What is different functions: `malloc()` and `kmalloc()`?

What is different functions: malloc() and kmalloc()? They differ only in that:

  • the malloc() can be called in user-space and kernel-space, and it allocates a physically fragmented memory area
  • but kmalloc() can be called only in kernel-space, and it allocates physically contiguous memory chunk

or something else?

kmalloc() use pointer in virtual or physical addressing and what kmalloc() is different from __ get_free_pages()?


I answer to the second question, assuming that you are using Linux OS. Regarding to the first one please have a look to my comment.

kmallocuses get_free_page to get the memory. The way in which the pages are collected depends on the second parameter ( GFP_ATOMIC GFP_KERNEL ... in which GFP means GET FREE PAGE). The advantage of kmalloc on the GFP is that it can fit multiple allocations into a single page.

some of the options for kmalloc are:

GFP_USER - Allocate memory on behalf of user. May sleep.
GFP_KERNEL - Allocate normal kernel ram. May sleep.
GFP_ATOMIC - Allocation will not sleep. May use emergency pools. For example, use this inside interrupt handlers.
GFP_HIGHUSER - Allocate pages from high memory.
GFP_NOIO - Do not do any I/O at all while trying to get memory.
GFP_NOFS - Do not make any fs calls while trying to get memory.
GFP_NOWAIT - Allocation will not sleep.
GFP_THISNODE - Allocate node-local memory only.
GFP_DMA - Allocation suitable for DMA. Should only be used for kmalloc caches. Otherwise, use a slab created with SLAB_DMA.

Apart from this get_free_page and kmalloc are very similar. _get_free_pages differs from get_free_page because it gives the pointer to the first byte of a memory area that is potentially several (physically contiguous) pages long. Another function that is again very similar to get_free_page is get_zeroed_page(unsigned int flags) which gets a single page like get_free_page but zeroes the memory

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