Differences between VM structs

Differences between VM structs

[Kirk True]

Hi all,

I apologize to ask these questions here, but I got no response from the 
newbies list...

     1. Regarding non-contiguous memory allocation, what is the need to 

        have *virtually* contiguous but not *physically* contiguous
        pages?
     2. UtLVMM says that vmalloc is only used in the kernel for storing
        swap information - yet it's used by a bunch of drivers which
        are considered part of the kernel; is it just semantics?
     3. Is vmalloc called from user-mode ever?
     4. Can you state a succint/brief comparison of the difference
        between kmalloc, malloc, and vmalloc with usage examples of each?
     5. Anonymous memory is memory that is *not* backed by a file, such
        as the stack or heap space, right? And mmap is called when
        mapping files into memory, right? The why does mmap deal with
        anonymous memory (sorry, I'm totally confused here)?

TIA!
Kirk

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Re: Differences between VM structs

[Marco Cova]

On Mon, Sep 08, 2003 at 11:21:38AM -0700, William Lee Irwin III wrote:
> On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
> >     4. Can you state a succint/brief comparison of the difference
> >        between kmalloc, malloc, and vmalloc with usage examples of each?
> 
> No.

Maybe this can be helpful:
http://www.xml.com/ldd/chapter/book/ch07.html

Also, check out this link for a description of mmap:
http://www.xml.com/ldd/chapter/book/ch13.html

HTH,
Marco

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Re: Differences between VM structs

[William Lee Irwin III]

On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
>     1. Regarding non-contiguous memory allocation, what is the need to 
>        have *virtually* contiguous but not *physically* contiguous
>        pages?

Fragmentation happens.


On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
>     2. UtLVMM says that vmalloc is only used in the kernel for storing
>        swap information - yet it's used by a bunch of drivers which
>        are considered part of the kernel; is it just semantics?

No, its usage has probably expanded. Drivers are generally not supposed
to try to use it directly. It's used for things like vmap() too nowadays.


On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
>     3. Is vmalloc called from user-mode ever?

No function in the kernel can be called directly from usermode.


On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
>     4. Can you state a succint/brief comparison of the difference
>        between kmalloc, malloc, and vmalloc with usage examples of each?

No.


On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
>     5. Anonymous memory is memory that is *not* backed by a file, such
>        as the stack or heap space, right? And mmap is called when
>        mapping files into memory, right? The why does mmap deal with
>        anonymous memory (sorry, I'm totally confused here)?

mmap() needed very few extensions to handle the anonymous case.


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Re: Differences between VM structs

[Raghu R. Arur]

> On Mon, Sep 08, 2003 at 09:26:59AM -0700, Kirk True wrote:
> >     5. Anonymous memory is memory that is *not* backed by a file, such
> >        as the stack or heap space, right? And mmap is called when
> >        mapping files into memory, right? The why does mmap deal with
> >        anonymous memory (sorry, I'm totally confused here)?
>
> mmap() needed very few extensions to handle the anonymous case.


 What are these extensions in mmap() that need to handle anonymous pages??
 Thanks a lot,
 Raghu
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Re: Differences between VM structs

[William Lee Irwin III]

At some point in the past, I wrote:
>> mmap() needed very few extensions to handle the anonymous case.

On Mon, Sep 08, 2003 at 02:26:50PM -0400, Raghu R. Arur wrote:
>  What are these extensions in mmap() that need to handle anonymous pages??
>  Thanks a lot,
>  Raghu

MAP_ANONYMOUS.


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