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Subject[RFC][PATCH] flexible array implementation v3
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Changes from v2:
- renamed some of the index functions
- added preallocation function
- added flex_array_free_parts() for use with
statically allocated bases
- killed append() function

Changes from v1:
- to vs too typo
- added __check_part_and_nr() and gave it a warning
- fixed off-by-one check on __nr_part_ptrs()
- added FLEX_ARRAY_INIT() macro
- some kerneldoc comments about the capacity
with various sized objects
- comments to note lack of locking semantice

--

Once a structure goes over PAGE_SIZE*2, we see occasional
allocation failures. Some people have chosen to switch
over to things like vmalloc() that will let them keep
array-like access to such a large structures. But,
vmalloc() has plenty of downsides.

Here's an alternative. I think it's what Andrew was
suggesting here:

http://lkml.org/lkml/2009/7/2/518

I call it a flexible array. It does all of its work in
PAGE_SIZE bits, so never does an order>0 allocation.
The base level has PAGE_SIZE-2*sizeof(int) bytes of
storage for pointers to the second level. So, with a
32-bit arch, you get about 4MB (4183112 bytes) of total
storage when the objects pack nicely into a page. It
is half that on 64-bit because the pointers are twice
the size. There's a table detailing this in the code.

There are kerneldocs for the functions, but here's an
overview:

flex_array_alloc() - dynamically allocate a base structure
flex_array_free() - free the array and all of the
second-level pages
flex_array_free_parts() - free the second-level pages, but
not the base (for static bases)
flex_array_put() - copy into the array at the given index
flex_array_get() - copy out of the array at the given index
flex_array_prealloc() - preallocate the second-level pages
between the given indexes to
guarantee no allocs will occur at
put() time.

We could also potentially just pass the "element_size"
into each of the API functions instead of storing it
internally. That would get us one more base pointer
on 64-bit.

