What is the difference between vmalloc and kmalloc

Question

I ve googled around and found most people advocating the use of kmalloc  as you re guaranteed to get contiguous physical blocks of memory  However  it also seems as though kmalloc can fail if a contiguous physical block that you want can t be found  What are the advantages of having a contiguous block of memory  Specifically  why would I need to have a contiguous physical block of memory in a system call  Is there any reason I couldn t just use vmalloc  Finally  if I were to allocate memory during the handling of a system call  should I specify GFP ATOMIC   Is a system call executed in an atomic context      GFP ATOMIC   The allocation is high-priority and   does not sleep  This is the flag to   use in interrupt handlers  bottom   halves and other situations where you   cannot sleep       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

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The kmalloc    amp  vmalloc   functions are a simple interface for obtaining kernel memory in byte-sized chunks    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

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In short  vmalloc and kmalloc both could fix fragmentation  vmalloc use memory mappings to fix external fragmentation   kmalloc use slab to fix internal frgamentation  Fot what it s worth  kmalloc also has many other advantages

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Linux Kernel Development by Robert Love  Chapter 12  page 244 in 3rd edition  answers this very clearly   Yes  physically contiguous memory is not required in many of the cases  Main reason for kmalloc being used more than vmalloc in kernel is performance  The book explains  when big memory chunks are allocated using vmalloc  kernel has to map the physically non-contiguous chunks  pages  into a single contiguous virtual memory region  Since the memory is virtually contiguous and physically non-contiguous  several virtual-to-physical address mappings will have to be added to the page table  And in the worst case  there will be  size of buffer page size  number of mappings added to the page table   This also adds pressure on TLB  the cache entries storing recent virtual to physical address mappings  when accessing this buffer  This can lead to thrashing

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You only need to worry about using physically contiguous memory if the buffer will be accessed by a DMA device on a physically addressed bus  like PCI   The trouble is that many system calls have no way to know whether their buffer will eventually be passed to a DMA device  once you pass the buffer to another kernel subsystem  you really cannot know where it is going to go  Even if the kernel does not use the buffer for DMA today  a future development might do so   vmalloc is often slower than kmalloc  because it may have to remap the buffer space into a virtually contiguous range  kmalloc never remaps  though if not called with GFP ATOMIC kmalloc can block   kmalloc is limited in the size of buffer it can provide  128 KBytes    If you need a really big buffer  you have to use vmalloc or some other mechanism like reserving high memory at boot         This was true of earlier kernels  On recent kernels  I tested this on 2 6 33 2   max size of a single kmalloc is up to 4 MB   I wrote a fairly detailed post on this    mdash  kaiwan   For a system call you don t need to pass GFP ATOMIC to kmalloc    you can use GFP KERNEL  You re not an interrupt handler  the application code enters the kernel context by means of a trap  it is not an interrupt

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What are the advantages of having a contiguous block of memory  Specifically  why would I need to have a contiguous physical block of memory in a system call  Is there any reason I couldn t just use vmalloc   From Google s  I m Feeling Lucky  on vmalloc   kmalloc is the preferred way  as long as you don t need very big areas  The trouble is  if you want to do DMA from to some hardware device  you ll need to use kmalloc  and you ll probably need bigger chunk  The solution is to allocate memory as soon as possible  before memory gets fragmented

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Short answer  download Linux Device Drivers and read the chapter on memory management   Seriously  there are a lot of subtle issues related to kernel memory management that you need to understand - I spend a lot of my time debugging problems with it   vmalloc   is very rarely used  because the kernel rarely uses virtual memory  kmalloc   is what is typically used  but you have to know what the consequences of the different flags are and you need a strategy for dealing with what happens when it fails - particularly if you re in an interrupt handler  like you suggested

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One of other differences is kmalloc will return logical address  else you specify GPF HIGHMEM   Logical addresses are placed in  low memory   in the first gigabyte of physical memory  and are mapped directly to physical addresses  use   pa macro to convert it   This property implies kmalloced memory is continuous memory   In other hand  Vmalloc is able to return virtual addresses from  high memory   These addresses cannot be converted in physical addresses in a direct fashion  you have to use virt to page function

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On a 32-bit system  kmalloc   returns the kernel logical address  its a virtual address though  which has the direct mapping  actually with constant offset  to physical address  This direct mapping ensures that we get a contiguous physical chunk of RAM  Suited for DMA where we give only the initial pointer and expect a contiguous physical mapping thereafter for our operation   vmalloc   returns the kernel virtual address which in turn might not be having a contiguous mapping on physical RAM  Useful for large memory allocation and in cases where we don t care about that the memory allocated to our process is continuous also in Physical RAM

[c] What is the difference between vmalloc and kmalloc?

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