Mach provides a so-called external pager mechanism. This mechanism serves to separate managing memory from managing content. Mach does the former while user-space processes do the latter.
To associate a memory object with a portion of a task's address space,
vm map is invoked on a capability designating
the task and passing a reference to the memory object
and the offset at which to install it. (The first time
a task maps an object, Mach sends an initialization message
to the server including a control capability, which it uses
to supply pages to the kernel.) This is essentially
the same as mapping a file into an address space on UNIX
When a task
method and the page to read. The memory manager then fetches
or creates the content as appropriate and supplies it to
Mach using the
Creating and Mapping a Memory Object
The following illustrates the basic idea:
________ / \ | Mach | \________/ /| / |\ \ (C) vm_map / / m_o_ready (E)\ \ (D) memory_object_init / |/ (F) return \ \| ________ ________ / \ -----> / \ | Client | (A) open | Server | \________/ <----- \________/ (B) memory_object
(A) The client sends an
open RPC to the server.
(B) The server creates a memory object (i.e., a port receive right), adds it to the port set that it is listening on and returns a capability (a port send right) to the client.
(C) The client attempts to map the object into its address space using
vm map RPC. It passes a reference to the port that
the server gave it to the vm server (typically Mach).
(D) Since Mach has never seen the object before, it queues a
memory_object_init on the given port along with a send right (the
memory control port) for the manager to use to send messages to the
kernel and also as an authentication mechanism for future
interactions: the port is supplied so that the manager will be able to
identify from which kernel a given
memory_object_* IPC is from.
(E) The server dequeues the message, initializes internal data
structures to manage the mapping and then invokes the
memory_object_ready method on the control object.
(F) The kernel sees that the manager is ready, sets up the appropriate
mappings in the client's address space and then replies to the
vm map RPC indicating success.
There is nothing stopping others from playing the kernel. This is not a security problem: clients must trust the server from whom they obtain memory objects and also the servers with whom they share the object. Multiple memory managers are a reality that should be dealt with gracefully: they are useful for network transparent mappings etc.
Resolving Page Faults
(G) Client ________ resumed / \ | Mach | (A) Fault +----|------+ | \ (B) m_o_request (C) store_read ____|___ \_____|__/ |\ \| ________ _________ / +---\-------+ \ / \ / \ | Client | (F) | Server |<===>| storeio | \________/ m_o_supply \________/ \_________/ (E) return data | ^ | | (D) device_read v | ________ / Device \ | Driver | \________/ | ^ | | v ____________ / Hardware \
(A) The client does a memory access and
memory_object_request method on the associated
capability. (In addition to the page to supply, it also supplies the
control port so that the server can determine which kernel
sent the message.)
(B) The manager dequeues the message. On the Hurd, this is translated
store_read: a function in the libstore library which is used to
transparently manage block devices. The server starts off as
a separate process, however, if the server has the appropriate
permission, the backing object can be contacted directly by the
server. This layer of indirection is desirable when, for instance, a
storeio running as root may want to only permit read only access to a
resource, yet it cannot safely transfer its handle to the client. In
this case, it would proxy the requests.
(C) The storeio server contacts, for instance, a network block device (the NBD server in GNU/Linux), a file, a memory object, etc.to do the read. This could also be a
(D) The device driver allocates anfrom the and reads the data into it. Once all of the operations are complete, the device returns the data to the client unmapping it from its own address space at the same time.
(E) The storeio server transfers the page to the server. The page is still anonymous.
(F) The manager does a
memory_object_supply transferring the page to
the kernel. Only now is the page not considered to be anonymous but
(G) The kernel caches the page, installs it in the client's virtualand finally, resumes the client.
Paging Data Out
Change manager Pager m_o_return store_write \ _________ (B) __(A)__ (C) ________ (D) _______ S | / Default \ / \ / \ / \ W |<=>| Pager |<=>| Mach |==>| server |<=>| storeio |<=> A | \_________/ \________/ \________/ \_______/ P | /
(B) Once the kernel has selected a page that it would like to evict, it changes the manager from the server to the default pager. This way, if the server does not deallocate the page quickly enough, it cannot cause a denial of service: the kernel will just later double page it to swap (the default pager is part of the tcb).
(C) Mach then invokes
memory_object_return method on the control
object. The server is expected to save the page free it in a timely
fashion. The server is not required to send a response to the kernel.
(D) The manager then transfers the data to the storeio server which
eventually sends it to disk. The device driver consumes the memory
doing the equivalent of a
GNU Hurd Usage
Read about the Hurd's I/O path.