reasoning about resizing a lot harder.
For example, consider the case where a slice is resized to a size larger than its creation size. It would be surprising if the slice would now uncover parent pages that were not visible to it before. On the other hand, a slice does not own any pages directly, so it would be awkward if it were to fork zero pages in the extended range. There is a similar problem if the parent is resized up to a larger size. Since the motivating use case requires resizes to propagate between parent and child, this would require resizing any slices as well, which again could be surprising.
VMO resizability is controlled by
ZX_RIGHT_WRITE today. Having a separate
ZX_RIGHT_RESIZE affords tighter enforcement opportunities in the future. For
example, it can allow users to create writable non-resizable VMO handles to
share with a client. It is not possible to create such a VMO handle today; a
writable VMO handle is also resizable (if the VMO was created resizable).