The last improvement that I thought about was letting
the individual array members span pages. In this
implementation, if you have a 2049-byte object, it
will only pack one of them into each "part" with
no attempt to pack them. At this point, I don't think
the added complexity would be worth it.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
---
linux-2.6.git-dave/include/linux/flex_array.h | 45 ++++
linux-2.6.git-dave/lib/Makefile | 2
linux-2.6.git-dave/lib/flex_array.c | 240 ++++++++++++++++++++++++++
3 files changed, 286 insertions(+), 1 deletion(-)
diff -puN /dev/null include/linux/flex_array.h
--- /dev/null 2008-09-02 09:40:19.000000000 -0700
+++ linux-2.6.git-dave/include/linux/flex_array.h 2009-07-22 10:52:04.000000000 -0700
@@ -0,0 +1,45 @@
+#ifndef _FLEX_ARRAY_H
+#define _FLEX_ARRAY_H
+
+#include <linux/types.h>
+#include <asm/page.h>
+
+#define FLEX_ARRAY_PART_SIZE PAGE_SIZE
+#define FLEX_ARRAY_BASE_SIZE PAGE_SIZE
+
+struct flex_array_part;
+
+/*
+ * This is meant to replace cases where an array-like
+ * structure has gotten to big too fit into kmalloc()
+ * and the developer is getting tempted to use
+ * vmalloc().
+ */
+
+struct flex_array {
+ union {
+ struct {
+ int element_size;
+ struct flex_array_part *parts[0];
+ };
+ /*
+ * This little trick makes sure that
+ * sizeof(flex_array) == PAGE_SIZE
+ */
+ char padding[FLEX_ARRAY_BASE_SIZE];
+ };
+};
+
+#define FLEX_ARRAY_INIT(size, total) { { {\
+ .element_size = (size), \
+ .nr_elements = 0, \
+} } }
+
+struct flex_array *flex_array_alloc(int element_size, int total, gfp_t flags);
+int flex_array_prealloc(struct flex_array *fa, int start, int end, gfp_t flags);
+void flex_array_free(struct flex_array *fa);
+void flex_array_free_parts(struct flex_array *fa);
+int flex_array_put(struct flex_array *fa, int element_nr, void *src, gfp_t flags);
+void *flex_array_get(struct flex_array *fa, int element_nr);
+
+#endif /* _FLEX_ARRAY_H */
diff -puN /dev/null lib/flex_array.c
--- /dev/null 2008-09-02 09:40:19.000000000 -0700
+++ linux-2.6.git-dave/lib/flex_array.c 2009-07-22 10:51:23.000000000 -0700
@@ -0,0 +1,240 @@
+/*
+ * Flexible array managed in PAGE_SIZE parts
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright IBM Corporation, 2009
+ *
+ * Author: Dave Hansen <dave@linux.vnet.ibm.com>
+ */
+
+#include <linux/flex_array.h>
+#include <linux/slab.h>
+#include <linux/stddef.h>
+
+struct flex_array_part {
+ char elements[FLEX_ARRAY_PART_SIZE];
+};
+
+static inline int __elements_per_part(int element_size)
+{
+ return FLEX_ARRAY_PART_SIZE / element_size;
+}
+
+static inline int nr_base_part_ptrs(void)
+{
+ int element_offset = offsetof(struct flex_array, parts);
+ int bytes_left = FLEX_ARRAY_BASE_SIZE - element_offset;
+ return bytes_left / sizeof(struct flex_array_part *);
+}
+
+/**
+ * flex_array_alloc - allocate a new flexible array
+ * @element_size: the size of individual elements in the array
+ * @total: total number of elements that this should hold
+ *
+ * Note: all locking must be provided by the caller.
+ *
+ * We do not actually use @total to size the allocation at this
+ * point. It is just used to ensure that the user does not try
+ * to use this structure for something larger than it can handle
+ * later on.
+ *
+ * The maximum number of elements is defined as: the number of
+ * elements that can be stored in a page times the number of
+ * page pointers that we can fit in the base structure or (using
+ * integer math):
+ *
+ * (PAGE_SIZE/element_size) * (PAGE_SIZE-8)/sizeof(void *)
+ *
+ * Here's a table showing example capacities. Note that the maximum
+ * index that the get/put() functions is just nr_objects-1.
+ *
+ * Element size | Objects | Objects |
+ * PAGE_SIZE=4k | 32-bit | 64-bit |
+ * ----------------------------------|
+ * 1 byte | 4186112 | 2093056 |
+ * 2 bytes | 2093056 | 1046528 |
+ * 3 bytes | 1395030 | 697515 |
+ * 4 bytes | 1046528 | 523264 |
+ * 32 bytes | 130816 | 65408 |
+ * 33 bytes | 126728 | 63364 |
+ * 2048 bytes | 2044 | 10228 |
+ * 2049 bytes | 1022 | 511 |
+ * void * | 1046528 | 261632 |
+ *
+ * Since 64-bit pointers are twice the size, we lose half the
+ * capacity in the base structure. Also note that no effort is made
+ * to efficiently pack objects across page boundaries.
+ */
+struct flex_array *flex_array_alloc(int element_size, int total, gfp_t flags)
+{
+ struct flex_array *ret;
+ int max_size = nr_base_part_ptrs() * __elements_per_part(element_size);
+
+ /* max_size will end up 0 if element_size > PAGE_SIZE */
+ if (total > max_size)
+ return NULL;
+ ret = kzalloc(sizeof(struct flex_array), flags);
+ if (!ret)
+ return NULL;
+ ret->element_size = element_size;
+ return ret;
+}
+
