Cannot get memory allocated from `flex_array_alloc` when requesting a relatively big size in linux kernel
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1
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I'm doing some linux kernel development.
And I'm going to allocate some memory space with something like:
ptr = flex_array_alloc(size=136B, num=1<<16, GFP_KERNEL)
And ptr turns out to be NULL every time I try.
What's more, when I change the size to 20B or num to 256，there's nothing wrong and the memory can be obtained.
So I want to know if there are some limitations for requesting memory in linux kernel modules. And how to debug it or to allocate a big memory space.
Thanks.
And kzalloc has a similar behavior in my environment. That is, requesting a 136B * (1<<16) space failed, while 20B or 1<<8 succeed.
c linux linux-kernel kernel kernel-module
asked Nov 20 at 14:51
Dai Zhang
85
add a comment |
up vote
1
down vote
favorite
I'm doing some linux kernel development.
And I'm going to allocate some memory space with something like:
ptr = flex_array_alloc(size=136B, num=1<<16, GFP_KERNEL)
And ptr turns out to be NULL every time I try.
What's more, when I change the size to 20B or num to 256，there's nothing wrong and the memory can be obtained.
So I want to know if there are some limitations for requesting memory in linux kernel modules. And how to debug it or to allocate a big memory space.
Thanks.
And kzalloc has a similar behavior in my environment. That is, requesting a 136B * (1<<16) space failed, while 20B or 1<<8 succeed.
c linux linux-kernel kernel kernel-module
asked Nov 20 at 14:51
Dai Zhang
85
With respect to your edit:kzallocis a variant ofkmallocwhich zeros out the allocation.kmallocis "the normal method of allocating memory for objects smaller than page size in the kernel." If you want to allocate larger objects, you need to usevmalloc/vzalloc. For the limitations of flexible arrays, see my (edited) answer.
– rici
Nov 20 at 15:53
add a comment |
up vote
1
down vote
favorite
up vote
1
down vote
favorite
I'm doing some linux kernel development.
And I'm going to allocate some memory space with something like:
ptr = flex_array_alloc(size=136B, num=1<<16, GFP_KERNEL)
And ptr turns out to be NULL every time I try.
What's more, when I change the size to 20B or num to 256，there's nothing wrong and the memory can be obtained.
So I want to know if there are some limitations for requesting memory in linux kernel modules. And how to debug it or to allocate a big memory space.
Thanks.
And kzalloc has a similar behavior in my environment. That is, requesting a 136B * (1<<16) space failed, while 20B or 1<<8 succeed.
c linux linux-kernel kernel kernel-module
asked Nov 20 at 14:51
Dai Zhang
85
I'm doing some linux kernel development.
And I'm going to allocate some memory space with something like:
ptr = flex_array_alloc(size=136B, num=1<<16, GFP_KERNEL)
And ptr turns out to be NULL every time I try.
What's more, when I change the size to 20B or num to 256，there's nothing wrong and the memory can be obtained.
So I want to know if there are some limitations for requesting memory in linux kernel modules. And how to debug it or to allocate a big memory space.
Thanks.
And kzalloc has a similar behavior in my environment. That is, requesting a 136B * (1<<16) space failed, while 20B or 1<<8 succeed.
c linux linux-kernel kernel kernel-module
c linux linux-kernel kernel kernel-module
asked Nov 20 at 14:51
Dai Zhang
85
asked Nov 20 at 14:51
Dai Zhang
85
edited Nov 20 at 15:36
asked Nov 20 at 14:51
Dai Zhang
85
asked Nov 20 at 14:51
Dai Zhang
85
asked Nov 20 at 14:51
Dai Zhang
85
85
With respect to your edit:kzallocis a variant ofkmallocwhich zeros out the allocation.kmallocis "the normal method of allocating memory for objects smaller than page size in the kernel." If you want to allocate larger objects, you need to usevmalloc/vzalloc. For the limitations of flexible arrays, see my (edited) answer.
– rici
Nov 20 at 15:53
add a comment |
With respect to your edit:kzallocis a variant ofkmallocwhich zeros out the allocation.kmallocis "the normal method of allocating memory for objects smaller than page size in the kernel." If you want to allocate larger objects, you need to usevmalloc/vzalloc. For the limitations of flexible arrays, see my (edited) answer.
– rici
Nov 20 at 15:53
With respect to your edit:
kzalloc is a variant of kmalloc which zeros out the allocation. kmalloc is "the normal method of allocating memory for objects smaller than page size in the kernel." If you want to allocate larger objects, you need to use vmalloc/vzalloc. For the limitations of flexible arrays, see my (edited) answer.– rici
Nov 20 at 15:53
With respect to your edit:
kzalloc is a variant of kmalloc which zeros out the allocation. kmalloc is "the normal method of allocating memory for objects smaller than page size in the kernel." If you want to allocate larger objects, you need to use vmalloc/vzalloc. For the limitations of flexible arrays, see my (edited) answer.– rici
Nov 20 at 15:53
add a comment |
1 Answer
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There are two limits to the size of an array allocated with flex_array_allocate. First, the object size itself must not exceed a single page, as indicated in https://www.kernel.org/doc/Documentation/flexible-arrays.txt:
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.
Second, there is a maximum number of elements in the array.
