Guaranteeing Forward Progress of I/O Operations
Some drivers, such as storage drivers for the system's paging device, must perform at least some of their supported I/O operations without failure, to avoid losing critical system data. One potential cause of a driver failure is a low-memory situation. If the framework or the driver cannot allocate enough memory to handle an I/O request, one or the other might have to fail the I/O request by completing it with an error status value.
In versions of KMDF prior to version 1.9, the framework always fails an I/O request if it cannot allocate a framework request object for an I/O request packet (IRP) that the I/O manager has sent to the driver. To provide drivers the ability to process I/O requests during low-memory situations, versions 1.9 and later of the framework provide a guaranteed forward progress capability for I/O queues.
This capability enables the framework and the driver to pre-allocate memory for sets of request objects and request-related driver context buffers, respectively. The framework and driver use this pre-allocated memory only when the amount of system memory is low.
Features of Guaranteed Forward Progress
By using the framework's guaranteed forward progress for I/O queues, a driver can:
Ask the framework to pre-allocate a set of request objects to use with a specific I/O queue during low-memory situations.
Provide a callback function that pre-allocates request-specific resources that the driver can use when it receives pre-allocated request objects from the framework during low-memory situations.
Provide another callback function that allocates driver-specific resources for an I/O request when a low-memory situation has not been detected. If this callback function's allocation fails because of a low-memory situation, it can indicate whether the framework should use one of its pre-allocated request objects.
Specify which I/O requests require the use of pre-allocated request objects. Options include using pre-allocated objects for all IRPs, using them only if a paging I/O operation is in progress, or having an additional driver callback function examine each IRP to determine whether to use a pre-allocated object.
If your driver implements guaranteed forward progress for one or more of its I/O queues, the driver will be better able to successfully process I/O requests during low-memory situations. You can implement guaranteed forward progress for a device's default I/O queue, and for any I/O queue that your driver configures by calling WdfDeviceConfigureRequestDispatching.
The framework's guaranteed forward progress capability works for your driver only if both your driver and the driver's I/O targets implement guaranteed forward progress. In other words, if a driver implements guaranteed forward progress for a device, all lower-level drivers in the device's driver stack must also implement guaranteed forward progress.
Enabling Guaranteed Forward Progress for an I/O Queue
To enable guaranteed forward progress for an I/O queue, your driver initializes a WDF_IO_QUEUE_FORWARD_PROGRESS_POLICY structure and then calls the WdfIoQueueAssignForwardProgressPolicy method. If the driver calls WdfDeviceConfigureRequestDispatching to configure an I/O queue, it must do so before it calls WdfIoQueueAssignForwardProgressPolicy.
When the driver calls WdfIoQueueAssignForwardProgressPolicy, it can specify the following three event callback functions, all of which are optional:
A driver's EvtIoAllocateResourcesForReservedRequest callback function allocates and stores request-specific resources for request objects that the framework is reserving for low-memory situations.
The framework calls this callback function each time that it creates a reserved request object. The driver should allocate request-specific resources for one I/O request, typically by using the reserved request object's context space.
A driver's EvtIoAllocateRequestResources callback function allocates request-specific resources for immediate use. It is called immediately after the framework has received an IRP and created a request object for the IRP.
If the callback function's attempt to allocate resources fails, the callback function returns an error status value. The framework then deletes the newly created request object and uses one of its reserved request objects. In turn, the driver's request handler uses request-specific resources that its EvtIoAllocateRequestResources callback function previously allocated.
A driver's EvtIoWdmIrpForForwardProgress callback function examines an IRP and tells framework whether to use a reserved request object for the IRP or to fail the I/O request by completing it with an error status value.
The framework calls this callback function only if the framework is unable to create a new request object and you have indicated (by setting a flag in the driver's WDF_IO_QUEUE_FORWARD_PROGRESS_POLICY structure) that you want the driver to examine IRPs during low-memory situations. In other words, your driver can assess each IRP and decide if it is one that must be processed even during low-memory situations.
When your driver calls WdfIoQueueAssignForwardProgressPolicy, it also specifies the number of reserved request objects that you want the framework to pre-allocate for low-memory situations. You can choose the number of request objects that are appropriate for your device and driver. To prevent reduced performance, your driver should typically specify a number that approximates the number of I/O requests that the driver and device can handle in parallel.
However, if your driver's call to WdfIoQueueAssignForwardProgressPolicy and its EvtIoAllocateResourcesForReservedRequest callback function pre-allocate too many reserved request objects or too much request-specific resource memory, your driver can actually contribute to the low-memory situations that you are attempting to handle. You should test the performance of your driver and device, and include low-memory simulations, to determine the best numbers to choose.
Before WdfIoQueueAssignForwardProgressPolicy returns, the framework creates and reserves the number of request objects that the driver has specified. Each time that it reserves a request object, the framework immediately calls the driver's EvtIoAllocateResourcesForReservedRequest callback function so that the driver can allocate and save request-specific resources, in case the framework actually uses the reserved request objects.
