2009-07-30 05:04:18 +07:00
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#ifndef _FLEX_ARRAY_H
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#define _FLEX_ARRAY_H
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#include <linux/types.h>
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reciprocal_divide: update/correction of the algorithm
Jakub Zawadzki noticed that some divisions by reciprocal_divide()
were not correct [1][2], which he could also show with BPF code
after divisions are transformed into reciprocal_value() for runtime
invariance which can be passed to reciprocal_divide() later on;
reverse in BPF dump ended up with a different, off-by-one K in
some situations.
This has been fixed by Eric Dumazet in commit aee636c4809fa5
("bpf: do not use reciprocal divide"). This follow-up patch
improves reciprocal_value() and reciprocal_divide() to work in
all cases by using Granlund and Montgomery method, so that also
future use is safe and without any non-obvious side-effects.
Known problems with the old implementation were that division by 1
always returned 0 and some off-by-ones when the dividend and divisor
where very large. This seemed to not be problematic with its
current users, as far as we can tell. Eric Dumazet checked for
the slab usage, we cannot surely say so in the case of flex_array.
Still, in order to fix that, we propose an extension from the
original implementation from commit 6a2d7a955d8d resp. [3][4],
by using the algorithm proposed in "Division by Invariant Integers
Using Multiplication" [5], Torbjörn Granlund and Peter L.
Montgomery, that is, pseudocode for q = n/d where q, n, d is in
u32 universe:
1) Initialization:
int l = ceil(log_2 d)
uword m' = floor((1<<32)*((1<<l)-d)/d)+1
int sh_1 = min(l,1)
int sh_2 = max(l-1,0)
2) For q = n/d, all uword:
uword t = (n*m')>>32
q = (t+((n-t)>>sh_1))>>sh_2
The assembler implementation from Agner Fog [6] also helped a lot
while implementing. We have tested the implementation on x86_64,
ppc64, i686, s390x; on x86_64/haswell we're still half the latency
compared to normal divide.
Joint work with Daniel Borkmann.
[1] http://www.wireshark.org/~darkjames/reciprocal-buggy.c
[2] http://www.wireshark.org/~darkjames/set-and-dump-filter-k-bug.c
[3] https://gmplib.org/~tege/division-paper.pdf
[4] http://homepage.cs.uiowa.edu/~jones/bcd/divide.html
[5] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1.2556
[6] http://www.agner.org/optimize/asmlib.zip
Reported-by: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: Jesse Gross <jesse@nicira.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Andy Gospodarek <andy@greyhouse.net>
Cc: Veaceslav Falico <vfalico@redhat.com>
Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
2014-01-22 08:29:41 +07:00
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#include <linux/reciprocal_div.h>
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2009-07-30 05:04:18 +07:00
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#include <asm/page.h>
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#define FLEX_ARRAY_PART_SIZE PAGE_SIZE
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#define FLEX_ARRAY_BASE_SIZE PAGE_SIZE
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struct flex_array_part;
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/*
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* This is meant to replace cases where an array-like
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* structure has gotten too big to fit into kmalloc()
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* and the developer is getting tempted to use
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* vmalloc().
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*/
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struct flex_array {
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union {
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struct {
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int element_size;
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int total_nr_elements;
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2011-05-27 06:25:02 +07:00
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int elems_per_part;
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reciprocal_divide: update/correction of the algorithm
Jakub Zawadzki noticed that some divisions by reciprocal_divide()
were not correct [1][2], which he could also show with BPF code
after divisions are transformed into reciprocal_value() for runtime
invariance which can be passed to reciprocal_divide() later on;
reverse in BPF dump ended up with a different, off-by-one K in
some situations.
This has been fixed by Eric Dumazet in commit aee636c4809fa5
("bpf: do not use reciprocal divide"). This follow-up patch
improves reciprocal_value() and reciprocal_divide() to work in
all cases by using Granlund and Montgomery method, so that also
future use is safe and without any non-obvious side-effects.
Known problems with the old implementation were that division by 1
always returned 0 and some off-by-ones when the dividend and divisor
where very large. This seemed to not be problematic with its
current users, as far as we can tell. Eric Dumazet checked for
the slab usage, we cannot surely say so in the case of flex_array.
Still, in order to fix that, we propose an extension from the
original implementation from commit 6a2d7a955d8d resp. [3][4],
by using the algorithm proposed in "Division by Invariant Integers
Using Multiplication" [5], Torbjörn Granlund and Peter L.