+static int fa_element_to_part_nr(struct flex_array *fa, int element_nr)
+{
+ return element_nr / __elements_per_part(fa->element_size);
+}
+
+/**
+ * flex_array_free_parts - just free the second-level pages
+ * @src: address of data to copy into the array
+ * @element_nr: index of the position in which to insert
+ * the new element.
+ *
+ * This is to be used in cases where the base 'struct flex_array'
+ * has been statically allocated and should not be free.
+ */
+void flex_array_free_parts(struct flex_array *fa)
+{
+ int part_nr;
+ int max_part = nr_base_part_ptrs();
+
+ for (part_nr = 0; part_nr < max_part; part_nr++)
+ kfree(fa->parts[part_nr]);
+}
+
+void flex_array_free(struct flex_array *fa)
+{
+ flex_array_free_parts(fa);
+ kfree(fa);
+}
+
+static int fa_index_inside_part(struct flex_array *fa, int element_nr)
+{
+ return (element_nr % __elements_per_part(fa->element_size));
+}
+
+static int index_inside_part(struct flex_array *fa, int element_nr)
+{
+ int part_offset = fa_index_inside_part(fa, element_nr);
+ return part_offset * fa->element_size;
+}
+
+static int __check_part_nr(struct flex_array *fa, int part_nr)
+{
+ if (part_nr >= nr_base_part_ptrs()) {
+ WARN(1, "bad flexible array part number: %d >= %d",
+ part_nr, nr_base_part_ptrs());
+ return -EINVAL;
+ }
+ return 0;
+}
+
+static struct flex_array_part *
+__fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
+{
+ struct flex_array_part *part = NULL;
+ if (__check_part_nr(fa, part_nr))
+ return NULL;
+ part = fa->parts[part_nr];
+ if (!part) {
+ part = kmalloc(FLEX_ARRAY_PART_SIZE, flags);
+ if (!part)
+ return NULL;
+ fa->parts[part_nr] = part;
+ }
+ return part;
+}
+
+/**
+ * flex_array_put - copy data into the array at @element_nr
+ * @src: address of data to copy into the array
+ * @element_nr: index of the position in which to insert
+ * the new element.
+ *
+ * Note that this *copies* the contents of @src into
+ * the array. If you are trying to store an array of
+ * pointers, make sure to pass in &ptr instead of ptr.
+ *
+ * Locking must be provided by the caller.
+ */
+int flex_array_put(struct flex_array *fa, int element_nr, void *src, gfp_t flags)
+{
+ int part_nr = fa_element_to_part_nr(fa, element_nr);
+ struct flex_array_part *part;
+ void *dst;
+
+ part = __fa_get_part(fa, part_nr, flags);
+ if (!part)
+ return -ENOMEM;
+ dst = &part->elements[index_inside_part(fa, element_nr)];
+ memcpy(dst, src, fa->element_size);
+ return 0;
+}
+
+/**
+ * flex_array_prealloc - guarantee that array space exists
+ * @start: index of first array element for which space is allocated
+ * @end: index of last (inclusive) element for which space is allocated
+ *
+ * This will guarantee that no future calls to flex_array_put()
+ * will allocate memory. It can be used if you are expecting to
+ * be holding a lock or in some atomic context while writing
+ * data into the array.
+ *
+ * Locking must be provided by the caller.
+ */
+int flex_array_prealloc(struct flex_array *fa, int start, int end, gfp_t flags)
+{
+ int start_part = fa_element_to_part_nr(fa, start);
+ int end_part = fa_element_to_part_nr(fa, end);
+ int part_nr;
+ struct flex_array_part *part;
+
+ for (part_nr = start_part; part_nr <= end_part; part_nr++) {
+ part = __fa_get_part(fa, part_nr, flags);
+ if (!part)
+ return -ENOMEM;
+ }
+ return 0;
+}
+
+/**
+ * flex_array_get - pull data back out of the array
+ * @element_nr: index of the element to fetch from the array
+ *
+ * Returns a pointer to the data at index @element_nr. Note
+ * that this is a copy of the data that was passed in. If you
+ * are using this to store pointers, you'll get back &ptr.
+ *
+ * Locking must be provided by the caller.
+ */
+void *flex_array_get(struct flex_array *fa, int element_nr)
+{
+ int part_nr = fa_element_to_part_nr(fa, element_nr);
+ struct flex_array_part *part;
+ int offset;
+
+ if (__check_part_nr(fa, part_nr))
+ return NULL;
+ if (!fa->parts[part_nr])
+ return NULL;
+
+ part = fa->parts[part_nr];
+ offset = index_inside_part(fa, element_nr);
+ return &part->elements[index_inside_part(fa, element_nr)];
+}
diff -puN lib/Makefile~fa lib/Makefile
--- linux-2.6.git/lib/Makefile~fa 2009-07-22 10:02:25.000000000 -0700
+++ linux-2.6.git-dave/lib/Makefile 2009-07-22 10:02:25.000000000 -0700
@@ -12,7 +12,7 @@ lib-y := ctype.o string.o vsprintf.o cmd
idr.o int_sqrt.o extable.o prio_tree.o \
sha1.o irq_regs.o reciprocal_div.o argv_split.o \
proportions.o prio_heap.o ratelimit.o show_mem.o \
- is_single_threaded.o plist.o decompress.o
+ is_single_threaded.o plist.o decompress.o flex_array.o

lib-$(CONFIG_MMU) += ioremap.o
lib-$(CONFIG_SMP) += cpumask.o
_

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