Re: Packing data in kernel memory

[John Richard Moser <nigelenki@comcast.net>]

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Randy.Dunlap wrote:
> On Tue, 13 Jun 2006 22:18:55 +0200 (MEST) Jan Engelhardt wrote:
> 
>>> Subject: Packing data in kernel memory
>>>
>> Can't you just use mlock(), if you want to keep it in RAM?
>>
>> Or do you need it in kernel memory, because you need it in the lowmem area? 
>> Or for interaction with other kernel code?
>>
>>> Is there a way to pack and store arbitrary data in the kernel, or do I
>>> need to roll my own?
> 
> Sounds a bit like a slab cache to me.
> 

OK cool, can I make that non-swappable?  I'm going to be trying to do
this between where kernel swaps a page out and swapped page actually is
written to disk.  The result will be a "Swap Zone," in-memory storage of
pages that the rest of the kernel thinks have been swapped to disk.
(Code here will use the swap interface, so the rest of the kernel thinks
it's just swapping; I'll handle whether to pull it out of memory or off
disk behind that)

The need for packing pages comes because eventually (using above
infrastructure) I'll be taking sets of 32KiB of data and compressing it;
I don't want to pad up to 4095 excess unused bytes if that stuff
compresses to 28KiB+1 :)  (more likely 16KiB+1 +/-8KiB)

This is all, of course, assuming I ever figure out how the heck to get
in the middle of the swapping process.  I'm looking at mm/page_io.c
swap_writepage() and friends and scratching my head.  I have no idea wtf...


>> Write a device driver, kmalloc some buffer, and copy data via a write 
>> function from userspace to that buffer. Should be trivial.
>>
>>> 1 excess pages, 4 units wasted memory.
>> Of course, kmalloc only works up to some boundary AFIACS.
> 
> 128 KB on some arches.  More on a few IIRC.
> 
> ---
> ~Randy
> 

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