Re: Possible cure for memory fragmentation.

[James Courtier-Dutton <James@superbug.demon.co.uk>]

Christoph Lameter wrote:
> On Wed, 21 Dec 2005, James Courtier-Dutton wrote:
> 
> 
>>I am suggesting we add a new memory allocation function into the kernel
>>called kremalloc().
>>
>>The purpose of any call to kremalloc() would mean that:
>>a) One really needs the memory already allocated, so don't loose it.
>>b) One does not mind if the memory location moves.
>>
>>Now, the kernel driver module that has previously allocated a memory block,
>>could at a time convenient to itself, allow the memory to be moved. It
>>would simple call kremalloc() with the same size parameter as it originally
>>called kmalloc(). The mm would then notice this, and then, if that location
>>had been tagged with (1), the mm could then happily move it, and the kernel
>>driver module would be happy. If it was not tagged with (1) the mm would
>>simply return, so very little overhead.
> 
> 
> Moving regular mapped kernel memory is not trivial. See my page migration
> patchsets.
> 
> Slab memory cannot be resized since the memory is managed in portions 
> of fixed sizes. So if these size boundaries are violated then the 
> kremalloc would degenerate into a kfree and a kmalloc. kremalloc 
> would be:
> 
> void *kremalloc(void *p, int oldsize, int newsize, gfp_t f)
> {
> 	void *new;
> 
> 	if (newsize < sizeboundary)
> 		return p;
> 
> 	new = kmalloc(size, f);
> 
> 	memcpy(new, old, oldsize);
> 
> 	kfree(p);
> 
> 	return new;
> }
> 
> 
> 

I think you missed the point I was trying to make.
The driver does not call kremalloc with a different size. It calls it 
with the SAME size. Then if the kernel thinks it would benefit from 
moving the allocation to a different location, it can do:

new = kmalloc(size, f);
if (!new)
	return p;
memcpy(new, old, oldsize);
kfree(p);

If the kernel does not wish to move it, kremalloc returns without having 
done anything.

This would effectively allow for dynamic recovery from a fragmented 
memory layout.

James

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