Document the flex_array library.

Using flexible arrays in the kernel

Last updated for 2.6.31

Jonathan Corbet <corbet@lwn.net>

Large contiguous memory allocations can be unreliable in the Linux kernel.

Kernel programmers will sometimes respond to this problem by allocating

pages with vmalloc().  This solution not ideal, though.  On 32-bit systems,

memory from vmalloc() must be mapped into a relatively small address space;

it's easy to run out.  On SMP systems, the page table changes required by

vmalloc() allocations can require expensive cross-processor interrupts on

all CPUs.  And, on all systems, use of space in the vmalloc() range

increases pressure on the translation lookaside buffer (TLB), reducing the

performance of the system.

In many cases, the need for memory from vmalloc() can be eliminated by

piecing together an array from smaller parts; the flexible array library

exists to make this task easier.

A flexible array holds an arbitrary (within limits) number of fixed-sized

objects, accessed via an integer index.  Sparse arrays are handled

reasonably well.  Only single-page allocations are made, so memory

allocation failures should be relatively rare.  The down sides are that the

arrays cannot be indexed directly, individual object size cannot exceed the

system page size, and putting data into a flexible array requires a copy

operation.  It's also worth noting that flexible arrays do no internal

locking at all; if concurrent access to an array is possible, then the

caller must arrange for appropriate mutual exclusion.

The creation of a flexible array is done with:

    #include <linux/flex_array.h>

    struct flex_array *flex_array_alloc(int element_size,

					unsigned int total,

					gfp_t flags);

The individual object size is provided by element_size, while total is the

maximum number of objects which can be stored in the array.  The flags

argument is passed directly to the internal memory allocation calls.  With

the current code, using flags to ask for high memory is likely to lead to

notably unpleasant side effects.

Storing data into a flexible array is accomplished with a call to:

    int flex_array_put(struct flex_array *array, unsigned int element_nr,

    		       void *src, gfp_t flags);

This call will copy the data from src into the array, in the position

indicated by element_nr (which must be less than the maximum specified when

the array was created).  If any memory allocations must be performed, flags

will be used.  The return value is zero on success, a negative error code

otherwise.

There might possibly be a need to store data into a flexible array while

running in some sort of atomic context; in this situation, sleeping in the

memory allocator would be a bad thing.  That can be avoided by using

GFP_ATOMIC for the flags value, but, often, there is a better way.  The

trick is to ensure that any needed memory allocations are done before

entering atomic context, using:

    int flex_array_prealloc(struct flex_array *array, unsigned int start,

			    unsigned int end, gfp_t flags);

This function will ensure that memory for the elements indexed in the range

defined by start and end has been allocated.  Thereafter, a

flex_array_put() call on an element in that range is guaranteed not to

block.

Getting data back out of the array is done with:

    void *flex_array_get(struct flex_array *fa, unsigned int element_nr);

The return value is a pointer to the data element, or NULL if that

particular element has never been allocated.

Note that it is possible to get back a valid pointer for an element which

has never been stored in the array.  Memory for array elements is allocated

one page at a time; a single allocation could provide memory for several

adjacent elements.  The flexible array code does not know if a specific

element has been written; it only knows if the associated memory is

present.  So a flex_array_get() call on an element which was never stored

in the array has the potential to return a pointer to random data.  If the

caller does not have a separate way to know which elements were actually

stored, it might be wise, at least, to add GFP_ZERO to the flags argument

to ensure that all elements are zeroed.

There is no way to remove a single element from the array.  It is possible,

though, to remove all elements with a call to:

    void flex_array_free_parts(struct flex_array *array);

This call frees all elements, but leaves the array itself in place.

Freeing the entire array is done with:

    void flex_array_free(struct flex_array *array);

As of this writing, there are no users of flexible arrays in the mainline

kernel.  The functions described here are also not exported to modules;

that will probably be fixed when somebody comes up with a need for it.