Montgomery, that is, pseudocode for q = n/d where q, n, d is in
u32 universe:
1) Initialization:
int l = ceil(log_2 d)
uword m' = floor((1<<32)*((1<<l)-d)/d)+1
int sh_1 = min(l,1)
int sh_2 = max(l-1,0)
2) For q = n/d, all uword:
uword t = (n*m')>>32
q = (t+((n-t)>>sh_1))>>sh_2
The assembler implementation from Agner Fog [6] also helped a lot
while implementing. We have tested the implementation on x86_64,
ppc64, i686, s390x; on x86_64/haswell we're still half the latency
compared to normal divide.
Joint work with Daniel Borkmann.
[1] http://www.wireshark.org/~darkjames/reciprocal-buggy.c
[2] http://www.wireshark.org/~darkjames/set-and-dump-filter-k-bug.c
[3] https://gmplib.org/~tege/division-paper.pdf
[4] http://homepage.cs.uiowa.edu/~jones/bcd/divide.html
[5] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1.2556
[6] http://www.agner.org/optimize/asmlib.zip
Reported-by: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: Jesse Gross <jesse@nicira.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Andy Gospodarek <andy@greyhouse.net>
Cc: Veaceslav Falico <vfalico@redhat.com>
Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
2014-01-22 08:29:41 +07:00
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struct reciprocal_value reciprocal_elems;
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2009-08-27 04:29:21 +07:00
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struct flex_array_part *parts[];
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2009-07-30 05:04:18 +07:00
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};
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/*
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* This little trick makes sure that
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* sizeof(flex_array) == PAGE_SIZE
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*/
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char padding[FLEX_ARRAY_BASE_SIZE];
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};
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};
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2009-09-22 07:04:33 +07:00
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/* Number of bytes left in base struct flex_array, excluding metadata */
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#define FLEX_ARRAY_BASE_BYTES_LEFT \
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(FLEX_ARRAY_BASE_SIZE - offsetof(struct flex_array, parts))
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/* Number of pointers in base to struct flex_array_part pages */
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#define FLEX_ARRAY_NR_BASE_PTRS \
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(FLEX_ARRAY_BASE_BYTES_LEFT / sizeof(struct flex_array_part *))
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/* Number of elements of size that fit in struct flex_array_part */
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#define FLEX_ARRAY_ELEMENTS_PER_PART(size) \
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(FLEX_ARRAY_PART_SIZE / size)
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/*
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* Defines a statically allocated flex array and ensures its parameters are
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* valid.
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*/
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#define DEFINE_FLEX_ARRAY(__arrayname, __element_size, __total) \
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struct flex_array __arrayname = { { { \
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.element_size = (__element_size), \
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.total_nr_elements = (__total), \
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} } }; \
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static inline void __arrayname##_invalid_parameter(void) \
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{ \
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BUILD_BUG_ON((__total) > FLEX_ARRAY_NR_BASE_PTRS * \
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FLEX_ARRAY_ELEMENTS_PER_PART(__element_size)); \
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}
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2009-07-30 05:04:18 +07:00
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2009-08-27 04:29:22 +07:00
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struct flex_array *flex_array_alloc(int element_size, unsigned int total,
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gfp_t flags);
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int flex_array_prealloc(struct flex_array *fa, unsigned int start,
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2011-04-29 02:55:52 +07:00
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unsigned int nr_elements, gfp_t flags);
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2009-07-30 05:04:18 +07:00
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void flex_array_free(struct flex_array *fa);
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void flex_array_free_parts(struct flex_array *fa);
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2009-08-27 04:29:22 +07:00
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int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
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2009-07-30 05:04:18 +07:00
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gfp_t flags);
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2009-09-22 07:04:30 +07:00
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int flex_array_clear(struct flex_array *fa, unsigned int element_nr);
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2009-08-27 04:29:22 +07:00
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void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
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2009-09-22 07:04:31 +07:00
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int flex_array_shrink(struct flex_array *fa);
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2009-07-30 05:04:18 +07:00
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2010-08-10 07:20:56 +07:00
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#define flex_array_put_ptr(fa, nr, src, gfp) \
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2010-11-30 03:47:09 +07:00
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flex_array_put(fa, nr, (void *)&(src), gfp)
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2010-08-10 07:20:56 +07:00
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void *flex_array_get_ptr(struct flex_array *fa, unsigned int element_nr);
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2009-07-30 05:04:18 +07:00
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#endif /* _FLEX_ARRAY_H */
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