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| -rw-r--r-- | Documentation/filesystems/seq_file.txt | 2 | ||||
| -rw-r--r-- | Documentation/flexible-arrays.txt | 99 |
2 files changed, 100 insertions, 1 deletions
diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt index b843743aa0b..0d15ebccf5b 100644 --- a/Documentation/filesystems/seq_file.txt +++ b/Documentation/filesystems/seq_file.txt @@ -46,7 +46,7 @@ better to do. The file is seekable, in that one can do something like the following: dd if=/proc/sequence of=out1 count=1 - dd if=/proc/sequence skip=1 out=out2 count=1 + dd if=/proc/sequence skip=1 of=out2 count=1 Then concatenate the output files out1 and out2 and get the right result. Yes, it is a thoroughly useless module, but the point is to show diff --git a/Documentation/flexible-arrays.txt b/Documentation/flexible-arrays.txt new file mode 100644 index 00000000000..84eb26808de --- /dev/null +++ b/Documentation/flexible-arrays.txt @@ -0,0 +1,99 @@ +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. |