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Full Discussion: Kmalloc and malloc
Top Forums UNIX for Dummies Questions & Answers Kmalloc and malloc Post 302463130 by Corona688 on Friday 15th of October 2010 09:31:43 PM
Old 10-15-2010
Quote:
Originally Posted by dragonpoint
If user space heap and kernel space heaps are diff, can the allocated kernel/ user areas be accessed by mapping in either case all times?
Heap isn't a special kind of memory, even to the process. It's all just memory as far as they're concerned. Take a look at The Paging Game to get a rough idea of what virtual memory does.

But it's not exactly direct. User space sees its own memory as a flat memory space but it's not: It's organized in 4K chunks in no particular order and might even be on disk, not in memory, when asked for. It's the kernel's job to keep that mess straight.

The kernel almost never reaches into process space to extract data without being told to do so. That's what read() and write() calls do when you think about it -- tell the kernel what areas of user memory to extract from, or to write into, for some operation or other. Once it's been told, copy_to_user and copy_from_user calls translate memory in and out of user space. Otherwise, the kernel's job is mostly set things up and let them happen in userspace.

Last edited by Corona688; 10-15-2010 at 10:38 PM..
 

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iomap(7)						 Miscellaneous Information Manual						  iomap(7)

NAME
iomap - physical I/O address mapping SYNOPSIS
DESCRIPTION
The mechanism allows the mapping (thus direct access) of physical I/O addresses into the user process address space. For PA-RISC machines, the physical I/O address space begins at and extends to The special (device) files for devices are character special files using the dynamic major number allocation scheme. The minor number for devices is of the form: The physical I/O address is formed by prefixing 0xAAAA with 0xF, and by appending 0x000 (this forces the I/O address to be page-aligned). The size of the region to be mapped is given by the expression M*(2^S) 4K pages. For example, the minor number for a device starting at that occupies 64MB is The driver must be explicitly added to the file, the kernel rebuilt, and the system subsequently rebooted prior to first using I/O space is always mapped with both read and write access rights, regardless of the actual permissions on the device special file. Multiple processes can have concurrently a single device opened and mapped. It is the responsibility of the processes to synchronize their access. Successive calls to to map the same I/O space must be identical to the first mapping. Identical mappings have the same address and size. Note that a process can additionally share I/O space (mapped by with a kernel driver. However, this is only possible if the driver maps in the I/O space with user read/write access rights using the appropriate driver I/O mapping services. Any I/O space mapped by drivers with kernel read/write access rights cannot be concurrently mapped by processes using No or system calls are supported by the driver. The function is used to control the device. The following requests are defined in Map the device into user address space at the location specified by the pointer to which the third argument to points. If the argument points to a variable containing a null pointer, the system selects an appropriate address. then returns the user address where the device was mapped, storing it at the address pointed to by the third argument (see below). Multiple processes can concurrently have the same device mapped. Unmap the device from the user address space. shuts down the file descriptor associated with the device. If the close is for the last system wide open on the device, the device is also unmapped from the user address space; otherwise it is left mapped into the user address space (see above). WARNINGS
Be extremely careful when creating and using devices. Inappropriate accesses to I/O devices or RAM can result in a system crash. ERRORS
The address field was out of range, or the request was invalid. Not enough memory could be allocated for the mapping. Device was already mapped and this mapping was not identical to the initial mapping (same address, size and access rights). Read and write calls are unsupported. No such device at the address specified by the minor number. Required resources for mapping could not be allocated. Inappropriate request for this device type; fildes is not a file descriptor for an device file. EXAMPLES
Consider the following code fragment: ... ... where is an open file descriptor for the device special file and is the address originally requested by the program. If is a null pointer, the system selects a suitable address then returns the selected address in addr. If the value in addr is not a null pointer, it is used as a specified address for allocating memory. If the specified address cannot be used, an error is returned (see SEE ALSO
mknod(1M). OBSOLETED iomap(7)
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