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Subject[RFC][PATCH] flexible array implementation v4
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Remaining issues:
- How should we deal with out-of-range indexes, especially
in flex_array_get() which returns void*? ERR_PTR()?
BUG_ON()? return NULL (current behavior)?
- Should care be taken not to allow a flex_array_get() to
an index where no flex_array_put() was done?
- Should we decay further than just packing things into the
'base' page? Should we actually kmalloc() less than a
page at times when it will fit?

Changes from v3:
- Now store ->total_nr_elements, and enforce stores,
accesses and preallocation limits below that.
- Decay to an "elements_fit_in_base()" mode. No second-
level pages are allocated and all elements are packed
into the flex_array->parts[] array that we used to use
only for second-level pointers. Thanks hpa.

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 32-bit.

I've been testing this by running it in userspace. The
header and patch that I've been using are here, as well
as the little script I'm using to generate the size table
which goes in the kerneldocs.

http://sr71.net/~dave/linux/flexarray/

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
---
linux-2.6.git-dave/include/linux/flex_array.h | 46 ++++
linux-2.6.git-dave/lib/Makefile | 2
linux-2.6.git-dave/lib/flex_array.c | 269 ++++++++++++++++++++++++++
linux-2.6.git-dave/lib/go.sh | 17 +
4 files changed, 327 insertions(+), 7 deletions(-)
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 2009-07-23 08:21:07.000000000 -0700
@@ -0,0 +1,46 @@
+#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 too big to fit into kmalloc()
+ * and the developer is getting tempted to use
+ * vmalloc().
+ */
+
+struct flex_array {
+ union {
+ struct {
+ int element_size;
+ int total_nr_elements;
+ 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), \
+ .total_nr_elements = (total), \
+} } }
+
+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 2009-07-23 08:21:07.000000000 -0700
@@ -0,0 +1,269 @@
+/*
+ * 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 bytes_left_in_base(void)
+{
+ int element_offset = offsetof(struct flex_array, parts);
+ int bytes_left = FLEX_ARRAY_BASE_SIZE - element_offset;
+ return bytes_left;
+}
+
+static inline int nr_base_part_ptrs(void)
+{
+ return bytes_left_in_base() / sizeof(struct flex_array_part *);
+}
+
+/*
+ * If a user requests an allocation which is small
+ * enough, we may simply use the space in the
+ * flex_array->parts[] array to store the user
+ * data.
+ */
+static inline int elements_fit_in_base(struct flex_array *fa)
+{
+ int data_size = fa->element_size * fa->total_nr_elements;
+ if (data_size <= bytes_left_in_base())
+ return 1;
+ return 0;
+}
+
+/**
+ * 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.
+ *
+ * @total is used to size internal structures. If the user ever
+ * accesses any array indexes >=@total, it will produce errors.
+ *
+ * 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. This
+ * basically means that you get 4MB of storage on 32-bit and 2MB on
+ * 64-bit.
+ *
+ *
+ * Element size | Objects | Objects |
+ * PAGE_SIZE=4k | 32-bit | 64-bit |
+ * ---------------------------------|
+ * 1 bytes | 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 | 1022 |
+ * 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;
+ ret->total_nr_elements = total;
+ 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();
+
+ if (elements_fit_in_base(fa))
+ return;
+ 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 struct flex_array_part *
+__fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
+{
+ struct flex_array_part *part = fa->parts[part_nr];
+ if (!part) {
+ /*
+ * This leaves the part pages uninitialized
+ * and with potentially random data, just
+ * as if the user had kmalloc()'d the whole.
+ * __GFP_ZERO can be used to zero it.
+ */
+ 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;
+
+ if (element_nr >= fa->total_nr_elements)
+ return -ENOSPC;
+ if (elements_fit_in_base(fa))
+ part = (struct flex_array_part *)&fa->parts[0];
+ else
+ 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;
+ int end_part;
+ int part_nr;
+ struct flex_array_part *part;
+
+ if (start >= fa->total_nr_elements || end >= fa->total_nr_elements)
+ return -ENOSPC;
+ if (elements_fit_in_base(fa))
+ return 0;
+ start_part = fa_element_to_part_nr(fa, start);
+ end_part = fa_element_to_part_nr(fa, end);
+ 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 index;
+
+ if (element_nr >= fa->total_nr_elements)
+ return NULL;
+ if (!fa->parts[part_nr])
+ return NULL;
+ if (elements_fit_in_base(fa))
+ part = (struct flex_array_part *)&fa->parts[0];
+ else
+ part = fa->parts[part_nr];
+ index = 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 2009-07-23 07:48:13.000000000 -0700
+++ linux-2.6.git-dave/lib/Makefile 2009-07-23 07:48:13.000000000 -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
diff -puN lib/go.sh~fa lib/go.sh
--- linux-2.6.git/lib/go.sh~fa 2009-07-23 08:07:33.000000000 -0700
+++ linux-2.6.git-dave/lib/go.sh 2009-07-23 08:21:07.000000000 -0700
@@ -1,14 +1,19 @@
#!/bin/sh
set -e
for bits in 32 64; do
- gcc-3.4 -m$bits -I ../include -o flex_array.$bits flex_array.c ;
+ gcc-3.4 -m$bits -I include -o flex_array.$bits lib/flex_array.c ;
done

+func fa
+{
+ ./flex_array.$1 | grep ^max | awk '{print $4}'
+}
+
for i in 1 2 3 4 32 33 2048 2049; do
- m32="$(./flex_array.32 $i 100)"
- m64="$(./flex_array.64 $i 100)"
+ m32="$(fa 32 $i 100)"
+ m64="$(fa 64 $i 100)"
printf " * %6d bytes | %7d | %7d |\n" $i $m32 $m64;
done
-m32="$(./flex_array.32 4 100)"
-m64="$(./flex_array.64 8 100)"
-printf " * void * | %7d | %7d |\n" $i $m32 $m64;
+m32="$(fa 32 4 100)"
+m64="$(fa 64 8 100)"
+printf " * void * | %7d | %7d |\n" $m32 $m64;
_

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