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What is the meaning of GFP flags in kmalloc? For instance GFP_KERNEL, GFP_ATOMIC?
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* GFP flags are commonly used throughout Linux to indicate how memory. * should be allocated. The GFP acronym stands for get_free_pages(), * the underlying memory allocation function.
The kmalloc() function guarantees that the pages are physically contiguous (and virtually contiguous). The vmalloc() function works in a similar fashion to kmalloc() , except it allocates memory that is only virtually contiguous and not necessarily physically contiguous.
The most commonly used allocation flag is GFP_KERNEL means that allocation is performed on behalf of a process running in the kernel space. This means that the calling function is executing a system call on behalf of a process.
GFP_ATOMIC prevents sleeping by telling the memory allocation code that it's not allowed to sleep to satisfy the allocation - that's all. If the memory allocation code needs to sleep, and GFP_ATOMIC has been passed, then it will return an error to the caller instead.
GFP = Get Free Pages = __get_free_pages.
These flags are flags passed to functions that allocate memory, such as __get_free_pages and kmalloc, telling them what can and can't be done while allocating.
For example, GFP_ATOMIC means no context-switch must happen while allocating (which means paging isn't possible).
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