kmalloc()

• void * kmalloc (size_t size, int flags) : 在kernel配置記憶體位置
• INCLUDE    :include/linux/slab.h
• FLAG: 

          size:欲分配大小
          flags:

Action Modifiers( 通常使用後面提到的type modifier)    

__GFP_WAIT	The allocator can sleep.
__GFP_HIGH	The allocator can access emergency pools.
__GFP_IO	The allocator can start physical I/O.
__GFP_FS	The allocator can start filesystem I/O.
__GFP_COLD	The allocator should use cache cold pages.
__GFP_NOWARN	The allocator will not print failure warnings.
__GFP_REPEAT	The allocator will repeat the allocation if it fails, but the allocation can potentially fail. This depends upon the particular VM implementation.
__GFP_NOFAIL	The allocator will indefinitely repeat the allocation. The allocation cannot fail.
__GFP_NORETRY	The allocator will never retry if the allocation fails.
__GFP_COMP	Add compound page metadata. Used internally by thehugetlb code.
__GFP_ZERO	Add compound page metadata. Used internally by thehugetlb code.
__GFP_NOMEMALLOC	Don't use emergency reserves.
__GFP_HARDWALL	Enforce hardwall cpuset memory allocs.
__GFP_THISNODE	No fallback, no policies.
__GFP_RECLAIMABLE	Page is reclaimable.
__GFP_NOTRACK	Don't track with kmemcheck.
__GFP_NO_KSWAPD
__GFP_OTHER_NODE	On behalf of other node.

Example:

ptr = kmalloc(size, __GFP_WAIT | __GFP_IO | __GFP_FS);

 

Zone Modifiers ( 選擇要從哪分配空間 預設是從ZONE_NORMAL)

__GFP_DMA	Allocate only from ZONE_DMA
__GFP_HIGHMEM	Allocate from ZONE_HIGHMEM or ZONE_NORMAL
__GFP_DMA32
__GFP_MOVABLE

 

Type Flags( 定義了action 及 zone modifier)

GFP_ATOMIC	The allocation is high priority and must not sleep. This is the flag to use in interrupt handlers, in bottom halves, while holding a spinlock, and in other situations where you cannot sleep.
GFP_NOIO	This allocation can block, but must not initiate disk I/O. This is the flag to use in block I/O code when you cannot cause more disk I/O, which might lead to some unpleasant recursion.
GFP_NOFS	This allocation can block and can initiate disk I/O, if it must, but will not initiate a filesystem operation. This is the flag to use in filesystem code when you cannot start another filesystem operation.
GFP_KERNEL	This is a normal allocation and might block. This is the flag to use in process context code when it is safe to sleep. The kernel will do whatever it has to in order to obtain the memory requested by the caller. This flag should be your first choice.
GFP_USER	This is a normal allocation and might block. This flag is used to allocate memory for user-space processes.
GFP_HIGHUSER	This is an allocation from ZONE_HIGHMEM and might block. This flag is used to allocate memory for user-space processes.
GFP_DMA	This is an allocation from ZONE_DMA. Device drivers that need DMA-able memory use this flag, usually in combination with one of the above.

以下列出各Type modifier的組合:

GFP_ATOMIC	__GFP_HIGH
GFP_NOIO	__GFP_WAIT
GFP_NOFS	(__GFP_WAIT | __GFP_IO)
GFP_KERNEL	(__GFP_WAIT | __GFP_IO | __GFP_FS)
GFP_USER	(__GFP_WAIT | __GFP_IO | __GFP_FS)
GFP_HIGHUSER	(__GFP_WAIT | __GFP_IO | __GFP_FS | __GFP_HIGHMEM)
GFP_DMA	__GFP_DMA

建議的用法:

Situation	Solution
Process context, can sleep	Use GFP_KERNEL
Process context, cannot sleep	Use GFP_ATOMIC, or perform your allocations with GFP_KERNEL at an earlier or later point when you can sleep
Interrupt handler	Use GFP_ATOMIC
Softirq	Use GFP_ATOMIC
Tasklet	Use GFP_ATOMIC
Need DMA-able memory, can sleep	Use (GFP_DMA | GFP_KERNEL)
Need DMA-able memory, cannot sleep	Use (GFP_DMA | GFP_ATOMIC), or perform your allocation at an earlier point when you can sleep

Example:

char *reloc_lp0;
reloc_lp0 = kmalloc(size,GFP_KERNEL);
WARN_ON(!reloc_lp0);
if (!reloc_lp0) {
    pr_err("%s: Failed to allocate reloc_lp0\n",__func__);
   goto out;
}
...
kfree(reloc_lp0);

Reference:
include/linux/gfp.h
http://www.makelinux.net/books/lkd2/ch11lev1sec4
http://www.google.com.tw/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CF8QFjAE&url=http%3A%2F%2Fant.comm.ccu.edu.tw%2Fcourse%2F96_Driver%2F0_Lectures%2FChap7-Getting%2520Hold%2520of%2520Memory_milk.ppt&ei=TMFqT_ysHqWfmQW5obyWBg&usg=AFQjCNGvVEk9-Io481khM_SqoFd-WsvwqQ&sig2=qvOIAsidZzvw15vpq0JlrQ