Both limitations are the result of the implementation technique:
…the need for memory from
vmalloc()can be eliminated by piecing together an array from smaller parts…
A flexible array holds an arbitrary (within limits) number of fixed-sized objects, accessed via an integer index.… Only single-page allocations are made…
The array is "pieced" together by using an array of pointers to individual parts, where each part is one system page. Since this array is also allocated, and only single-page allocations are made (as noted above), the maximum number of parts is slightly less than the number of pointers which can fit in a page (slightly less because there is also some bookkeeping data.) In effect, this limits the total size of a flexible array to about 2MB on systems with 8-byte pointers and 4kb pages. (The precise limitation will vary depending on the amount of wasted space in a page if the object size is not a power of two.)
answered Nov 20 at 15:20
rici
151k19131195
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
1
down vote
accepted
There are two limits to the size of an array allocated with flex_array_allocate. First, the object size itself must not exceed a single page, as indicated in https://www.kernel.org/doc/Documentation/flexible-arrays.txt:
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.
Second, there is a maximum number of elements in the array.
Both limitations are the result of the implementation technique:
…the need for memory from
vmalloc()can be eliminated by piecing together an array from smaller parts…
A flexible array holds an arbitrary (within limits) number of fixed-sized objects, accessed via an integer index.… Only single-page allocations are made…
The array is "pieced" together by using an array of pointers to individual parts, where each part is one system page. Since this array is also allocated, and only single-page allocations are made (as noted above), the maximum number of parts is slightly less than the number of pointers which can fit in a page (slightly less because there is also some bookkeeping data.) In effect, this limits the total size of a flexible array to about 2MB on systems with 8-byte pointers and 4kb pages. (The precise limitation will vary depending on the amount of wasted space in a page if the object size is not a power of two.)
answered Nov 20 at 15:20
rici
151k19131195
add a comment |
up vote
1
down vote
accepted
There are two limits to the size of an array allocated with flex_array_allocate. First, the object size itself must not exceed a single page, as indicated in https://www.kernel.org/doc/Documentation/flexible-arrays.txt:
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.
Second, there is a maximum number of elements in the array.
Both limitations are the result of the implementation technique:
…the need for memory from
vmalloc()can be eliminated by piecing together an array from smaller parts…
A flexible array holds an arbitrary (within limits) number of fixed-sized objects, accessed via an integer index.… Only single-page allocations are made…
The array is "pieced" together by using an array of pointers to individual parts, where each part is one system page. Since this array is also allocated, and only single-page allocations are made (as noted above), the maximum number of parts is slightly less than the number of pointers which can fit in a page (slightly less because there is also some bookkeeping data.) In effect, this limits the total size of a flexible array to about 2MB on systems with 8-byte pointers and 4kb pages. (The precise limitation will vary depending on the amount of wasted space in a page if the object size is not a power of two.)
answered Nov 20 at 15:20
rici
151k19131195
add a comment |
up vote
1
down vote
accepted
up vote
1
down vote
accepted
There are two limits to the size of an array allocated with flex_array_allocate. First, the object size itself must not exceed a single page, as indicated in https://www.kernel.org/doc/Documentation/flexible-arrays.txt:
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.
Second, there is a maximum number of elements in the array.
Both limitations are the result of the implementation technique:
…the need for memory from
vmalloc()can be eliminated by piecing together an array from smaller parts…
A flexible array holds an arbitrary (within limits) number of fixed-sized objects, accessed via an integer index.… Only single-page allocations are made…
The array is "pieced" together by using an array of pointers to individual parts, where each part is one system page. Since this array is also allocated, and only single-page allocations are made (as noted above), the maximum number of parts is slightly less than the number of pointers which can fit in a page (slightly less because there is also some bookkeeping data.) In effect, this limits the total size of a flexible array to about 2MB on systems with 8-byte pointers and 4kb pages. (The precise limitation will vary depending on the amount of wasted space in a page if the object size is not a power of two.)
answered Nov 20 at 15:20
rici
151k19131195
There are two limits to the size of an array allocated with flex_array_allocate. First, the object size itself must not exceed a single page, as indicated in https://www.kernel.org/doc/Documentation/flexible-arrays.txt:
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.
Second, there is a maximum number of elements in the array.
Both limitations are the result of the implementation technique:
…the need for memory from
vmalloc()can be eliminated by piecing together an array from smaller parts…
A flexible array holds an arbitrary (within limits) number of fixed-sized objects, accessed via an integer index.… Only single-page allocations are made…
The array is "pieced" together by using an array of pointers to individual parts, where each part is one system page. Since this array is also allocated, and only single-page allocations are made (as noted above), the maximum number of parts is slightly less than the number of pointers which can fit in a page (slightly less because there is also some bookkeeping data.) In effect, this limits the total size of a flexible array to about 2MB on systems with 8-byte pointers and 4kb pages. (The precise limitation will vary depending on the amount of wasted space in a page if the object size is not a power of two.)
answered Nov 20 at 15:20
rici
151k19131195
edited Nov 20 at 15:53
answered Nov 20 at 15:20
rici
151k19131195
answered Nov 20 at 15:20
rici
151k19131195
answered Nov 20 at 15:20
rici
151k19131195
151k19131195
add a comment |
add a comment |
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With respect to your edit:
kzallocis a variant ofkmallocwhich zeros out the allocation.kmallocis "the normal method of allocating memory for objects smaller than page size in the kernel." If you want to allocate larger objects, you need to usevmalloc/vzalloc. For the limitations of flexible arrays, see my (edited) answer.– rici
Nov 20 at 15:53