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.
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Answer
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.)