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-Using flexible arrays in the kernel
-Last updated for 2.6.32
-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.
-
-It is also possible to define flexible arrays at compile time with:
-
- DEFINE_FLEX_ARRAY(name, element_size, total);
-
-This macro will result in a definition of an array with the given name; the
-element size and total will be checked for validity at compile time.
-
-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 nr_elements, gfp_t flags);
-
-This function will ensure that memory for the elements indexed in the range
-defined by start and nr_elements 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. Flexible array elements are normally initialized to the
-value FLEX_ARRAY_FREE (defined as 0x6c in <linux/poison.h>), so errors
-involving that number probably result from use of unstored array entries.
-Note that, if array elements are allocated with __GFP_ZERO, they will be
-initialized to zero and this poisoning will not happen.
-
-Individual elements in the array can be cleared with:
-
- int flex_array_clear(struct flex_array *array, unsigned int element_nr);
-
-This function will set the given element to FLEX_ARRAY_FREE and return
-zero. If storage for the indicated element is not allocated for the array,
-flex_array_clear() will return -EINVAL instead. Note that clearing an
-element does not release the storage associated with it; to reduce the
-allocated size of an array, call:
-
- int flex_array_shrink(struct flex_array *array);
-
-The return value will be the number of pages of memory actually freed.
-This function works by scanning the array for pages containing nothing but
-FLEX_ARRAY_FREE bytes, so (1) it can be expensive, and (2) it will not work
-if the array's pages are allocated with __GFP_ZERO.
-
-It is possible to remove all elements of an array 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.