When one of the driver's request handlers receives an I/O request from the I/O queue, it can call the WdfRequestIsReserved method to determine whether the request object is one that the framework pre-allocated for low-memory situations. If this method returns TRUE, the driver should use resources that its EvtIoAllocateResourcesForReservedRequest callback function reserved.
If the framework uses one of its reserved request objects, it returns the object to its set of reserved objects after the driver completes the request. The framework saves the request object, and any context space that the driver created by calling WdfDeviceInitSetRequestAttributes or WdfObjectAllocateContext, for reuse if another low-memory situation occurs.
How the Framework and Driver Support Guaranteed Forward Progress
Following are the steps that the driver and framework perform to support guaranteed forward progress for an I/O queue:
The driver calls WdfIoQueueAssignForwardProgressPolicy.
In response, the framework allocates and stores the number of request objects that the driver specifies. If the driver previously called WdfDeviceInitSetRequestAttributes, each allocation includes context space that WdfDeviceInitSetRequestAttributes specified.
In addition, if the driver has provided an EvtIoAllocateResourcesForReservedRequest callback function, the framework calls the callback function each time that it allocates and stores a request object.
The framework receives an I/O request packet (IRP) that the I/O manager is sending to the driver.
The framework attempts to allocate a request object for the IRP. If the I/O queue that the driver created for the request type supports guaranteed forward progress, the next step depends on whether the allocation succeeds or fails:
The request object allocation succeeds.
If the driver provided an EvtIoAllocateRequestResources callback function, the framework calls it. If the callback function returns STATUS_SUCCESS, the framework adds the request to the I/O queue. If the callback function returns an error status value, the framework deletes the request object that it just created and uses one of its pre-allocated request objects. When the driver's request handler receives the request object, it determines whether the request object was pre-allocated and therefore whether it should use the driver's pre-allocated resources.
If the driver did not provide an EvtIoAllocateRequestResources callback function, the framework adds the request to the I/O queue, just as if the driver had not enabled guaranteed forward progress.
The request object allocation fails.
What the framework does next depends on the value that the driver provided for the ForwardProgressReservedPolicy member of the WDF_IO_QUEUE_FORWARD_PROGRESS_POLICY structure. This member informs the framework when to use a reserved request: always, only if the I/O request is a paging I/O operation, or only if the EvtIoWdmIrpForForwardProgress callback function indicates that a reserved request should be used.
In all cases, the driver's request handlers can call WdfRequestIsReserved to determine whether the framework has used a reserved request object. If so, the driver should use the request resources that its EvtIoAllocateResourcesForReservedRequest callback function allocated.
Guaranteed Forward Progress Scenario
You are writing a driver for a storage device that might contain the system's paging file. It is important that read operations from and write operations to the paging file succeed.
You decide to create separate I/O queues for read and write operations, and to enable guaranteed forward progress for both of these I/O queues. You decide to create a third I/O queue for all other request types without enabling guaranteed forward progress.
Your driver stack and device are capable of processing four write operations in parallel, so you set the TotalForwardProgressRequests member of the WDF_IO_QUEUE_FORWARD_PROGRESS_POLICY structure to 4 before calling WdfIoQueueAssignForwardProgressPolicy.
You decide that guaranteeing forward progress is only important if your driver's device is the paging device, so your driver sets the ForwardProgressReservedPolicy member of the WDF_IO_QUEUE_FORWARD_PROGRESS_POLICY structure to WdfIoForwardProgressReservedPolicyPagingIO.
Because your driver requires a framework memory object for each read request and each write request, you decide that your driver should pre-allocate some memory objects to use for its calls to WdfIoTargetFormatRequestForRead and WdfIoTargetFormatRequestForWrite in low-memory situations.
Therefore, the driver provides an EvtIoAllocateResourcesForReservedRequest callback function for the read queue and another one for the write queue. Each time that the framework calls one of these callback functions, the callback function calls WdfMemoryCreate and saves the returned object handle for low-memory situations. Because the callback function receives a handle to a pre-allocated request object, it can parent the memory object to the request object. (A driver for a DMA device might also pre-allocate framework DMA objects.)
The request handlers for the read and write queues must determine whether each received request object is one that the framework reserved for low-memory situations. A request handler can call WdfRequestIsReserved, or it can compare the request object handle with the ones that the EvtIoAllocateResourcesForReservedRequest callback function received previously.
The driver also provides an EvtIoAllocateRequestResources callback function for the read queue and another one for the write queue. The framework calls one of these callback functions when it receives a read or write request from the I/O manager and successfully creates a request object. Each of these callback functions calls WdfMemoryCreate to allocate a memory object for a request. If the allocation fails, the callback function returns an error status value to notify the framework that a low-memory situation has just occurred. The framework, detecting the error return value, deletes the request object that it just created and uses one of its pre-allocated objects.
This driver does not provide an EvtIoWdmIrpForForwardProgress callback function, because it does not need to examine individual read or write IRPs before the framework adds them to an I/O queue.
Remember that when a driver implements guaranteed forward progress for a device, all lower-level drivers in the device's driver stack must also implement guaranteed forward progress.